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Problem: Why I am not getting 24 volts to the contactor? Where does the 24 volts come from?

Happy AC

Problem: Why I am not getting 24 volts to the contactor? Where does the 24 volts come from?

Answer: The 24 volts comes from a low voltage transformer. Please see the pictures of the two transformers below.  The transformer has a primary (high voltage side) and a secondary (low voltage side).  The high voltage side takes 110 to 240 volts AC and transforms the voltage to 24 to 28 volts AC. If the contactor is not getting the 24 volts to the contactor coil then it could be a time delay relay (if the unit is equipped), thermostat problem, transformer, safety control (low on refrigerate) or wiring problem. I would like to suggest that you turn the power off to the unit and check all your connections to make sure they are good and tight. You can turn the power back on and set the thermostat so the system is calling for cooling (AC ON).  You could test the contactor with a volt meter to see if you are getting 24 to 28 volts AC to the contactor 24 volt coil. If you are not getting any voltage to the contactor coil then this could be a blown fuse on your furnace or air handler control board. Please see the picture of the control with fuse below.  If the fuse on your control board is blown then you probably have a short in the thermostat wires or another control in the furnace or outdoor unit.  We sell an universal time delay relay on the following page: Please click here if you would like to see the universal time delay relay we sell.  We sell transformers on the following page: Please click here if you would like to see the Low Voltage Transformers we sell.  Many air conditioners and heat pumps have low and high pressure safety switches on them to protect the unit from damage if the system pressure is too low (low refrigerant charge) or too high (dirty outdoor unit, slow or bad condenser fan motor). If your unit is low on charge (I find most often) the safety switch will not allow the unit to start (contactor will not engage). You will need to call a service tech in order to check the charge on your unit. We sell contactors on the following page: Please click here if you would like to see the AC contactors that we sell. Please send us your unit’s model number if you would like for us to look it up and see which OEM contactor your unit requires. Our email address is: support@arnoldservice.com  Below we have a real good informative YouTube video made by grayfurnace man which shows how to troubleshoot and test a contactor. If you have any questions we will be glad to answer them. Please email us: support@arnoldservice.com or comment in our comments section below. We would love to help you out and have your business!

 

Universal Low Voltage Transformer  Carrier Bryant Furnace Transformer HT01CN001

Troubleshooting Gas & Electric Furnaces

148 thoughts on “Problem: Why I am not getting 24 volts to the contactor? Where does the 24 volts come from?

  1. I have 36E24 210 EF32CW201A gas valve. Contractors changed my gas valve 11 times (more then $800 plus $98-110 for each visit). Each new gas valve works only 10- 30 min and fails. They are trusted professionals, but have no idea what happened each time!
    I tested voltage. It gives 28v and some other time 32v and even 58v. Is it possible that high voltage damages each new valve? What would be your suggestions? Not another new gas valve please 🙂

    1. Hi George! I hope that all of the gas valves were under warranty! I would think that you have a control board problem because it sounds like the control board is sending voltage that is too high for the gas valve and it burning the gas valve coil up. I would suggest checking all the low and high voltage wires and connections to make sure they are not shorting out against each other. If all the wires and connections look good then I would recommend a new control board to hopefully eliminate the gas valve problem. I hope you can find out what is wrong and get it fixed soon! Steve

      1. Hi Steve, thank you for your fast response. It is good to know that your web page almost in real time provides professional oppinion

      2. Thank you so very much for your kind words! I hope you have a great and blessed Holiday Season! Steve

  2. My fan runs but neither the furnace or ac will come on. And i can’t shut off the fan except by killing power to the entire unit. I replaced the thermostat and nothing changed. I get a reading of 15 volts at the control board connecting the red wire to green or white or blue.. I also get 15 volts at the location of the thermostat when checking between red and either green or white or blue. The voltage seems to be enough to start the fan. But there seems to be no way to signal anything.

    1. Hi Leigh! The voltage always runs to C (common). You would need to determine what color wire is the common (C) wire. The two wires that come off your low voltage secondary on your transformer should go directly to R and C (com) on your control board. Many thermostats do not have the C connection on the thermostat. If you do not have a C (com) connection on your thermostat then you will need to test with your voltmeter down at the furnace’s control board. With the thermostat calling for heat to be ON you should have 24 to 28 volts AC between the R and C and between W and C. If you turn the fan on you should have 24 to 28 volts between G and C. With the AC ON you should have 24 to 28 volts AC between R and C, G and C and Y and C. If you are not getting 24 volts and only 15 volts then you might have a low voltage problem somewhere. This could be your low voltage transformer that is weak. A shorted thermostat wire or low voltage control. It could be a fuse or an open rollout or limit that is causing the problem. I hope you can find and fix the problem soon! Steve

  3. Hi Steve, I have a dual Lennox AC system (model G61MPV-36B-070-1) on a two storage house and the upstairs unit lost the 24V at the contactor. Checked the circuit board at the furnace and it has a 5 + 1 flash code, “rollout circut open -reset rollout switch, check wiring, and recycle power to the furnace”. Could you please help? Thank you, David

    1. Hi David! If you are not getting 24 volts to the contactor then I would like to suggest that you set your thermostat so that the thermostat is calling for cooling. Then tape the furnace blower door switch on so you can test with a voltmeter while the furnace is in cooling mode. You should be getting 24 to 28 volts AC between the R and C (common) wires. If you are not getting 24 volts between R and C then you probably have a low voltage transformer problem or a blown low voltage fuse. You should also be getting 24 volts between the Y and C (com) connections on your control board. Sometimes installers do not attach the Y wire on the control board and just have the Y wire attached loosely to the two wires that go out to your outdoor unit with wire nuts. You should be getting 24 to 28 volts AC between the Y and C (com) wire with your thermostat set to call for cooling. If you are not getting the 24 volts then you either have a thermostat problem or a thermostat wire problem. If you are getting 24 volts between Y and C then continued to check between those two wires on out to your outdoor unit. Where you stop getting 24 volts AC is where the problem is located. You might have a bad contactor. We sell contactors on the following page: https://arnoldservice.com/product-category/air-conditioner-parts/ac-contactors-heat-pump-contactors/ I hope you can easily find and fix the problem. Steve

  4. Hello. My Ac unit is running intermittently; it would work one day and not the next day. It started last week; I noticed that air (from the blower in the garage) was flowing inside the house, but the fan ac inside the condenser outside the house was not turning and making grinding noise (compressor was on). I immediately turned it off. One by one, I replaced the parts with new, one by one, the capacitor, fan motor, and the contactor. The ac would still not turn on without manually engaging the contactor, so I figured the issue might be inside the house. So I took out the thermostat and jumped the wires. I also checked the control board in the blower and found that the fuse is okay and getting low voltage coming in (from the thermostat) and going out to the contactor in the ac unit. I checked the contactor, and I was getting a high voltage reading of ~240V coming from the disconnect box, but no low voltage when I place my probe on both sides of the contactor. However, when I place one probe on one side of the contactor and the other probe to a metal structure of the ac, I’m getting a low voltage reading of ~27 volts. The same ~27 volts reading when I do it on the other side of the contactor. Thus I presume this is indicative that the contactor is receiving low voltage but not engaging. Also, the pressure switch wires from the pipe compressor are not connected to the contactor, so I don’t think that causing it not to run. Yesterday, It was 105 deg. F outside and ac wasn’t working despite many attempts to override the thermostat. I even waited several minutes to see if it just a delay issue, but nothing, so I turn it off. This morning when it is still cool outside, I turned on the thermostat and the ac ran. I’m still confused on what is the issue. Is it a bad wiring connection (I checked, no broken wires), contactor (brand new), thermostat, or control board? Would you know what is going on and advise how to repair? Thanks.

    1. Hi Even! You should have 24 to 28 volts AC on the coil of the contactor. This most certainly sounds like you have a low voltage problem where the contactor is not getting the required 24 volts to energize the contacts. Going from each side of the contactor to ground and getting 24 volts does not mean that you are getting 24 volts to the contactor coil. You should get 24 to 28 volts AC between the two wires that go to your contactor coil. I would suggest turning your thermostat down so that the thermostat is calling for cooling (for the AC to be ON). Start at your control board. You should have 24 volts AC between the Y and C terminals on the control board. Sometimes installers do not attach the Y wire to the furnace’s control board but attach it outside the furnace or near the control board. If you are not getting 24 volts between the Y and C (com) wires when the thermostat is calling for cooling to be on then you must have either a thermostat problem or a thermostat wire problem. If you are getting 24 volts between Y and C then continue to follow those two wires out to your outdoor unit. Start testing where the two thermostat wires go into the outdoor unit’s control box. If you are getting 24 volts there then continue testing for 24 volts. Where you stop getting 24 volts is where the problem is. Some units have a delay timer on them and that might be the problem that is not allowing the contactor to get 24 volts. Some electric companies install devices that allow the electric company to control your unit during the summer months. If you have one of these devices then this could be the problem. I hope you can easily find and fix the problem soon so you can stay cool. God bless you and your family today and always! Steve

      1. I have done all that…when I test R & Y : 28 VAC. When I tested the transformer leads I have the correct voltages. When I tested y&C no voltage. Fuse is good. I don’t have any power to the connector. Everything works except the AC. Lennox g11. I guess I can say it is the board or faulty ground but I can not find the ground on the board. Any suggestion.

