Central A/C Compressor/Condensor/Fan Not Turning On

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Old 06-06-12, 06:56 PM
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Forgot to say..thanks hvactech and Grady for responding so quickly to my PM. You guys are what make this such a great site!
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Old 06-06-12, 07:04 PM
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Location: Columbia, SC
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Yup and yup.

I have a digital stat and he said we were 99% there. I still had the jumper in place when he got here. He said we were absolutely on the right trail, just that the jumper wasn't the right way to go about it. If you are going to bypass, bypass it all the way.

Thanks for everyone's great assistance and willingness to give up your evenings for my sake.

My kids, my dogs and especially my wife thank you.
Old 06-11-12, 07:51 PM
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Vic, you know as well as any, that's why we're here. To help folks. But thanks for the thank ya anyway.

Allen, Your guy was right. I should have had you remove the wires & jumper them together for testing purposes then either replace the delay or permanently conneceted the wires together. In any case, you are more than welcome.
Old 09-07-12, 04:50 PM
Join Date: Sep 2012
Posts: 3
no 24v to contactor

This thread was very helpful to me. My capacitor failed about 2 months ago, and all has been fine after I replaced it...until yesterday. After troubleshooting and google searching I identified two problems:

1. the fan motor is bad. If I applied some good pressure to it with a screw driver it would only spin about a 1/4 turn before it stopped. If I manually engaged the contactor and gave it a spin, it might make a couple revolutions before stopping. Picked up a new fan from Grainger today and installed it, so fan motor problem is resolved. Manually engaging the contactor causes the fan & condenser to quickly kick in and operate properly.

2. the contactor isn't engaging when the thermostat kicks in to cool. After reading this thread and others, I tested and found that there is no 24v coming in to the contactor. The supply wire has 2 wires, red and white. When cooling is off, both wires read 0 VAC. When cooling is on, the red wire (going to the contactor) reads 0 but the white wire (running to the condenser I think) reads ~24 VAC. I find it strange that the fan motor and something else would die at the same time, but that appears to be the case.
Where does the 24v wire that goes to the contactor come from? Is it controlled by the furnace blower control board, or from the thermostat zone board? I checked both and can't tell. The 24v transformer is in the furnace blower panel, and it runs 24v power to my Honeywell minizone panel (from which the thermostats are hooked up to). So it seems like it could come from either. I need to determine where the wire runs to so I can test to see if it is getting voltage on that end. If so, then I know I've got a break somewhere along the line (which doesn't seem likely to me). If not, then I guess I'll need to call for service on the board which isn't supplying the 24v power when it should.

Sorry for the long post, but I like to be thorough. Any advice would be greatly appreciated. The wife is unhappy with the CA heat and no A/C...

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Last edited by robertkara; 09-07-12 at 05:25 PM. Reason: added pic of zone control board
Old 09-07-12, 05:42 PM
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you measure 24V between the 2 wires, not one to ground. one wire is hot the other is common.
Old 09-07-12, 06:50 PM
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thanks. Yeah, I'm an electrical novice. OK, here's where I'm at. I verified power is going to the cooling relay from the thermostat which then goes to a board on the furnace blower which then goes out to the contactor. All of that looks good. I'm happy to rule out everything inside the house. Outside, 24v is coming in properly through the white wire at 27 VAC. It is then connected to a yellow wire which goes inside the unit to a ceramic looking thing (ugh, hate saying that...) on the condenser and immediately returns on a yellow wire which comes back outside the unit, then immediately connects to a smaller black wire which goes into the condenser unit and back out a black wire which is then connected to the contactor. When I disconnect the black wire from the contactor and measure its VAC, I get around 13 VAC. So somehow I'm dropping from 24 VAC to 13 VAC through those two runs to the condenser, and am guessing the low voltage is the reason the contactor is not engaging.
If I jumper direct from the 24 VAC in to the contactor, it immediately engages and everything kicks in. So it looks like I can rule out a bad contactor as well. This is a bit worrisome to me, as I'm hoping it isn't a problem with the condenser motor.
Old 09-07-12, 07:00 PM
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sounds like the circuit runs through pressure switches and it is likely you have a problem that will require a visit from a contractor. Having said that, make sure that there is not a rubber end on one of the pressure switches that the circuit runs through. If the motor had failed it would have tripped the high pressure switch, and it may be a manual reset switch.
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Old 09-07-12, 09:20 PM
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My family thanks you!! Reset switch did the trick. Thank you for the quick responses. Extremely helpful.
Old 08-16-13, 11:12 AM
Join Date: Aug 2013
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+1 for this thread. It helped me fix my AC Condenser.

