Hi Chris,
I just tested the pressostat with a paper clip and what happens is that
the engine idle changes, the idle also changes when I unplug
and replug the pressostat.

On the 1991 240 when I click on the a/c switch, the idle changes, then a second later I hear the compressor click. This does not happen on the 93.

On the 1993 240, which has the a/c problem, when I click on the a/c switch
a second later the engine idle changes, but the compressor does not make a
clicking sound.

I am going to check the wiring on the compressor.

Also, on the a/c relay switch (which has four prongs) prong #1 always
has juice, then when I switch the a/c on, prong #2 gets juice, then a second
or two later prong #3 gets juice, Prong #4 never gets juice.
Do you know if prong #4 is just there for fun...hehehe?
Thanks,
Joseph
New Mexico