Likely the AC compressor is toast -- try turning the clutch by hand --
if it's abnormally stiff, you need to replace it -- this is a big job,
sadly, and the system usually is full of crap that will kill a new
compressor pretty fast. More likely is a bad belt, though.
The AC compressor is not operated by vacuum, but by the electronics in
the pushbutton unit. It does cycle more than you think.
Check the operation of the shutoff solenoid, and check for oil in the
lines to it (brown and brown with a blue stripe) -- if oil is present,
the diaphram is leaking and needs to be replaced. You must also
replace the rubber connectors at the shutoff switch on the keyswitch
and flush the lines out to get rid of the oil in ther -- it can cause
the switch to stick, causing low performance when the shutoff solenoid
doesn't release completely.
Peter