      2. Hi Jesse! You should be getting 24 to 28 volts between Y and C when your thermostat is calling for cooling. The ground on the control board is the “C” (com) terminal. From your low voltage transformer “R” is the power wire from the transformer and “C” is the common or ground wire from the transformer. If you are not getting 24 volts between Y and C when the thermostat is calling for cooling then you probably have a bad thermostat or a broken or loose thermostat wire. You can turn off the power to your furnace and take the R (red) G (green) and Y (yellow) wires and tie them together with a wire nut. Turn the power back on to your furnace. The blower should come on and your AC should work. If it does work with the 3 wires tied together then you have a thermostat problem. If it does not work then you have another problem that you will need to troubleshoot and find out where the problem is located. I hope you can find and fix the problem soon. Steve

  5. Hi Steve, my ac condenser doesn’t come on and will only work when I depress the plunger on the contractor. I used a multimeter to check if it was getting 24 volts and it isn’t getting any voltage. Last fall before it got cold I installed a nest smart thermostat so I checked to see if there was any issues with the wiring. I checked and I am getting close to 27 volts when touching the R and C contacts. I went to my furnace and checked the 3 amp fuse to see if it had blown and it is good. So, the thermostat is getting 24 volts but the contacted on my ac condenser isn’t. Any suggestions on what it could be? The AC worked last summer and I replaced the thermostat since then so that was my initial suspicion but the smart thermostat tells me I’ve wired it correctly and I get 24 volts. Any help would be greatly appreciated.

    1. Hi Joel! I would suggest that you make sure that you are getting 24 to 28 volts AC between the Y and C (common) wires when the thermostat is calling for cooling to be ON. The “Y” wire/connection is what sends voltage to the outdoor unit’s contactor to be energized. If you are not getting 24 volts between Y and C then you either have a thermostat problem or a thermostat wiring problem. If you are getting 24 volts between Y and C when the thermostat is calling for cooling to be ON then you might have contactor problem or a low-pressure safety switch problem. On some of the air conditioners, there is a low-pressure switch that will not allow the unit to come on if the unit is low on refrigerant. Your unit could be low on refrigerate. If your unit is a heat pump sometimes the defrost control boards can cause the contactor to not get 24 volts. I would like to suggest that you check the contactor’s coil voltage to see if you are getting 24 volts to the contactor coil. If the contactor coil is getting 24 volts and not engaging then you need a new contactor. We sell contactors on the following page: https://arnoldservice.com/product-category/air-conditioner-parts/ac-contactors-heat-pump-contactors/ I hope you can easily find and fix the problem. Steve

  6. Hi,

    I have a heat pump that starts up very intermittently when calling for cooling. When it’s not working, the thermostat is not getting 24v to r+y and r+o and the defrost board and contactor is also not getting 24v. Any ideas what else I can check? I’m wondering if the issue could be a bad transformer or a bad control board in the air handler.

    1. Hi Arthur! If your thermostat is not getting 24 volts between R and C (common) then you probably either have a bad transformer or a blown low voltage fuse. All voltages should be measured between the power wire and the C (common wire transformer ground) So with the thermostat calling for cooling you should have 24 to 28 volts between R and C, Y and C, G and C and O and C. If you are getting 24 volts between R and C and not any voltage between Y and C and other wires that I just mentioned then you probably have a bad thermostat. I hope you can easily find and fix the problem. Steve

  7. hello Steve ac not working i have replaced start capsitor/ contactor /thermostat. i also have 8 volts at thermostat and 38 volts at transformer also the thermostat is the 2nd one can you help

    1. Hi Thoms! This sounds like you might have a short in one of your thermostat wires. You should have 24 to 28 volts AC between the R and C terminals on your control board. I would suggest that you disconnect the thermostat wires that go to your outdoor unit and see if this corrects the low voltage problem. Most of the time shorted thermostat wires occur on the outdoor unit because they are exposed to the sun and animals who chew through the wires. If that does not correct the low voltage problem then I would suggest that you disconnect all low voltage wires and see if you are getting 24 to 28 volts AC between R and C (com) at the control board. If you are not getting 24 volts after all thermostat wires are disconnected then you must have a low voltage transformer problem and you will need a new transformer. We have an air conditioner troubleshooting flow chart that I hope will help you determine where the problem is: https://arnoldservice.com/wp-content/uploads/2014/12/air-conditioner-troubleshooting1.pdf We have lots of air conditioning and heat pump troubleshooting questions with answers and Youtube videos on the following page: https://arnoldservice.com/air-conditioning-heat-pump-troubleshooting-questions-answers-index/ We also have an air conditioning and heat pump troubleshooting simplified page that I hope will help you find and fix the problem: https://arnoldservice.com/air-conditioning-heat-pump-troubleshooting-simplified/ We also have lots of troubleshooting advice on the following page: https://arnoldservice.com/troubleshooting/ If you have any specific questions please let me know. I hope you have a great day! Steve

      1. Heres a fun one, thermostat had no low voltage, control board puts out 27v, around 8pm when things cool down, magically i get 24v and the condensor and system runs perfect. So it only cuts out the low voltage during the heat of the day, 10am thru 8pm. Is this really just a poor connection somewhere that shrinks when it cools and BAM it shrinks and connects again? Weirder still is the house is 2 air handlers and 2 condensors completely seperate, but both are cutting out low voltage during the day. One i get only 24 from the control board, the other it makes it to the thermostat but no low voltage to the condensor. Then like i said magically they both suddenly get power and kick on once the sun goes down. So this would be 2 seperate bad connections in 2 different places at the same time that are expanding and shrinking back??? I cant think of anything else. Am i crazy. Times arent exact so its not a program in the thermostat, just vanishing and reappearing voltages. 20yr old house original wiring that has been reconnected to multiple units over the years so i think im right but… Any advice? Other than fishing new wire lol.

      2. Hi Jeff! Yes, this is really strange! Either you have a weak transformer/transformers that that are not producing enough low voltage during the day or your utility company is cutting the voltage down too much during the day. I would suggest that you test your low voltage transformer (between R and C Com) to see what low voltage is being produced during the time the units are off and not working correctly. If the transformers are producing, outputting below 24 volts then I would suggest that you check the primary voltage to make sure you are getting at least 110 volts to the primary. Please be careful to not get shocked. If the voltage is below 110 then you either have a loose connection or your utility company is cutting the voltage down to conserve electricity. For both units to not work between the same time periods this leads me to think that the utility electric company is cutting back the voltage to conserve their resources. If you are getting 110 on the primary and lower than 24 on the secondary then you might want a higher VA transformer that is over 40 VA. Maybe the transformer is not strong enough to support the unit’s requirements. I am just guessing at possible problems. Yes, I would suggest that you turn the power off to your units and check all low voltage and high voltage connections to make sure they are good and tight. Any loose connections can cause high amp readings and cause problems. I hope you can find and fix the problem soon! God bless you and your family. Steve

  8. Good morning Steve!
    I was troubleshooting my 24v wiring but I found something that I’m not sure how is happening; My outside unit wasn’t working(was not getting 24v) so after troubleshooting for a long time…when I land my Y wire going outside unit I don’t read 24v from C to Y at my control board screws BUT when I only have a thermostat Y wire I do read 24v from C to Y…i have my wires loose at my outside unit just to make sure my contractor wasn’t the problem but it’s still happening…I tied my Y wire going to my outside unit together with my G screw to see if I had a week transformer or to see if it would work like that and it did…I get my 24v outside…do you know what’s wrong?…. it seems like just by adding 1 wire to my Y terminal at my control board I don’t read 24v from Y to C anymore…thank you

    1. Hi Cristian! This sounds like you might have some bad thermostat wires that go to your outdoor unit. Many times the outdoor wires will go bad due to the fact that they are exposed to the heat of the sun, sunlight, and animals. I would like to suggest that you either test your thermostat wires that go to your outdoor unit by disconnecting the wires at both ends (both control board and outdoor unit end). Set your meter to ohms and make sure that you are not getting any resistance between the two wires. If you are getting any resistance then the wires are shorting out and you need new thermostat wire. Other than having a wire problem you could have a thermostat problem that has a weak relay in your thermostat where the volts drop to zero when a load is applied (outdoor contactor) or a weak transformer. I hope you can easily find and fix the problem. Steve

  9. I’m getting 24v on the Y and C wires if they are off the contactor when I set it to cooling. But once I plug them to the contactor the voltage drops. If I need to replace the wires coming from the circuit board all the way outside to the condenser unit, what gauge of wire should I use? The distance from my furnace is probably be between 20-25ft.

    1. Hi Vinci!
      Sounds like you might have a contactor that is bad or grounded out. You might need a new contactor, thermostat, thermostat wires or low voltage transformer. You might try plugging the wires into a new contactor and see if you get the same results. If you do then I would look into a new transformer, thermostat wires or a new thermostat. To rule out the thermostat as being the problem you should be able to tie the red (R) and Yellow (Y) and Green (G” wires together. The air conditioner and blower motor should come on. If they do not then you have a thermostat wire or transformer problem. I hope you can easily find and fix the problem. Steve

      1. i found this info very useful…thank you …your comment got me out of a mess….

      2. Hi Paul! Glad to hear that this information helped you out! Thanks so very much for taking your time to thank us! God bless you and your family today and always! Steve

  10. Hi Steve!

    The blower motor on my air handler has sometimes failed to start on both heat and cold. It will try turning but ends up slowly rotating back and forth for an infinite amount of time (Motor/Module is ECM 2.3) I removed the 16 pin harness and saw there was 29 volts at R and C. However, when I run the fan, I receive only 15 volts between G and C.

