I believe there's a high and low pressure switch (separate switches) behind
the rear left panel
(next to the High pressure port), on my 87.

this also jibes with my experience (failure) of my system. I had mixed 134
and 12, and this clogged the expansion valve. In my 87', once you turned the
AC on, the pressure backed up on the high side, and triggerred the high
speed fan. If you shut the van off, the fan would run until the battery died
(8-10 hours). This is obviously not a high temp switch, if it was temp, it
would shut off again when the temp dropped. When I replaced the expansion
valve, flushed the system, and re-charged, everything operates normally
(including the high speed fan switch).

Maybe there is a high temp switch also?

John