Alan Douglas wrote:


So it stalls first, then the unloader bleeds off pressure at the
compressor and allows it to restart?

Yes, that's correct.

I'm puzzled, but at this point I would disable the switch and unloader
and see if the compressor runs up to 120 as it used to.

It's a mechanical unloader, integrated with the check valve, so
disabling it would be difficult. However, it's pretty obvious that the
unloader only opens after the motor stalls when the check valve
closes.

You said the motor current is only 5 or 6 amps when it stalls?
Locked-rotor current for a 1/3HP motor should be much more than that,
at least 30 amps (I haven't looked it up). You're sure it's getting
full line voltage at that point? I don't see how it could be.

There's definitely no current spike when it stalls, nor does it trip
the breaker, dim the lights, etc. Is it getting full voltage? It is at
the point the motor leads connect to the pressure switch. I can't
easily measure any closer than that because the leads run into the
motor and there's no easy way to get to them, especially when it's
running. But, it looks like I need to disassemble the motor and check
things out inside, so that'll be the next step.

Thanks!