PeterD wrote:

Well, he did say (in another post) he's bypassed the pressure switch
for a test run, and the symptoms persist. I think he's got a high
resistance point in the wiring too, but I'm not sure where.

Yes, bypassing the pressure switch doesn't make any diffence. You may
well be right about a high resistance point.

Maybe monitor voltage at the motor terminals might give a clue.

I can't easily get to the internal terminals. The closest I can
measure is at the pressure switch, and the voltage looks fine there.

As Alan says, locked rotor should result in very high current/power
draws. It is very noticable, and unless the compressor is on a circuit
that is vastly overfused result in a blown fuse/tripped breaker almost
immediately.

It's on a standard outlet circuit with a 20A breaker. It hasn't
tripped. Nor have there been any of the signs that you mention for
high current draw.

Can you (the OP) post specicifications from the motor's name plate,
and the compressor assembly? FWIW, no compressor has a fractional HP
motor, most are motors rated at 1 to 3 HP on small compressors.

1/3 HP is what's on the nameplate. These days, it would probably be
rated at 1 or 2 "marketing HP" :-)

The compressor itself has long lost it's label. But, it's a single
cylinder reciprocating, belt driven compressor with about a 20 gallon
tank, a 3 inch pulley on the motor and a 9 inch on the compressor.

The motor was made by GE, and is labeled:

Model: 5KC42JG391AX
Volts: 115/230
Ph: 1
Amps: 6.0/3.0
HP: 1/3
HZ: 60
RPM: 1725

It's running on 120 volts.