According to my 1991 GM C/K Pickup Electrical Diagrams,
When the ignition Switch is in RUN, battery voltage is applied through the A/C-HTR Fuse(brown wire) to the coil and contacts of the AC Clutch Relay.
With A/C ON selected, Terminal 966(Dk Blue/White) of the Heater and A/C Controller is grounded which energizes the A/C Clutch Relay.
The contacts of the Relay close and battery voltage is applied(Lt Green) through the closed contacts and the normally closed A/C Accumulator Switch to the coil of the A/C Compressor Clutch(via Dk Green wire).
>>There is a junction here. The Dark Green wire *also* goes to PIN B8 [Circuit 59] in the '91 ECM labeled Air Conditioning)<<
The coil is energized and the A/C Compressor Clutch engages(when voltage goes out Blk wire to sheet metal ground).
sooooooo
ala Connector Part Information
A/C Compressor Clutch Supply Voltage [also Circuit 59, how fancy is that??] is STILL dark green..we are getting closer...
It is Pin 18 on the C2 RED connector. 
Last edited by r6z4o6; 06-13-2008 at 02:43 PM.
1991 Silverado ECLB
LM7-T56
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