As soon as the thermostat starts the a/c, the circuit breaker trips. A new capacitor didn't help. I don't have a meter handy and I probably wouldn't really know how to use it beyond basics.

I was wondering if disconnecting the HERM and FAN terminals from the cap would effectively take them out of the picture. If it does, since the breaker still tripped with either or both disconnected, what would be a good next step? I looked at the cables and couldn't find any obvious short. The contactor seems to work (connecting and disconnecting the yellow cable from the thermostat engages the relay).

All the other possible cause that came to mind don't seem to justify a breaker tripping right away - I understand that dirty fins (they are not that dirty) or refrigerant (the liquid line is relatively cool and I don't see any ice buildup) do trip breakers over time as they overload. An issue with the fuse in the disconnect box (it's a big one with two battery-sized fuses) or the breaker itself is possible but starts being very unlikely

Any suggestion? I would gladly pay a professional to take a look, but the sooner I can get someone over is 10 days so I'm trying to see if there is something I can do in the meantime...

 

First call yesterday was the same as you describe. All checks of condenser were normal and total amp draw of system well within spec. Put a thermal image camera on main circuit box and found an excessively hot leg on the breaker in question. Turned out to be a loose wire connection causing resistance/heat. With proper connections the heat and tripping were eliminated.
I'm not saying this is your problem, only that this is one of many things that could cause a trip. Checking for a loose wire is easy if you know how to do it safely.

 

Welcome to the forums.

No.... that does not remove those components from the system.
You will need a meter to check for ohms/continuity.
My guess..... a shorted compressor.

 

Thank you. I opened the unit, unplugged the compressor and the breaker tripping went away. The fan is spinning, the thermostat is controlling the unit and everything but the compressor seems to work. I'm guessing I'm done investigating and I should start bracing for the bill on a new compressor, correct? Anything else I can look at (I have a meter now)?

Quick question in the meantime: we have our system on so that at least some air is circulating from the furnace fan. Any reason to shut it down, either completely or just the outside unit (which is just the fan really)?

Thanks!

 

Check the compressor for a short. It could be in the load wiring going to the compressor.
Depending on equipment age it may not be wise to replace a compressor. And instead put the money towards a new unit.
A compressor replacement can be a few thousand dollars. And once the new one is running, it’s likely you’ll find the cause of why the compressor failed in the first place.

 

The plot thickens. The compressor pins all have continuity with no shorts I could detect, and I'm measuring with what's probably a crappy meter C-S at 1.8, C-R at 0.9 and R-S at 2.6. Is this enough to assume the compressor may be fine and the problem may be elsewhere?

 

Continuity and a short to ground are two different subjects.
What is resistance to ground?

 

Every single compressor pin measures open line to a sanded copper pipe in the unit. Is that the right way to check?

 

Yes.... that shows no internal shorts to ground.
Your pin measurements are in proper proportion but are all low. That could be just the meter.

I'd recommend posting a few pictures of your contactor/capacitor wiring so we can see if there is a visual issue..... How-to-insert-pictures

 

Thanks! I'll try to post some pics from the PC later. One thing I noticed is that I have continuity between the fan and compressor wires, measured at the capacitor (fan and herm). Is that OK?

 

Yes....... that is correct.

 

Here is a picture of the contactor/capacitor - let me know if something jumps up as unusual.

 

No..... I see nothing unusual. Make sure the power is off to the condenser. Using your ohmmeter...... check from the black wire and the yellow wire on top of the contactor to ground.