    I took the harness off and checked the signal directly on the board. Again, I was getting only 15 volts on G and C. Where do I go from there in order to trace the low voltage issue? (As a side note, I checked the wires going to my thermostat and I am getting 29 volts from G and C) Should I try cleaning the contract from G on the board to the harness? Do I need a new control board?

    Any help is appreciated. Thank you.

    1. Hi Marc! This sounds like you need a new motor module. My furnace with an ECM motor was doing the same thing that you describe and a new motor module fixed the problem. In most ECM motors the motor modules are what goes bad and not the motor. I am sorry, but I am not familiar with testing the ECM motors using a voltmeter. I use a tool similar to the one sold on the following page to test ECM motors https://www.supplyhouse.com/Genteq-Motors-GA447-TECINspect-Service-Tool?gclid=Cj0KCQjw4-XlBRDuARIsAK96p3DDGqlef10aLEYKIHCoy1m9DK28B4sIit0pXTJ4AH3zHsA5uZdck40aArYQEALw_wcB I hope you can easily find and fix the problem. Hope you have a Great and Blessed Easter Weekend! Steve

      1. Thanks for the info!

        I was thinking that it was the motor module till I saw the board was sending 15 volts to the motor module at the “G” #15 pin. That was with the harness removed from the module and reading directly on the control board’s pins.

        Could that be the issue if the module is not receiving proper voltage? Or is the 15 volts coming out of the control board to the module harness sufficient for the blower motor to turn on?

        Have a good and blessed Easter as well.

      2. Hi Marc! I am sorry, but I do not know anything about the ECM motors. You would need to ask someone who is knowledgable about the motors to get the right answer or search online. AC Service tech has many great videos that you help you troubleshoot the motor. Here is a link to the AC Service tech channel: https://www.youtube.com/results?search_query=ecm+motor+troubleshooting I would suggest that you use the Youtube search box to search for ECM motor troubleshooting. Sorry that I can not help you out. Thank you so very much for your Easter blessings! Steve

  11. Hello my 24 volt stays on making the condensing unit to run without calling for ac please help

    1. Hi! More than likely you might have a thermostat problem or thermostat wires could be shorted causing the unit to run all the time. If you have a heat pump it could be a problem with the defrost control board. I would like to suggest that you turn off the power to your furnace or air handler and disconnect the “Y” connection on your thermostat. Turn the power back on and see if your outdoor unit still comes on. It should not come on. If it still comes on then you probably have shorted thermostat wires. If it does not come on then more than likely you have a thermostat problem. I hope you can easily find and fix the problem. Steve

  12. Hi Steve,
    I am in the process of updating my thermostat I have in the basement on a Lennox Pulse 21 gas Furnace. I am replacing a old two wire unit with one that requires 24 volt constant. The old thermostat had the red and white wires only. I have opened up the furnace to check the connection and see that the red is going to R and the White wire is going to W no other wires are connected but there is the RWYGC connections. Following your directions I tested the voltage from R to C and got 44 Volts. Can you help me? This thermostat because of it’s location does not control the air conditioning system and as I indicated not other wires are connected on this board. The old thermostat worked okay I just wanted a digital read rather than the inaccurate swinging lever.

    1. Hi Ed! If the new thermostat requires a common wire to make it work then you will need to run new cable to the new thermostat. You would need a third wire connected to the C or common connection on your furnace’s control board. If it were me I would take the new thermostat back and get an electronic thermostat that does not require the C (common connection) because running new thermostat wires can be a pain. I have a White-Rogers Sensi wi-fi thermostat that runs off batteries and does not require the C connection. We can run our thermostat from our Smart Phone and it is nice. You might look into getting another thermostat unless you want to run new thermostat wire. I hope you have a great day! Steve

  13. HI i have a york roof top predator recently i noticed contractor was chattering and the motor was warbling.So i replaced the contractor when blower goes on it vibrates and the contractor wont pull in if i push contractor in manually the motor run smooth the motor is a three phase

    1. Hi! It sounds like the contactor is not getting enough voltage to the coil. I would suggest that you check the coil voltage and make sure it is getting a minimum of 24 volts if it is a 24-volt coil and 220 volts if you have a 220-volt coil. If the coil is not getting enough voltage the contactor will chatter and it will burn the contactor up quick. I hope you can easily find and fix the problem. Steve

  14. Hi. I just tried to replace an old round Honeywell mercury thermostat with a new Honeywell round one. I bought the right one and had the settings all correct but the heat was not coming on when I was trying to call for it. It tested the voltage at the thermostat and I’m getting 44 volts instead of the 24ish volts. I think this is causing the thermostat to not work. I’m dumbfounded. Any suggestions?

    1. Hi Ken! You should not be able to get voltage at the thermostat unless you have 5 wires with a Common wire coming from the transformer. If you had the 5th common wire the only two you should get 24 volts out of would be between R (red) and C (common). If you do not have a 5 thermostat wire setup then you should be able to turn the power off to the furnace, tie the red and white wires together at the thermostat and the gas valve should come on. Disconnect the white and red wires. If you want the air conditioner to come on you should be able to join red (power wire) to the most of the time Y, yellow and G green wire and the AC and blower should come on. We have a post about wiring thermostats on the following page: https://arnoldservice.com/problem-i-purchased-a-new-programmable-thermostat-my-thermostat-wiring-only-has-only-three-wires-the-thermostat-instructions-call-for-a-red-power-wire-a-yellow-green-and-white-wire-my-three-wire/ I hope this helps you out. If you have any other questions please let me know by emailing me at arnoldservice@gmail.com. Hope you get it fixed! Steve

  15. Hi Steve, I have an older Lennox Heat pump that is getting 24 volts to the control board but loses voltage down to 18 or 19 at the contactor. A/C works when directly wired from thermostat to contactor. this is the second control board put on this unit, worked for around 9 months then began to have the same issue the previous one had, any suggestions?

    1. Hi Chris! Any short or loose connection in the low voltage system could cause this voltage drop. This could include a slight short or loose connection in the thermostat wires, thermostat, control board, transformer, reversing valve solenoid, defrost board or contactor. Sorry, but this could be hard to find. If the control board fixes the problem then I would say the control board is at fault. I would suggest trying to by-pass the control board and wire directly to the contactor and see if the problem goes away. If it does then you will know it is another control board problem. I hope you can easily find and fix the problem. Steve

  16. Hi Steve! I get only 9 volts to the contactor on my outside compressor unit when my thermostat is calling for cooling. Is this a weak low voltage transformer? I have replaced the thermostat and contactor and checked the wiring at the transformer. Thank you

    1. Hi Bob! This could be a weak transformer, a loose or shorted thermostat wire, a short in a component like a contactor or control board, a weak battery in the thermostat or a bad relay in the thermostat. I would suggest that you test the voltage with the AC turned off and see if you are getting 24 to 28 volts AC between the R and C terminals. This could be hard to troubleshoot. If you aren’t getting 24 to 28 volts AC between R and C (com) then I would say you have a weak transformer. I would then turn the AC on and see if you are getting 24 volts between R & C. If the voltage is lower than 24 volts with the AC turned on then I would try disconnecting one of the low voltage wires on the contactor and see if the voltage improves. If the voltage goes up over 24 volts with the contactor disconnected then you might have a bad contactor or a weak transformer. I would suggest making sure all your connections are good and tight. I hope you can easily find and fix the problem. Steve

  17. Hi Steve,
    I had a bad transformer, I replaced with new one, connected 120 v at primary, I got 26v at secondary, after connected secondary wires to the board, The voltage became to 9 v, I checked all wire tight, do you know what happened?

    1. Hi Daniel! You probably have a short in the thermostat wires or in your low voltage controls somewhere. We have a post on the following page that discusses this problem: https://arnoldservice.com/problem-my-24-volt-transformer-continues-to-burn-up-this-is-the-second-transformer-what-could-be-the-problem/ Here is another post that might help you out: https://arnoldservice.com/problem-furnace-air-conditioner-heat-pump-will-not-come-on/ I hope you can easily find and fix the problem. Steve

  18. My connector does not get the 24 volt but when I disconnect the 24v wires from the connector and check the voltage on the wires they do have 24 volt, I connect it again to the connector an no 24v. Any idea what that could mean?

    1. Hi Sam! You might have a bad coil on your contactor (requires a new contactor), loose thermostat wire connection, thermostat with weak batteries, bad thermostat relay (might need a new thermostat) or a weak low voltage transformer (might need a new transformer). I hope you can find an easy inexpensive soluction. Steve

  19. Hi Steve! I have a Train 10Ton packaged unit my first stage of cooling is coming on but the second stage of cooling is not , am not getting 24V to my contactor coil, can the board be bad.

