HVAC Compressor Troubleshooting

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Old 05-20-18, 07:37 PM
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HVAC Compressor Troubleshooting

Need some guidance please. I'm pretty handy at fixing my own stuff but am stumped at this point. A little history. One of the HVAC systems in my house stopped working at the end of last summer. The air handler stopped running. The compressor outside was still humming. I flipped breakers and shut system down for the season. Let it sit all winter. I went to air handler a few weeks ago and the circuit panel was powered but fan did not come on. Replaced the capacitor. No fix. Just a buzzing noise. Spun fan by hand and powered unit. It started and then slowly came to a stop and started to buzz again. Replaced fan motor. Powered unit and viola, all started fine. Except, I could not hear any refrigerant flow. went outside and fan was running but compressor was not. Here's where I need help. Bought a meter that tests capacitance. Capacitor looked fine and had readings across all 3 terminals. Unit in single phase BTW. I did the following checks on the compressor leads. Disconnected from their terminals. Herm to common, start to common, and herm to start were not shorted. The resistances followed what I think is normal. Herm to Common 1.1, Start to common 3.0, and herm to start 4.1. I then scratched some rust off of top of compressor and touched the scratch and the case of the housing and got a reading so I was sure i had contact with the compressor through the rust. I then checked each of the compressor leads , herm, common, start with the compressor housing where i scratched. None of them showed any continuity. Those tests, so I have read indicate the compressor is not bad. I also checked the contactor block. 240v at input and when unit is powered and fan is on, 240 at the output side to the compressor. Any help?
Thanks,
John
 
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Old 05-20-18, 07:59 PM
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Welcome to the forums.

Run to Common 1.1, Start to Common 3.0, and Run to Start 4.1.

I'd have to think it was an incorrect or bad connection.
Those resistances look a tad low but show valid windings and no short to ground.
With those resistances measured.... the compressor would have to start or hum VERY loudly.

Please be careful working around the wiring connections on a compressor.
Safety glasses should always be worn when messing around there.
 
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Old 05-20-18, 08:14 PM
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Thanks Pete.
I did notice last year before the air handler fan failed that the this condenser unit was humming louder than normal when running. Shouldn't be incorrect wiring, its been installed and work for about 15 years. I honestly expected to find open or shorted windings being the unit is 15 years old. But when the testing didn't show that I was stumped. What is a hard start kit? Is this a possible short term solution?

JR
 
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Old 05-20-18, 08:22 PM
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Your measurements are very close to a dead short. When you connect 240v to it.... something MUST happen. A breaker trips or the compressor will hum loudly and get very hot quickly.
 
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