In article <firstname.lastname@example.org>,
> I have a 92 Mustang GT with the A/C converted to R134. All is fine
> except after about 15 to 20 minutes of operation the
> compressor quits operating. It starts working again after
> a while. When the compressor quits working, the pressure switch on the
> filter dryer is still closed. Pressures are normal before and after the
> shut down. Is there another switch I am over looking or something
> thermally sensitive I am over looking?
Yes. There's a WOT (Wide Open Throttle) cutout relay between the clutch cycling
pressure switch and clutch field coil. The contacts may be bad or it's
being normally energized by the PCM during WOT or possibly other
conditions such as high engine coolant temperature at low and high engine
speeds. The relay is located in the engine compartment on the lower RH
fender apron, forward of the shock tower near the fuel pump relay.
OTOH, (and assuming the pressure switch is closed as you mentioned) you
may be losing power upstream due to an intermittent wiring connection, AC
power/blower switch, or downstream with the field coil and ground
connection. Get the multimeter out and do some additional testing.
Electrical flow would be:
Fuse-->A/C pwr/blwr sw-->Pressure sw-->WOT c/o relay-->Field coil/diode-->Gnd
There's your electrics. Good luck and HTH...