    1. Hi Luis! I do not know much about the new Trane 2 stage packaged units, because of the electronics and complex wiring. I would think that if your second stage of cooling is not coming on then I would think it could be the control board. If there are any diagnostic lights on the control board I would suggest trying to read the flash code and see what the board says is wrong. Sorry that I can not be any help in answering your question. If you find out what the problem is please let me know. Thanks! Steve

  20. Good afternoon Steve,
    I have an American standard ac system that has no power to the thermostat. I have proper voltage out of the transformer to the circuit board, but nothing is coming out. I removed the board to check the back and it doesn’t have any burnt spots on it. It looks like the board has lights on it, but they are not lit up. Could the board be bad and not appear so? I’ve checked the fuses and circuit breakers and they are all good.
    Thank you,
    Clint

    1. Hi Clint! Sorry to hear that you are having trouble with your air conditioner on the 4th of July! You should be getting 24 to 28 volts AC between the R and C (com) terminals on the control board and when the thermostat is calling for cooling you should have 24 to 28 volts between Y and C, G and C, and of course R and C. If you are not getting 24 volts to R and C then you probably have a transformer problem or a blown fuse on the transformer. Make sure that you blower door safety switch is taped down and the power switch on the furnace is “ON” and if you have a condensate pump make sure the safety switch has not cut the power off to the furnace. Let me know if you have any questions. I hope you can find the problem soon! Steve

  21. Hello Steve:

    I have read many of the posts here and they have cleared up most of my confusion.
    However, I would like to ask a specific question concerning the “not receiving 24 vac at the contactor coil.
    Since I am receiving the 24 vac at the thermostat, I am thinking one or both of the wires going to the contactor coil
    are either shorted or broken. My plan is to just bypass by connecting new wires directly from the thermostat to
    the contactor coil. Also, when I press the spring-loaded button on the contactor, the outside fan does come on,
    but only when I continually hold the button down. When I let up on the button, the fan shuts off. Not sure about
    the compressor because I cannot hear it over the noise of the fan.

    My question is if I do this bypass, is it possible that I could instead be bypassing the safety pressure valves?
    Thank you,
    Rayman

    1. Hi Mr. Parks! If one of the safety pressure switches are open then you might be low on refrigerant charge and the low pressure switch will not allow the contactor to close. I would suggest testing with a volt meter and see where you stop getting the 24 volts from the thermostat. Where you stop getting 24 volts then this is where the problem is. If you want to eliminate the thermostat wires from being the problem you could run new thermostat wires, but I hate for you to waste your time if it is an open pressure switch or a contactor coil that is bad. Here is a post we have that I hope will help you out: https://arnoldservice.com/problem-the-air-conditioner-outdoor-unit-will-not-come-on/ We have lots of air conditioning and heat pump troubleshooting questions with answers and Youtube videos on the following page: https://arnoldservice.com/air-conditioning-heat-pump-troubleshooting-questions-answers-index/ Let me know if you have any questions. Hope you have a great day! Steve

      1. Thank you for your advice. It has cleared up more confusion that I had. I want to rephrase my question to you:
        Is it possible that either of the safety pressure switches could cause the contactor coil NOT to receive the 24 vac? In other words, Is the 24 vac independent from the safety pressure switches?
        (sorry, that’s two questions).

      2. The high and low pressure safety switches are dependent of getting the voltage to the 24 volt contactor coil. If one of the safety switches are open, 24 volts will not be allowed to go to the contactor coil. If one of the switches are open then you might be low on refrigeration charge (Freon). You should be able to temporarily join the two wires together that go into the safety switch to by-pass it temporarily to see if this is the problem. I hope you can easily find and fix the problem. Ask as many questions as you want and I will be glad to try and answer them. Hope you have a great day and weekend! Steve

  22. When I set my thermostat to cool my contractor will not energize and I don’t get 24v on the contractor so the condenser unit will not turn on. However, if I set the thermostat to heat the contractor energizes and I’m getting about 25 v to the contactor. What do you think the problem could be?

    1. Hi Steve! This sounds like you might have a thermostat problem. Your heat pump thermostat should energized the yellow wire (y connection) in both the heating and cooling modes. You could also have a bad defrost control board or a weak transformer. On most heat pumps other than Rheem/Ruud the reversing valve solenoid is energized in the cooling mode. If you have a weak transformer the extra power used to energize the reversing valve solenoid might be robbing the power from a weak transformer and not giving the contactor enough voltage to energize. If the voltage going to the contactor is “0” then I would suspect a bad thermostat or defrost control board. I hope you can easily find and fix the problem. Steve

  23. I have 24 V to my fuse on my board but nothing to my common/red and have had the same problem with two new boards. Could it be a transformer issue?

    1. Hi Alex! Yes your 24 volt low voltage transformer should be directly connected to the R and C terminals on your control board and you should be getting 24 to 28 volts between the R and C terminals on your control board. If you see any terminal connections that are labeled secondary 1 or secondary 2 then you should be getting 24 volts between those two wires or if you can trace down the secondary wires on your transformer I would suggest that you test there. Please make sure the safety door switch is taped closed so you can do proper testing. Best of luck. I hope you can easily find and fix the problem. Steve

  24. On my Trane unit, I have 24 volts to the condensing unit, nothing at the contractor. The unit has not been running. If I manually activate the contractor, the compressor starts.

    It appears that there are pressure switches o the high and low coolant lines that aren’t making. Ian there a video showing the repair or replacement of these switches? I do not want to bypass them, because I do not know what I am doing, and I don’t want to bugger something up.

    1. Hi Danny! Most of the time pressure switches are good and are doing their job when they will not allow the unit to come on. I would suggest that you hook up some gauges to see if your unit is low on charge. If your unit is low on charge you will probably need to join the two wires together that go into the pressure switch in order to charge the unit and get the contactor energized. You can cut the wires, strip the insulation off and join the two wires together with wire nuts. This is what I found when I searched for videos on fixing high and low refrigeration pressure switches: https://www.youtube.com/results?search_query=ac+low+refrigerant+pressure+switch+hvac Most people leave this job to an HVAC tech because many of the pressure switches are brazed into the refrigeration piping and you would need the tools to reclaim the refrigerant and tools to braze the new pressure switch in properly. If your Trane unit is a heat pump then you might have a bad defrost control board that is causing the contactor on your unit to not be energized. I hope you can easily find and fix the problem. Steve

  25. Where on the control board do I lay down the two wires that connects to the ac contactor coil. U gave a Rheem gas furnace and there are wires from the thermostat that that connect to the board. No common wire laid down but I do have 24 volts from the transformer. I am thinking one of the wires probably connects to the Y (yellow) connection point. Where is the other need to be laid down.
    thanks

    1. Hi Dee! As you know the contactor coil needs 24 volts from your thermostat. You should have two thermostat wires that go out to your outdoor unit. One wire would connect to C (com) on the control board and the other wire would join together on the Y connection terminal of the control board with the yellow wire coming from your thermostat. I hope you have a great day! Steve

      1. Thanks for the reply Steve,
        Control board does not have anything layed down on the C. Could I come off the common on the low voltage side of the transformer?
        thanks

      2. Yes, you could hook one of the air conditioner wires to the common side of the low voltage transformer if the control board does not have a C (com) terminal. Steve

      3. Steve,
        Just thot of something. Although the C on the control board does not have a wire already laid down on it could the configuation of the board have the needed common already there available?
        Sorry
        thanks
        Dee

      4. Yes, the control board “C” connection should be directly connected to the common on your low voltage transformer. Steve

  26. Hi Steve,

    Thanks for helping everyone; lots of good information on your page! I have an older Trane heat pump WCY024F1 and I had a problem with the contactor sticking about a month ago causing the compressor fan to run constantly. After cleaning it a bit it was working great with the AC. Now with the cooler weather I switched over to heat and not getting 24V at the contactor coil, only with the heat though. If I switch back to AC and set the temp so it will turn on, I get 24V at the contactor coil and the AC works fine. In heat mode the thermostat does activate the indoor fan motor. The fuse is not blown and the LED on the control board is flashing indicating normal operation. What do you suspect could be the problem? The thermostat is a cheaper Honeywell RTH2300B that has been installed for over a year. After reading the sequence of operation and reviewing the wiring diagram for the Trane I am thinking that the thermostat is not making the internal connection to Y when in heat mode. The heat worked fine not long before this problem began. Thank you! Ben

    1. Hi Ben! Thanks so much for your kind words! I would suggest turning the thermostat to heat so that the heat pump should be running outside and measure with a volt meter set to “Volts AC” between the Y and C (com) wires. You should be getting 24 to 28 volts AC going to the contactor. If you do not get the 24 volts going to the contactor then you either have the thermostat set to emergency heat mode, a loose connection, broken thermostat wire, or a thermostat problem. You should have 24 volts between Y and C (to turn on the outdoor unit contactor), G and C (to turn on the fan), W and C (to turn on auxilary heat). Sounds like you have a thermostat problem. I hope you can easily find and fix the problem.
      Steve

      1. Your board could be faulty.

      2. Yes, The defrost control board if you have a heat pump could be faulty. Great advice Craig! Steve

  27. I have a Trane furnace TWV030B140A1 that is not sending power to the thermostat. The tstat is a Honeywell Smart WiFi. A/C has worked fine all summer – turned on heat and after about 20 minutes it turned off and tstat went blank. Narrowed it down to transformer. Replaced with new 240v one but now cannot get 24v out. I tested contactor and can get 120v, but cannot detect 24v anywhere on contactor. Fuse is good. Also cannot detect 24v on any wire wired to the small terminal block inside of furnace. Should I be checking in the fan unit outside?

