Carrier with a control circuit short to ground

[Milk man]

Carrier
38AE-064-510
U797544

Control fuse was blown.

Pulled the secondary wires from the transformer and started ohming to ground. (Secondary is grounded)

Seperating goobs of control wires. Start yanking some to see where they land and the short disapears. Didn't see and bare wires.

We fire up the compressors and still looking for short. The right side compressor stops running and the breaker is tripped. Left side still going and I throw the right side breaker and we loose the control fuse again.

Called it quits for the night since it wasn't critical to have it running tonight.

Any thoughts.

[cxagent]

The control fuse blowing when the circuit breaker was reset sounds like either two control transformers fighting or the transformer is fed too high a primary voltage. Either of those mean the transformers need to be on the same phase.

Good luck on tracing this one down. I would start looking for shorts to ground and bare wires first. Then check for transformer phasing.

[joey791]

Lynn, pull the coil wire on 2nd compressor, if it stays running you have narrowed it down, good luck, I bet its in the 2nd compressor circuit(right circuit).

Also ohm out the left contactor and compare the readings to the right contactor.

[Randy S.]

If the control circuit has one side intentionally grounded, then you will read all kinds of low ohms to ground as you poke around with a DC powered meter. Temporarily lifting the intentional ground will help you determine if there's a ground fault somewhere.

I spent hours one time tracking down what turned out to be an internally shorted contactor coil. No visible signs of anything, but that's what it turned out to be.

Let us all know what it was.

[Milk man]

This thing has 4 contactors, 2 time delay relays, 2 sol. drop relays. 3 condenser fan relays, oil pressure switch. electric unloaders and a whole lot of wiring.

[Milk man]

Originally posted by Randy S.
If the control circuit has one side intentionally grounded, then you will read all kinds of low ohms to ground as you poke around with a DC powered meter. Temporarily lifting the intentional ground will help you determine if there's a ground fault somewhere.

I spent hours one time tracking down what turned out to be an internally shorted contactor coil. No visible signs of anything, but that's what it turned out to be.

Let us all know what it was.

I first thought it was a holding relay draging down. I've seen relays do that. By unwiring the secondary of the transformer I removed the ground from the circuit. I was wondering if I was chasing my tail until the short disapeared.

[Milk man]

Control wire rubbed on the discharge muffler. Making intermidant contact. We were yanking that bundle when the short disapeared. I was hunting a discharge thermistor when I found it. We looked at the big ass romex connector on both compressors for the bad wire last night. Just didn't look at the whole thing.

It's very hard to find good help. At least that is what I tell my boss.

[Milk man]

Control wire rubbed on the discharge muffler. Making intermidant contact. We were yanking that bundle when the short disapeared. I was hunting a discharge thermistor when I found it. We looked at the big ass romex connector on both compressors for the bad wire last night. Just didn't look at the whole thing.

It's very hard to find good help. At least that is what I tell my boss.