If the dash vents switch to defrost at full throttle, you have a vacuum
leak -- there is a tank that holds vac for the system, but leaking
supply lines and/or leaking servos will cause it to cycle back to
defrost at wide open throttle. Usually this is leaking servos, as they
require condsiderably more vac that the system usually provides at WOT.
As for the temp rise, you should check the AC pressures at high rpm
(3000 -- only for a few seconds, please, the engine doesn't like it!).
Either excessive high side OR too low low side pressures will cut the
AC off. A faulty expansion valve will cause very low pressure by
preventing freon flow, and this can also cause the compressor to stop.
Usually this means a clog. Air in the system will cause excessive high
side pressure hot.
And, a bad evaporator temp sensor will cut the compressor off
inappropriately, too!
However, the very first thing I would do is replace the belt -- a
slipping belt will cause the AC compressor to drop off line until the
engine is restarte. This is to prevent a compressor lockup from making
the engine inoperable.
I don't know if there is a WOT cutout for the AC compressor. That
would be unusual, as even the old diesels don't have one!
Peter