    1. Hi! You should be getting 24 to 28 volts AC between the C (com) and R connections on your thermostat and control board. Please check the fuse and make sure it is screwed in good if you are not getting the 24 volts. I see that you say the fuse is good. There is no reason why if you have 220 going into the primary of the transformer that you are not getting 24 volts out of the secondary unless you have a bad transformer or the transformer is not getting power. Please see the following screen shot which shows the location of the fuse: https://www.screencast.com/t/6heyhPQM I would also check the primary to make sure you are getting 220 volts into the transformer and 24 to 28 volts out between C and R. If you are only getting 120 volts to the contactor between L1 and L2 then please check your circuit breaker to make sure it has not thrown. I would suggest turning the breaker all the way off and back on again. You should be getting 220 to 245 volts between L1 and L2 on the contactor. Sometimes, but seldom units have transformers both on the inside and outside. This could get quiet complex if you have two transformers and trying to figure out what transformer controls what. I hope you can easily find and fix the problem. Steve

      1. There is no voltage/power at all on tstat wires at tstat location. I checked the fuse quite a few times and had two other people check it for good measure – fuse is definitely good and screwed in tight. This is the second transformer – I had the same issue with not getting 24v out so I thought the first replacement had to be a bad part – apparently not. The common and “power” wire coming from furnace wiring (that were previously hooked up to blown transformer) both have 120v testing on them. When I hook those wires up to the common and the 240v wires on new transformer – both wires still read at 120v, but nothing on secondary wires. The only voltage reading I can get on the contactor itself is 120v from one red wire. No other wires have any readings. No voltage readings at all on anything wired to terminal block in furnace and no voltage readings on any other wire hooked up to contactor except for that red wire reading 120v.
        Old transfomer had the following wires connected: common and 230v and two secondary wires for 24v.

      2. One last thing – the old transformer was rated at the following: 75VA (200-230v PRI, 24v SEC). New transformer is rated at: 20VA (120/208/240 PRI, 24v SEC). Is that an issue – the old one being 75VA and the new one being 20VA?
        Advanced apologies – Im a single woman trying to do the best I can on a budget before my pipes freeze!

      3. Hi Ms. Weller! The new universal transformer that you have is too weak. You should have a minimum of 40 VA with a heat pump system and 75 VA would be best. 20 VA is not powerful enough. You need a new transformer that is rated over 40 VA. Also you should have 220 to 245 volts going to the primary of the transformer. If you do not then you have an electrical problem because the transformer should be tied into the power for your 220 volt heat pump air handler. Sorry that I can not be more help. You might need to call in a tech to install a new transformer and make sure the primary and secondary voltages are correct. I will say a prayer for you. God bless you. Steve

      4. Thank you, Steve! One last question on this – I know how potentially ridiculous this may sound, but on the 2 primary wires that I currently have testing at 120v each – does that count as the 240v (Im adding the two wire voltages together)? Or I need to see 220-245v on each single wire – not a combined total from 2 or more wires?
        Thank you again – super appreciate your knowledge and kindness.

      5. Thank you so much for your kind words. None of your questions are ridiculous. The two wires together should measure 220-245 volts AC. If you go from each individual wire to ground it should be 120 volts each. Sorry it took me so long to answer I had a doctor’s appointment this afternoon. Please ask questions anytime. Steve

  28. I seem to have low voltage to a Nest (1st Generation) a friend gave me. Nest support didn’t tell me what to do, aside from calling a HVAC person. What can I do to raise the voltage? I had a older thermostat with red and white solid wires (not stranded). There’s also a light blue wire in the wall. The furnace is gas fed hot water with baseboards around the house. I notice at the furnace end ALL the blue wires to the little control boxes for various areas of the home are not connected. I’m guessing the blue wire at my thermostat is not going to give me anything but wonder if that can be used to raise the voltage the Nest needs. Currently the Nest says the voltage is around 11 or 13. Any idea? Thank you!

    1. Hi Richard! I am sorry that I am not familiar with the Nest thermostat set-up. If you presently have a 24 volt hot water system and you are only getting 11 to 13 volts with the nest thermostat then I would think that you need a more powerful low voltage transformer with a higher VA. Most HVAC transformers are 40 VA. I would suggest maybe a 75 VA transformer. Here is what I found when I searched this topic on Google: https://www.google.com/search?source=hp&q=higher+VA+hvac+transformers&oq=higher+VA+hvac+transformers&gs_l=psy-ab.3…1487.14788.0.15156.28.27.0.0.0.0.156.3436.0j27.27.0….0…1.1.64.psy-ab..1.24.3038.0..0j35i39k1j0i131k1j0i20i264k1j0i22i30k1j33i21k1j33i22i29i30k1j33i160k1.0.Il_lliHQUpw I hope this helps you solve the problem. Steve

  29. I have an Amana AC unit. Initial problem No display on Thermostat and AC not working. Checked out unit found bad low voltage transformer. Replaced transformer and as soon as I applied power. transformer smoked. Was told could be a bad contactor. A direct short on the secondary. Well I replaced transformer with a fused output and the contactor. Keeps blowing fuses. I disconnected the load off the contactor and still bad. I disconnected the transformer from the board and it did not blow fuse which it shouldn’t. Tried isolating outputs from the board but no change. Bad Board ??

    1. Hi Frank! Most of the time through my experience, the problem with low voltage burn out transformers is caused by shorted thermostat wires. I discuss this problem on the following page: https://arnoldservice.com/problem-furnace-air-conditioner-heat-pump-will-not-come-on/ This could be a bad control board, but 90% of the time it is shorted thermostat wires. Here is another page where I discuss this problem: https://arnoldservice.com/problem-my-24-volt-transformer-continues-to-burn-up-this-is-the-second-transformer-what-could-be-the-problem/ I hope you can easily find and fix the problem. Steve

  30. Hello Steve,
    I don’t see 24V between C and Y when STAT is calling for cool cycle. I changed to a new STAT and problem started happening after few days. Now I don’t see any voltage even after I remove the 24V contactor leads from the board. If i measure resistance across the contactor leads it shows 0.6 Ohms. Is it short? I had to recently change out the relay that was shorted on the compressor unit and it was working fine until breaker started to trip an now I don’t see any voltage between C and Y. I did notice the capacitor was weak when I measured. Should changing the capacitor fix the problem or do I need “hardstart” kit. How do I fix the C and Y voltage problem?

    1. If you do not have 24 to 28 volts AC between the Y (yellow) and C (com) connections on your control board or Y and C thermostat wires when the thermostat is calling for cooling, then I would think you have a thermostat problem. You should be getting 24 volts AC between the R (red) and C (com) connections. The R and C connections are the power connections that comes directly from your low voltage transformer. R (red) is the hot wire from your transformer. C is the common or neutral wire coming from your low voltage transformer. The thermostat takes the power Red (R) connection and when the thermostat calls for cooling the thermostat acts as a switch making connections between R and Y, to energize the contactor, R and G to energize the fan relay to turn the blower on your furnace on. R to W to turn the gas valve or electric heat on. You should be able to turn the power off to your furnace take the red (R) wire off, the green (G) wire off and the yellow (Y) wire off and tie these three wires together with a wire nut or twist them together. Turn the power back on to your furnace and the AC and your furnace blower should work (Come ON). If it does work then you know you have a thermostat problem. If it does not work then you have a thermostat wire problem or you are not getting the 24 volts to the thermostat. You might want to check the fuse (if equipped) on the control board to make sure the fuse is not blown. If the contactor you replaced is not getting the proper voltage it could shutter and vibrate the contacts and cause the contacts to burn out. If you are only getting 0.6 ohms on your contactor coil then I would say the contactor coil is bad and you need a new contactor. I tested two of our contactors and got 11 ohms on one and 18 ohms on another. I hope you can easily find and fix the problem. Steve

  31. I recently replaced defrost control board on my outdoor unit and now neither air handler or outdoor unit come on when setting Tstat to cool. Previously HVAC tech had bypassed the defrost control to get the AC to work. Confirmed wiring is good (I have 2 identical outdoor units and compared the wiring) and manually pushing the contactor in caused the compressor and fan to come on so it sounds like there is no signal coming to the unit. Does the indoor hair handler control signal to the outdoor unit? Hopefully I provided enough info to troubleshoot, thanks

    1. Hi Paul! Sounds like as you say the low voltage is being cut off somewhere. The thermostat or a loose wire could be the problem. The thermostat wires from the thermostat usually junction inside the indoor air handler. I would suggest that you use a volt meter and test with the thermostat set to call for cooling between the C (com) and Y (yellow) connections on your thermostat. Continue testing all the way out to the outdoor unit’s contactor. Where you stop getting the 24 to 28 volts AC then this is where the problem is. This could also be a bad contactor if the contactor coil is getting 24 volts and not closing. I hope you can easily find and fix the problem. Steve

  32. Hi, Steve. I want to replace my Honeywell thermostat RTH7600 with a Honeywell wifi thermostat RTH6500. I discovered the control voltage is about 19.5VAC (on the C terminal). Honeywell says they need 20 to 28VAC to operate. The 7600 tolerates the low voltage but the 6500 does not. The house voltage is 120VAC. Is my solution to replace the low voltage transformer? Where is the transformer located?

    1. Hi Tom! You might want to purchase a transformer that has a higher VA (volts Amps) rating. Most transformers have 40 VA rating. So if your transformer is a 35 VA you might want to go to a 50 VA transformer or higher. Most transformers are located inside the furnace in the blower compartment. Some systems have two transfomers, one on the furnace and one on the outdoor AC unit, but that would be unusual. Most only have one. Here is a link to a good video which explains how to replace a transformer: https://www.youtube.com/watch?v=gwTDvAyKdrE I hope this helps you out. Steve

      1. Ok so I diagnosed a bad contactor today and replaced it all the wiring is correct. Prior to replacement I was showing 26v to the coil. I now have 0 voltage to the coil and traced it back to the transformer where I have 240 in and 0 out. Something else is going on here for my transformer to have burnt up. all the fuses are good. I have a replacement trans and 20′ of 5/18 wire. Will I burn out the new trans if I replace it.

