AC compressor clutch not getting power

Bernie Benz b.benz at
Thu Aug 18 16:56:42 EDT 2005

Ben,  do you have a Bentley?  Go to the schematic on pg 590 and note that
the AC Clutch relay is home at position #1 in the Aux relay panel, not in
the fuse panel.  This relay has 2 failure modes, BTDT.  An open supression
diode causes big negative voltage spikes superimposed on the electrical
system when the clutch drops, radio popping.  A shorted diode causes the
clutch to run all the time when the AC is on.  The diode is replaceable.
Check for X Bat voltage to the relay.


  Organization: SWANN
> Reply-To: Ben Swann <benswann at>
> Date: Thu, 18 Aug 2005 15:11:15 -0400
> To: <quattro at>, <200q20V at>
> Cc: brian at, Ben Swann <benswann at>
> Subject: AC compressor clutch not getting power
> Two '91 200Q20V sedans with apparantly the same problem.  The AC system has
> full charge -  approximately 90PSI static/35PSI low and 180 PSI high
> pressure side during operation and outside temp around 80 deg. F.  Climate
> control set to low and everything operates except for compressor clutch is
> not engaging.  Applying power directly to compressor engages clutch and
> system blows nice and cold.
> I noticed on the one I was just working on that there was no relay in
> position number 6 where the fuse box cover indicates AC relay should be.
> Inserting relay makes no difference.
> Any clues as to what is going on?  BTDT on common culprit(s)?
> Ben

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