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3-terminal (not 4) multifunction thermoswitch

Recently I posted about erratic A/C in my '89 100. The A/C cools very well
when the compressor cycles on, but once in a while--inexplicably--it
refuses to cycle on for extensive periods (hours). A local Audi A/C
mechanic concluded that freon level was OK, and  his guess ("guess", since
A/C was working OK at the time) was that the multifunction thermoswitch was
defective and sometimes sends a spurious overheat signal which keeps the
A/C off.

I passed this info along to the Audi mechanic who had done repairs on the
vehicle before I bought it. He stated that in the past he had sometimes
installed non-OEM thermoswitches, which had only 3 terminals instaed of the
4 in the OEM version. The use of these 3-legged switches he later found to
be correlated with A/C non-functioning. Sure enough, a 3-terminal switch
had indeed been installed in my Audi. He'll be giving me a free replacement
switch (OEM).

Has anyone encountered this problem with the use of a 3-terminal
thermoswitch? How come everything seems to work fine *most* of the time?

Phil Rose
'89 100

         *  Phil & Judy Rose     E-mail:              *
         *                       pjrose@servtech.com  *