      2. Hi Mark! You probably already know this, but for the transformer to burn out you must have a short to ground in either the thermostat wires or some control. You will need to find where the short is before installing a new transformer. Most of the time the shorts were found in the thermostat wires on the outdoor unit where sun or animals had chewed through the wires. I have also seen where thermostat wires were pinched or grounding out on the side (the frame) of the furnace or air handler. Finding the short in thermostat wires can be quiet time consuming because what I would do is disconnect all the thermostat wires at all ends then test with an ohm meter between any and all combinations of wires. You should not get any resistance between any two wires and if you do get resistance then this is where the short is. Sometimes it is almost easier to replace the thermostat wire and be done with it. Low voltage controls can have a short to ground too. The theromostat, contactor or if you have a heat pump the reversing valve solenoid could have a short. I would suggest trying to find the short then install the transformer. When you first turn on the transformer immediately check the transformer body to make sure it is not over-heating. If the transformer starts to heat up fast kill the power to the transformer so the new transformer will not burn up. It would be a good idea to place an inline fuse in the transformer circuit to blow the fuse before destroying the transformer. I hope you can easily find and fix the problem. Steve

      3. I got to the transformer. There is 27 VAC on the secondary, so it’s fine. I discovered that the common is not connected at the control board. Why would this be disconnected? If I connect it, what problem could occur?

      4. Hi Tom! I have no idea why the common wire would be disconnected other than it vibrated loose. Most boards have a primary 1 and 2 connections on the board and also a Secondary 1 and 2 connections. If you connected the common to the wrong connection it could most certainly burn the control board up. I would suggest looking at the wiring diagram which should be glued to the furnace blower door or stuck somewhere on the furnace to see where the common from the transformer is attached. I hope you can get it fixed soon! Steve

      5. Hi, Steve. The common from the thermostat was deliberately tied back at the heater unit control board. I went ahead and connected it and all is well. I’ve got mobile temperature control!

      6. Fantastic! Glad to hear that you got it fixed! I hope you have a great week! Steve

  33. I am having an issue with my air conditioning. I have a nest thermostat and the heat and fan work great. When I set it to cool the fan comes on and turns off but the A/C never initiates outside. It then gives me an error (e53 which tells me it’s a wiring issue). I’m not getting 24 volts from the contacts even after I replaced the fuse on the board. I can’t figure out why I’m not getting the 24 volts. Everything else checks out and I’ve checked my wires and they all have great contacts. Is there something I’m missing?

    1. Hi Marcus! I am sorry, but I do not know anything about the Nest Thermostat wiring or operation. Here is a page that I hope will help: https://www.google.com/#q=nest+thermostat+wiring+diagram&spf=1 I know that some of the people where having problems with the transformer on their equipment not being strong enough. You might need a higher VA transformer. I would suggest that you go to the Nest Thermostat support page and ask the professionals. Here is what I found when I googled this: https://www.google.com/#q=nest+thermostat+problem+solving&spf=660 I hope you can easily find and fix the problem. Steve

    2. One or more of the wires in the Main system is not connected properly. This is the reason why the fan is not getting the 24 volts it needs. Once you connect those wires in the unit properly, your fan will spin. Check the thermostat too, because the main unit needs to get the voltage from the thermostat to turn on. If the unit is powered by gas, get the help of a technician. You don’t want the fan to blow on too high because there is a pilot in the unit (as it has a dual purpose to provide both heat in the winter and AC in the Summer) and it may catch fire.

      1. Thanks so much for your input.

    3. I had this exact issue with Nest and e53 error. I found the two wires running from furnace control to the Compressor had been chewed (?) by mice, resulting in a short. I replaced the damaged wires, it still wouldn’t come on. Y wire would not get 24 V. Tried Steve’s advice to tie Y-R-G together at thermostat, and AC worked! Went out and bought a Honeywell thermo at Lowes, all works fine. So the shorted wires running to Y somehow damaged the Nest. Happy to get rid of that unit, not sure what all the complexity and cost buys you in the end. Also, we live in california, and HVAC doesn’t run for weeks/months at time during spring through fall. I read this can cause issues with Nest. Thanks Steve for your great answers on this site, helped me locate the issue, wish I would have come to your page earlier!

      1. Hi Dave! Thank you so very much for taking the time to explain how you fixed the problem with the Nest thermostat. I hope this will help other people who are having problems with their Nest thermostats. God bless you for sharing your experiences! Steve

  34. I have a rbcs2/bcs2 air handler and I can’t find the circuit board. Can you help me. I have look in it. But I did all the test you suggested but I can’t find the fuse. Thanks in advance

    1. Hi Mr. Willis! Some air handlers do not have circuit boards. I could not look up your furnace parts because I do not know what the Brand name is for your furnace. I tried “RBCS” in our Rheem parts program and nothing came up. Sorry that I can not help you out. Steve

  35. I have a somewhat similar problem: I got a newer wifi thermostat. I had to redo the wire so I have the 5th wire for C which was not required by the old thermostat. Everything works except that after the a few minutes of heating, the thermostat screen goes dark and the furnace blower stops, and then a few minutes later, the thermostat comes back and it takes a few minutes to send the signal to the furnace, and then it stops again. I checked the 24 volt power on the wire, the voltage is about 27-30, except after heating for a while, the voltage drops to zero and stay there for about a minute. My old thermostat used battery backup so I do not know if there were also voltage interruptions with the old thermostat. I wonder if this voltage interruption normal behavior? Is there a work around?

    1. Hi Yi! You should not have a voltage interruption in the 24 to 28 volts that are being supplied to the thermostat. You might have a loose wire or a faulty transformer. Please make sure the common wire is not grounding out to the other hot thermostat wires. Please make sure the battery back-up in the thermostat is in good condition. Sorry that I can not be of more help. Steve

  36. I have Carrier FB4A unit and it is no longer putting 24 VAC to the thermostat. So, the thermostat is not working and either is my heat. A contractor shorted the 24 VAC wires, and that is when it stopped working. What has burned out? is there a fuse somewhere? Does shorting the wires for a few seconds destroy the transformer?

    1. Hi Bob! Most of the air handlers have a fuse somewhere on the control board on inline from the transformer. If the wires are shorted out it can damage the thermostat and any low voltage controls if the fuse does not blow first. Most of the time the fuse will protect the low voltage controls. I hope! I would need your full air handler product number in order to look up parts. I would suggest turning the power off to the unit, pull the blower door which usually shows a wiring diagram. This should point you to the control board or a fuse location. I hope that it is only the fuse. I hope you can easily find and fix the problem. Steve

  37. Hi.. I am thinking of installing smart thermostats in my home. I have a Duracraft programmable for downstairs and a Honeywell dial therm upstairs . My question is the Duracraft has double A batteries and says it won’t work unless they are installed.. does that mean there is no low voltage energizing the thermostat?? Or do the batteries just supplement the therm controls and memory?…. We have a natural gas boiler … dual zones… 2 zone valves and just one circulator pump..
    So my main ? Is that are all thermostats powered by low voltage ? Thank u sir for your time…

    1. Hi Will! Some thermostats are power only by batteries and some thermostats are optional and you can power the thermostat with batteries or power from the furnace’s transformer. If your thermostat has a C or Com connection on it then this means it can be powered by the furnace’s transformer. Many of the thermostats that are powered by the furnace or air handler’s transformer use the batteries as a backup for the program (on programmable thermostats). My White-Rodgers Programmable thermostat is optional and can be power by batteries only or by the furnace’s transformer. I only have 4 wires going to my thermostat and I do not have the 5th common wire so I just use the batteries only to power my thermostat. I would suggest you try to get a Duracraft owner’s manual to see what your options are. I hope I have answered your question. Steve

  38. First, awesome site you are great for being so helpful to so many people. I’m confused about my problem because I have the same problem on two different units, where as all my problems in the past have been isolated to one or the other. My setup is I have a downstairs and upstairs thermostat, each has their own compressor heat pump unit outside and each has a separate air handler in the attic. I’m not getting 24 volts at either thermostat, so I can’t get anything to run. I had noticed that the air wasn’t coming though the vents downstairs but the compressor was running and that’s why I started looking and after straightening out the bent wires behind the thermostat the compressor shut off and never did come back on. I changed the batteries and saw the display go out and usually it stays on so I checked the voltage and got nothing, and nothing upstairs at that thermostat either. I’m just confused as to how they are both getting no voltage, is there a common place where incoming voltage would then be split and go to both air handlers?

    1. Hi Thomas!
      The two units should be controlled by two separate transformers. The transformers should be located inside the air handlers. The air handlers both should have fuses to protect the low voltage circuit. I am sorry you have to go in the attic, but I would suggest looking inside the air handler for a fuse. I hope you can find a fuse and an easy fix. I can not understand why both units went out at the same time. The two units should have separate thermostats with separate low voltage wiring. Please make sure that all your breakers are on. Thanks so much for your kind words! Sorry that I can not be of much help. I hope you can easily find and fix the problem. Steve

  39. My contactor won’t engage I checked the Transformer and it’s fine I checked the contactor it’s also fine I’m not getting the 24volts to the contactor and I replaced the capacitor. The fuses are good at the furnace but still nothing any ideas?

