Compressor not turning on. - http://www.hvacmechanic.com/ Forums
Original message
| trippintl0
 |  "Compressor not turning on."
, posted Tue 19 Aug 22:44  
My compressor is not turning on at all, but the condenser fan is turning on. Here's what I've checked:
The contactor is energized 24v. The line side of the contactor has 120v on each leg, 240v when measured across L1 and L2.
BUT on the load side of the contactor, if I measure T1 to ground, I get 120v. If I measure T2 to ground, I get 120v. But from T1 to T2 I'm only getting between 10.2 volts to 11.0 volts.
I'm confused. How is it I can get 120v separately on both lines but only 10v between the two???
I took apart the contactor and cleaned it already, and it was very dirty. Afterwards I got no resistance from L1 to T1, and L2 to T2, so I'm thinking that means its good. Yet still same issue with the voltage.
One weird thing I noticed is that the condenser fan motor has one line wired straight to L1 on the contactor, and the other line goes to T2. So only one leg is interrupted when power is turned off. I know sometimes the compressor has one line with power on for crankcase heat, but why the condenser fan motor? Maybe that's why the fan motor comes on but not the compressor.
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Replies:
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| Houston204
| "Re(1):Compressor not turning on." , posted Tue 19 Aug 23:28
If you verified that nothing was obstructing the contacts, replace thecontactor.
"Tip of the Day" Remember to remove power first.
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| | trippintl0
| "Re(2):Compressor not turning on." , posted Wed 20 Aug 00:15
Nothing is obstructing the contacts? Not sure what you mean by that. The contacts were dirty, but I took it all apart and cleaned them pretty well. And if the contactor is bad, then how come I can measure 120v from each side to ground when the contactor is energized? (Not arguing, I'm just trying to understand what I'm missing here.)
Also I disconnected the capacitors and tested them, they both are within spec (has dual capacitors on the compressor.) I disconnected the compressor and ohmed it to find about 5.2 ohms from start to common, 5.3 from start to run, but only 0 - 0.1 ohms from run to common. Maybe the run winding has a short? I should have checked each wire to ground while it was disconnected, but I didn't think of it.
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| | Houston204
| "Re(3):Compressor not turning on." , posted Wed 20 Aug 01:13
Backfeed is giving you 120 volts to ground when your contactor is pulled in but yu cannot measure 240VAC at T1 to T2 when it is present at L1 to L2.
You can prove the faulty leg by measuring L1 to T1 and L2 to T2 (should be 0 when pulled in, 240VAC would indicate the problem)
You can remove the small spring, remove to upper contacts and run it them across a file for a temporary fix. Scratch the other points with a small straight screwdriver.
In this single pole example, the blue line points to the spring area...
I would still replace the contactor for dependable operation.
"Tip of the Day" Remember to remove power first.
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| | trippintl0
| "Re(4):Compressor not turning on." , posted Wed 20 Aug 22:34
Yes! Thank you for your expertise and for your patience with me! I completely forgot what I learned about checking for voltage drop on a single line - 0 volts = good connection, any reading means a drop in voltage.
You were right, L2 was 0 volts from L2 to T2, but L1 to T1 measured 240v (with the contactor energized).
So when I measured from T1 to ground, I was actually measuring the 120v from T2, going through the compressor & capacitors, and back into T1. I assume that's what "backfeed" is.
Thank you for teaching me something that I didn't know (obviously there is still a lot that I don't know, lol).
And just so you know, the contactor is definitely going to be replaced. I just wanted to know for sure the problem first.
Again thank you for your help!
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