I have a 23 year-old dual-fuel 2.5-ton heat pump; it is a split system, air handler and furnace in the crawlspace, compressor outside. I use it very little in the winter, mostly heating with wood; I use it a lot in the summer. Here is what has happened:
Turned on A/C, after several weeks of idleness (pleasant springtime weather). Compressor did not come on (neither the compressor fan nor the compressor-proper).
Next day, tried to see if compressor would work in heat mode. It did. Then switched to cool mode, and it also worked. A/C has worked fine on the few occasions I've tried it since then.
Looked at compressor while running properly. Measured 24 vac signal to contactor coil and 36mv voltage drop across contacts (1-1/2 pole contactor).
What do you recommend ? Is the 36mv across contacts normal ? Sounds small, but at 20 amps, that's almost a watt dissipated in very little metal, so maybe not. (The compressor runs off a 30amp circuit). The contactor has not been replaced in many years. I am tempted to just replace it. Cheap part. DIY - I am electrical engr and have done lots of home wiring, so can do it without killing myself (turn off breaker, make sure capacitor is discharged).
But, do you think this is the most likely problem ? If not, I guess I just have to wait for it to fail again, and check to see if the coil is getting 24vac. Anything else I can check before then ?
Also, searching here, someone said capacitor is most likely cause of compressor failure. However, I wonder if the fact that my problem occurred once, and then went away, would modify this ?