    1. Hi Joel! I would suggest starting at the control board with the thermostat set to call for cooling. You will probably have to tape the door safety switch down so you can test the low voltage on the control board. You should have 24 to 28 volts between the Y and C (com) terminals on the control board with the thermostat calling for cooling. If you aren’t getting 24 volts between Y and C then you either have a thermostat problem or a thermostat wire problem. If you are getting 24 volts between Y and C then this is good. Continue to go and test out by the out door unit. I would suggest turning off the high voltage power to the outdoor unit because you are interested in testing the 24 volt contactor coil and other low voltage components only. You should be getting 24 volts to the same two wires that were coming off your indoor control board. If you aren’t then you have a broken wire. When you find where you stop getting the 24 volts then this is where your problem is located. We cover this problem in more detail on the following page: https://arnoldservice.com/problem-my-air-conditioners-contactor-will-not-engage-i-am-not-getting-24-volts-to-the-contactor-to-make-it-engage-what-could-be-the-problem/ I hope you can easily find and fix this problem. Steve

  40. Hello quick question on my circuit board I see where the thermostat is connected in getting 24 v to the drain pump but I’m not getting 24 v back out to the contactor do I need a new drain pump ?

    1. Hi Josue! If your condensate pump has a safety switch on it then maybe the condensate pump is full of water and not allowing the air conditioner to come on. I would suggest checking the pump to see if it is full. Many times contractors will run a condensate pump safety switch through the Y (yellow) wire coming from the thermostat to shut the AC off if the pump fails. You should be able to by-past the condensate pump safety switch to see if it works with by-passing it. Best of luck in finding and fixing the problem. Steve

  41. hi,,im new at this ac stuff,,but i have a question please..new therm,,new connector..but the ac dont come on like it should..the furnace fan runs all the time and i notice no cold air is coming out of the vent. but if i go outside,and hit the side of the outside unit,,the unit will come on and start cooling. now when it cools to temp it was set for,,the outside unit will cut off, but the furnace fans keeps going.. everything seems to work as it should, besides me having to manually start it with a tap on the outside unit. any help on what to do or try would be awsum..ty in advance ..jeff

    1. Hi Jeff! This most certainly sounds like a loose wire or a loose connection inside your outdoor unit since it starts working when you hit on the side of the unit. I would suggest turning off the power to the unit and check all connections to make sure they are good and tight. The furnace fan staying on could be caused by an open limit switch, control board relay that is stuck closed, shorted thermostat wires or a thermostat that is bad and sending voltage through the G (green) terminal all the time. We discuss this problem on the following page: https://arnoldservice.com/problem-your-fan-on-your-furnace-runs-all-the-time-and-will-not-shut-off/ I hope you can easily find and fix the problem. Steve

      1. hi steve,,sorry to bother you oncre again,,my syptoms are a bit diff from what i mentioned before…i failed to add a cpl things..lately ive been able to tap on the outside unit and most times it will come on…but now its gotten harder to start. it has new connector/contactor and a new blower motor. and a new digital thermostat ..everything seems to be connected correctly as i did it one wire at a time,,the blower motor was done from a prof…now my symtoms are diff ,,,when i cut on the ac,,and i walk outside to check the unit,,all im hearing is a clicking noise..the contacts are trying to pull in,,but not staying. i ussually get this at night time when its cooler outside..but during the daylight hours (when its warmer outside) it seems to cut on without much problems besides every so often having to tap on the side of the unit. making me think it has something to do with the temp outside. i was reading about being low on coolant ..says they wont cut on it low. but mine cuts on (most of the time) this thing is getting to me pretty bad lol…i checked the heat,,and it cuts on and off just fine as it should..is there anything i can check? as ive looked for shorts everywhere and cant find one. what about the transformer? do they work sometimes and not other times? as i dont get 24v from the contactor,,but when its running i do…any advice would be awsome..this thing is confusing me pretty bad ..ty in advance jeff

      2. Hi Jeff! You say that sometimes you walk outside and the contacts on the contactor are trying to pull in, but not staying in. Most of the time this problem is caused by a weak low voltage transformer, shorted thermostat wires or a thermostat that has low batteries or is defective. I have seen contactors destroyed in minutes when there is contactor chatter (not enough coil voltage to the contactor). You might need a new contactor if the contacts on the contactor are worn. We discuss contactor problems on the following page: https://arnoldservice.com/problem-my-air-conditioners-contactor-will-not-engage-i-am-not-getting-24-volts-to-the-contactor-to-make-it-engage-what-could-be-the-problem/ I would suggest that you try and check the coil voltage at the contactor when the problem occurs. You might need a new transformer with a higher VA rating, new thermostat wires if the voltage is not at least 24 volts AC or new thermostat batteries if your thermostat uses batteries. Most of the time a transformer with a 40 VA rating is sufficient but a 50 VA transformer would be better. I am sorry that I can not be of more help. I hope you can easily find and fix the problem.

  42. i have 24 vac from the transformer to the fuse, but only 5 vac coming to the r on the low voltage. the lights inside my unit lights and it telling me it is the low pressure switch. any help would be appreciate.thanks

    1. Hi Tony! You should have 24 to 28 volts AC if you test between the R and the C (com) terminals on your control board. Most of the time the low voltage transformer is connected directly to R and C but this could be interrupted by the fuse. I would suggest that you make sure that your fuse is in good shape. I would suggest tracing the wires from the transformer on the low voltage side and see where the hot and Com wires go. See if any other controls could be messing up (interrupting) the voltage. Best of luck in finding and fixing the problem. Steve

      1. thanks for quick response, i bypass the control board and hook the 24 vac from the xformer to the r on the thermostat (hope it does damage anything). have to buy new control board now.

      2. Hi Tony! I would not recommend by-passing the fuse and any other safety controls because this could damage your control board or any other low voltage controls on your furnace and air conditioner. If the fuse blows then this is a sign that you have a shorted wire or control somewhere in the system. We have a real good troubleshooting flow chart for air conditioners on the following page: https://arnoldservice.com/wp-content/uploads/2014/12/air-conditioner-troubleshooting1.pdf and one for furnaces on the following page: https://arnoldservice.com/wp-content/uploads/2012/12/furnacetroubleshootingflowchart1.jpg Best of luck in finding and fixing the problem. Steve

  43. My circuit breaker keeps tripping. The AC unit will run, for even up to a couple hours sometimes, then breaker trips. When unit is running, and after the breaker has tripped, I heard a clicking sound coming from my contactor. HVAC friend told me it is most likely a bad contractor. I replaced with a new one, and now my Nest thermostat is saying I have bad wiring. I connected everything the same way as the old one. I was getting 120VAC to the input, but not getting 24VAC across my low voltage tabs. I did a continuity check on the coil, and that was good. Condenser never came on. I put the old contractor back in, and the AC unit came on again, and I am waiting for it to trip as I type this. Possible the new contractor is bad? I don’t have a clue what else it could be. Help!!!

    1. Hi Scott! If your contactor chatters the problem more than likely is a weak transformer, or a thermostat problem. If your Nest thermostat has batteries please make sure they are in good shape. You should be getting a minimum of 24 volts AC to your contactor coil when your thermostat is calling for cooling. If you aren’t getting 24 volts to the contactor then this can be caused by a weak low voltage transformer. a weak relay in the thermostat or partially shorted thermostat wires. I would start with checking the voltage between R and C (com) at the control board and make sure you have 24 volts when the AC is running. You will probably need to tape down the furnace door safety switch so you can do this test. Contactor chatter and weak loose wire connections can cause a breaker to blow because this causes high amperage draw. It is very hard on contactors and I have seen contactor contacts get destroyed in a matter of minutes with contactor chatter. I would suggest checking all high voltage wires to make sure they are tight too if you feel comfortable with this. If you do not feel comfortable then please call in an HVAC tech or electrician. A circuit breaker that is arc on the inside or going bad can cause the breaker to trip. This sounds like your contactor is not getting the 24 volts needed to make a good connection. I hope you can easily find and fix the problem. Steve

  44. I am trying to make a training board with pc board, thermostat, contactor and transformer . Everything hooked up the way it supposed to be , I am getting 120vac at the board and 24 vac at the transformer but not getting 24 volts at thermostat or contactor coil. I ohm checked all low voltage wiring and has continuity . Any suggestion will be greatly appreciated ( split system)

    1. Hi Mr. Singh! You should have 24 to 28 volts AC between C (com) and R (Red) on your control board. These two terminals should be directly connected to the low voltage wires of the transformer to produce 24 volts for the low voltage controls on the furnace and thermostat. The R (red) connection should connect to the R on your thermostat and the thermostat acts like a switch between R and W (heat, gas valve is energized) R and Y (contactor on AC is energized) R and G (fan blower relay is energized. I hope you can easily find the problem and get it fixed. Steve

      1. Hi Steve , thanks four help, I am not getting 24 vac at contactor coil . I am only getting 24 v when I go to ground on one side of the terminal and 0 volt from the other side . Found out There’s t/s wire has no continuity to the contactor So I decided to ground the common side and everything came on . Can you please explain the theory? Will be greatly appreciated

      2. Hi Mr. Singh! You should get 24 volts from only one side of the contactor to ground like you tested. Electricity flows from hot to ground or neutral. The wires from your transformer should have two wires on the low voltage side. One hot (24 to 28 volts AC) and the other Ground or Common (C). It sounds like maybe the ground wire (C) that is going to the contactor is broken somewhere since you have to give the contactor an external ground to make it work. I would suggest checking the ground wire (C) from your control board in on your air hander or furnace to make sure it is not broken. I hope you can easily find and fix the problem. Steve

  45. Hi Steve,
    I’m having the same problem. The problem actually started on both AC units simultaneously. What started this mystery occurred after I installed new contactors, capacitors, and hard start kits for preventive maintenance. They were both working ok before that. Prior to starting the work, I tripped both AC breakers, but failed to shut off the furnace breakers. Once completed, neither system worked. The furnace fans are running consistently and the thermostats appear to be calling for cool, but neither capacitor will engage. Thinking I could have damaged the 24v transformers, I installed new ones on both units. No luck. On the furnace control board, I ran a volt meter between R and C and get 25 v. Across Y and C, I get 6v. Outside at the AC unit I get 6v all the way to the contactor terminals. The fuses on the control boards are ok. The two green lights on each board indicate the system in operating as normal. Because I’m not getting 25 v between Y and C, is it possible the board is damaged? Note that I ran a meter across the new transformers and get 25v. Any guidance you can provide would be most appreciated!