AC was running but blowing warm air, went outside condenser was not running, and there was a loud hum from the transformer.

I had a helper turn off the AC then back on again, I could hear the click from a relay (contactor).
The fan would move freely when spun with a stick.

Switched out the dual run cap ($40) and runs like a champ now.
Saved me a $150+ visit from a technician.

I'm not an AC guy, i work in IT. I know my way around electricity and how lethal it can be.
For any DIY job, don't work outside your comfort level.
Old 07-29-14, 05:48 PM
Join Date: Jul 2014
Location: United States
Posts: 1
Pretty good discussion, and thank you.
Not sure how much money I saved by reading this thread. My Fan and compressor unit quit on us last night and thanks to this thread I was able to debug and fix the unit. Let me mention that I'm pretty handy (fix my own cars, microvave, oven, fridge...you name it, chances are I've done it...and now I can add repairing my ac unit to the list).
After reading this thread, I ran out to get a multi meter and proceeded to find the contactor and cap. It's the middle of Summer and I could hear the system start but could only get hot air (or room temperature).
I probed the contactor and got 240 V, BUT NOT the 24V needed to trigger the capacitor. A manual push of the trigger made the fan and compressor run (what a relief since I looked at the compressor prices prior to finding this thread).
Next, I traced the wires that should bring 24v to the unit back into the house, cut power (circuit breakers from fuse box), disconnected the 2 wires that should provide 24v, hooked them up to a power source (happened to have a 24v adapter from an old phone), and I could hear the fan and compressor run outside the house. This really worried me and I thought I would need a new board. I traced the board diagram and realized that the thermostat is partly responsible for the voltage fed to those 2 wires.
I dis-assembled the thermostat and voila !!! I could clearly see and smell a fried capacitor. Let me mention that my thermostat was a mobile device that I purchased so that I can create separate zones, and not cool/heat the 1st floor and basement during night time and cut down my energy bills.
I hooked up the old thermostat and every thing was running great once again. I ordered a replacement for my wireless/mobile thermostat and will hook it back up as soon as I get it.
Sorry for the long essay...I hope it helps someone.
Old 08-14-15, 06:00 PM
Join Date: Aug 2015
Location: USA
Posts: 2
Thank You!!!


Recognize this thread is over a year old but if any of you are still up - thank you. I got home last night to a stifling house - fan worked but no compressor. Finally gave up trying to sleep at 2:00 am and spent the next 4 hours researching - this thread did the trick. After a few false starts finally found a supplier on the island that had both compressor and contactor - 15 minutes to put it back together and life is good again! I'm reasonably handy but this is the first time ever foray into HVAC -- you guys made it easy - and $47 for parts is a WHOLE lot better than a $500 repairman visit... Aloha!
Old 08-14-15, 06:03 PM
Join Date: Aug 2015
Location: USA
Posts: 2
Thank You correction

Sorry - meant to type capacitor. Compressor is working like a champ. Guess that's what pulling an all nighter at 50+ gets you... :-)
Old 08-18-15, 07:13 PM
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This thread has received a lot of views. If after reading this thread you don't get your system running.... start a new thread where we can help you directly.

This thread will be sent back to the archives where it will always be available for reference.
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