Air Conditioning - Contactor shorting

My single pole contactor on my York A/C fried yesterday, and I replaced it with a new twin pole contactor. Everything ran fine until about an hour ago, when the contactor started going out again. There doesn't seem to be anything that would cause it to short, like a bug or something. House does have aluminum wiring and an old FPE stab-lock breaker box. Trying to determine if I need an A/C guy or an electrician - can't afford to pay them both. Any suggestions would be greatly appreciated. Thanks.

My single pole contactor on my York A/C fried yesterday, and I replaced it with a new twin pole contactor. Everything ran fine until about an hour ago, when the contactor started going out again. There doesn't seem to be anything that would cause it to short, like a bug or something. House does have aluminum wiring and an old FPE stab-lock breaker box. Trying to determine if I need an A/C guy or an electrician - can't afford to pay them both. Any suggestions would be greatly appreciated. Thanks.

Could you explain a little better what you mean when you say the contactor is going out again? Do you mean the circuit breaker is tripping? Are wires burned off right st the contactor? Is the compressor trying to start but won't/can't?

You sure you have the right coil on the contactor???? After the compressor is on for a time it dont go on and off ridding the klixon like does it. That aluminum wiring did it go to the contactor???? If so did you put the paste on the wire???

Old contactor had large amount of black "soot" on it, was cycling in and out rapidly and making a lot of noise. "Soot" now developing on new one and making same racket - contactor opening and closing. Circuit breaker does not trip. In fact, when I threw the breakers (both on the main panel and the breaker next to the unit outside) to shut it down, contactor continued to open and close until I turned off the thermostat. Don't see any burning at wiring, just at the contactors.

Sounds like you do not have the full 24V coming out to the contactor.

Check your low voltage at the contactor and make sure you have 24V.

You could have a short in the low voltage side.

All the wiring is connected as it was on the old contactor, which was working fine until yesterday. The wiring into the unit is copper, the house has aluminum. I did replace the capacitor about a month ago.

Then like said check the 24V line . Check the tstat first with it off jump R to Y does the contactor stay on??? if so can be the tstat If not then check the 24V wires from tstat to the contactor

Don't know enough about what I'm doing to be sure I'm checking it correctly. If I push in the contactor and hold it in it runs smoothly. When I turn on the thermostat the contactor jumps in and out and makes a racket, compressor (or at least fan) runs.

Anybody still out there? Any chance the thermostat is the problem?

connected the R and Y in the thermostat and a/c runs smooth. I'm replacing thermostat. Thanks for the feedback. If that doesn't it, I'll be back!

Everything could be wired correctly and you could still have this problem, infact if you didn't wire it correctly your unit more than likely wouldn't work at all.

The transformer sends 24 volts to the coil of the contactor, which causes a magnetic field that will close the line voltage contacts. If for some reason your transformer isn't producing enough voltage it will not be able to hold the contactor closed and will act just as you've described it.

You need to use a volt meter.

Follow the small thermostat wire that enters the condenser.

While the unit is calling for cool, you need to put both your leads from the voltmeter where these two wires connect to the contactor. If you read anything less than 23.5 or 24 volts than try changing the transformer.

If you have healthy voltage present, than try placing a jumper wire between r and y at the thermostat. If your AC runs steady with the thermostat jumped, than there's probably a bad contact within the thermostat and you'd need to change the t stat

I had a unit once that burned out 3 compressors. When they finally sent me to the job I found (after sitting on the unit for 5 hours) That when the T-Stat was within a degree or two of being satisfied that it would go on and off rapidly. It turned out to be the T-Stat.

I would look there first.

What I would do is this. Remove the T-Stat and jump out R G Y and see if the unit runs without shutting down. If it does a new T-Stat is needed. If it does the same thing I would start looking at the low voltage wiring.