    1. Hi Paul! Sorry to hear that you are having this problem. Since you are not getting the required 24 volts between Y and C when the thermostat is calling for cooling this indicates that you either have a thermostat problem or a thermostat wire problem (wires could be shorted out or loose). You should be getting 24 volts between Y and C when calling for cooling. If not I would first check the thermostat to make sure it is working correctly. The thermostat is like a switch and when calling for cooling should take the R (red) 24 volt hot wire and connect it with the Y (most of the time yellow wire) which closes the contactor on your outdoor unit and starts you AC. I hope you can easily find and fix the problem.
      Steve

      1. hi I am having the same problem as Paul but after replacing the thermostat the low voltage at the contactor still exist (21v ac) but the transformer is at 25 v ac new contactor , thermostat and Capacitor. I am new to hvac with a 2 day YouTube education an help would be greatly appreciated

      2. Hi Robert! You might have a weak low voltage transformer. I would suggest that you have at least a 40 VA transformer installed if you do not have one already. Some systems require a 50 VA or more transformer. I would suggest that you turn your air conditioner on at the thermostat, then test the low voltage at the furnace or air handler’s control board. You should have 24 to 28 volts AC between R and C (com). Then test the two wires that go out to the air conditioner at your furnace’s control board which is usually Y (yellow) and C (com) If you have only 21 volts between Y and C and have 24 volts at R and C then you might have a partially shorted thermostat wire or a weak transformer. I would try disconnecting the contactor and test the voltage going to the contactor with it disconnected from the low voltage. If the voltage is up (24 volts) disconnected and back down connected then you either have a weak transformer, shorted thermostat wires or a contactor coil that is partially shorted out and using too much voltage. With everything turned off and just sitting idle most transformers produce in the 26 to 28 volt range. Best of luck in finding and fixing the problem. Steve

  46. Hello,

    my downstairs AC fan/compressor is not kicking on. I recently replaced the capacitor and contactor so those componets should be Ok. I checked the AC voltage coming into the contactor and am getting 250 AC volts into the contactor, however, i am not getting the lower 24 voltage into the “switch” of the contactor. it is reading 0 volts when i measure it. The thermostat seems to be working fine. i measure voltage of 24-25 volts across the R and Y columns behind the thermostat. I read to check the air handler R and C wiring but i cant seem to locate the air handler. The last technician to visit said i was a little low on refrigerant but that it should last. Im thinking that his estimate was a little off?

    any thoughts?

    1. Hi Mr. Zermeno!
      If you aren’t getting 24 volts to the contactor coil I would suggest starting back at the furnace or air handler control board. The air handler would be where your blower and blower motor are located. Near where you change the air filter. When the thermostat is calling for cooling you should be getting 24 to 28 volts AC between the Y (yellow) and C (com) on the control board. If you aren’t then you have a thermostat or thermostat wire problem. If you are getting the 24 volts between Y and C then continued to test with the volt meter going outside to the outdoor unit until you find where the voltage stops. Where the voltage stops then this is where your problem is located. Broken thermostat wires, Low pressure safety switches, high pressure switches and delay timers can go bad and cause the 24 volts to be absent from the contactor coil. I hope you can easily find and fix the problem. Steve

      1. hey steve, thanks for the quick reply. still struggling to understand the air handler control board location. its not the thermostat, right? :). i look around where I change my filters and do not see anything.

      2. Hi Mr. Zermeno!
        You might have a packaged unit which has the blower, furnace and air conditioner all in one. The entire unit sits outside and you would need to take off the panels to access the contactor and control board. Below is a picture of a packaged unit on the left. If you have a split system your unit will have a furnace or air handler downstairs, in a crawl space or in an attic and the air conditioning unit or heat pump will be located outside. On the right is a picture of a split system. I hope you can find the blower controls. Steve

  47. hello i am getting only 6 volts at the relay when the unit calls for cool havent been under the house yet,( its a friends house) would like to have an idea what to check , at the air handler, its an old unit 24 years i think so not sure yet what kind of control it has

    1. Hi Jeff! I would suggest testing between the R and C (com) terminals at the air handler with a volt meter set to “volts Ac”. You should be getting 24 to 28 volts AC between R and C. If you aren’t getting the 24 volts then you might have a shorted thermostat wire or a weak transformer. I would suggest trying to disconnect the two wires that go out to the AC unit and see if the voltage improves. If the voltage does not improve then you might need a new transformer. If the voltage does improve to 24 volts or more then you probably have a shorted thermostat wire or a grounded out contactor coil taking the voltage down. I hope you can easily find and fix the problem.
      Steve

  48. If I have a split system And thermostat is calling for cooling which is turning the air handler on but yet I’m not getting 24 to the condenser what should I look for

    1. The best answer that I have to this problem is in the answer to the above post. Most of the time when a contactor does not close it is one of the following: a broken or loose wire, blown fuse on the control board, thermostat problem, low refrigerant safety control problem, or the unit is low on charge or a bad contactor. I would suggest troubleshooting by starting at the furnace control board and test with a volt meter on terminals Y and C (com). You should be getting 24 to 28 volts on terminals Y and C when the thermostat is calling for cooling. If you aren’t then you have a thermostat problem or a thermostat wire problem. If this check out OK then move to the outside unit and test there. Where you stop getting the 24 volts is where the problem is located. Thanks so very much for your comment! I hope we can help others out in fixing their air conditioners! Hope you have a blessed day! Steve

  49. my contactor on the outside ac unit will not close. I have 24 volts on the low voltage wire coming from the furnace to the ac unit. if im not getting 24 volts at contactor coil does that mean the pressure switch is not allowing 24 volts to the contactor because my coolant is low?

    1. Hi! Thanks for asking the question. This could be a low pressure switch that is not allowing the contactor to close if you have one on your unit. This could also be a thermostat problem, a thermostat wire problem or a contactor low voltage coil problem (bad contactor). Some units have time delay circuit boards on them and these boards can cause the contactor to not get the 24 volts. About the only way to find out where the problem is would be to troubleshoot and test each component with a volt meter. I hope you can easily find and fix the problem. Steve

      1. I have the same problem. Just changed contactor. Have volts coming into the unit. Just nothing up top.
        I thought changing the contactor would work but it is the exact same problem my old contactor had. Voltage coming in. None going out.

        We had a pressure switch problem with the furnace last winter. It took the serviceman 20 minutes to “clear the pressure swtich”. Problem is that I didn’t see where the pressure switch is on my unit. My outdoor unit is Goodman Ck30-1a. My attic furnace (I have a split system) is Goodman GMP series. Both 1999 era.

        Any thoughts on what I might try? Oh…and where can I buy the 3amp fuse mentioned above. I will try that as well. Your web site is very helpful. I will read more tonight. Thank you so much for all of the great information on here. You’re helping lots of people.

      2. Hi Carole! We have lots of troubleshooting questions with answers on the following page: https://arnoldservice.com/air-conditioning-heat-pump-troubleshooting-questions-answers-index/ We have lots of HVAC troubleshooting and repair videos on the following page: https://arnoldservice.com/troubleshooting/hvac-troubleshooting-repair-videos/ There should be several topics which discuss contactor troubleshooting. If you are not getting 24 volts to the contactor coil then you either have a thermostat problem, thermostat wire problem or a safety control problem (high or low pressure switch, time delay relay). You would need to troubleshoot and test using a volt meter to find where the problem is. I hope the videos and troubleshooting questions and answers will help you find the problem. You should be able to test the fuse with a continuity tester or visually see if the fuse is good or not. You should be able to purchase a 3 amp fuse at a local hardware or Home Depot if you need one. Thanks so much for your question and kind words! Steve

        Steve

  50. my contractor is not getting the 24 volts,but my heating fan stays on.
    i work maintenance in an office building,all floors are wired the same,but this is the only one with this problem.

    1. Hi Brandon! This sounds like you have a stuck relay (relay contacts stuck together) on the heater where the fan runs all the time. If this is electric heat many of the fans are controlled by sequencers. You might have a stuck sequence or fan relay. I would suggest using an amprobe to see what control is supplying the voltage to make the fan motor run all the time. This could be a sequencer, control board, two thermostat wires shorted together or a thermostat. Best of luck in finding and fixing the problem. Steve

      1. could there be a problem with the transformer at all? the transformer is receiving proper voltage,just not putting proper voltage out. the thermostat works properly,shutting off and turning the unit on at set temps.

      2. Hi Brandon! If the unit is turning off and on then the transformer shouldn’t be the problem. The unit would not work at all if the transformer was burnt out or bad. I would suggest tracing the wires back to see what control is causing the fan to run all the time. When you find the control that is causing the fan to run, then this is more than likely where the problem is. Best of luck! Steve

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