What does the compressor keep running when air conditioner is off?

Answer 1

The most likely possibility would be that the main contactor (the big relay that connects and disconnects the power to the compressor and fan) is stuck. This could be caused by dirt/junk causing the plunger to stick, or more likely by contacts that are welded or fused together. It might be possible to manually unstick the contactor, BUT it's just going to stick again sooner or later, AND when you start taking panels off and digging around in the electrical section, you risk being ELECTROCUTED OR BADLY BURNED! Unless you have experience with this kind of equipment and ABSOLUTELY KNOW what you are doing, DON'T EVEN THINK ABOUT WORKING ON IT!!

It's also possible that the inside control wiring or circuitry that tells the compressor when to run is messed up. It could be damaged wiring, incorrect connections (Did someone just work on it?), or a circuit board or relay failure. If the signal to energize the contactor is always on, the outside unit never shuts down. Whatever the cause, it's potentially a very bad situation for the compressor.

One possible result: The pressure in the cooling coil side of the system could go low enough to cause lube oil to be pulled out of the compressor, leaving it starved for oil, and creating slugs of oil in the system.

Another possibility: With no air flowing over the evaporator coil (cooling coil in the house), the coil will cool to the point where less and less of the refrigerant will turn to gas before it returns to the compressor. Depending on the system piping and layout, this could result in slugs of liquid refrigerant entering the compressor, and since the liquid is not compressible, the valves, pistons, and other parts in the compressor will be damaged. Some units have low pressure sensors to keep the compressor from running if this happens (like for an indoor blower failure or clogged air filter), but if the contactor is stuck, this safeguard will have no effect.

If you really want to try to fix it, give the outdoor unit a good rap near the access panel. Not enough to dent or move it, just a good thump (once or twice only) with the heel of your hand. If that makes it shut off, it almost has to be the contactor. That has worked for me a couple of times.

If it's been years since the thing was serviced, tell the service tech to go ahead and clean the condenser coil and change all relays and capacitors. I have been replacing the relays and capacitors about every 6-7 years in my home system, and I am convinced that's one big reason that my 33 yr. old, 10 EER, Montgomery Ward-by-Rheem system is running as well now as it did when it was new. By the way, tell the tech you want all the old parts when they are finished. If there are X no. of parts listed on the ticket, there'd better be X no. of used parts that you receive from the tech. And if you get a competant and friendly technician, treat them nicely and offer them some refreshment. They can be worth their weight in gold.

Remember - I'm not trying to be insulting or condescending when I say this - Please Please PLEASE - Do NOT try to fix it yourself if you are not trained and experienced in working on heavy-duty electrical equipment. Your best and safest repair tool is your telephone.