Question about 1991 GMC Sierra

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1991 Sierra GMC AC control problem

When you push the ac on button on the control panel the inside blower comes on but the compressor clutch does not engage. The AC indicator on light flashes on and off for a short period and goes off. The blower continues to run. The low pressure switch checks ok. The refrigerant charge is ok. The fan clutch will work if power is connected directly to it. I have checked everythig that I know how to check except the compressor control relay and I am not sure where it is physically located. If the relay is not functioning properly will this cause the AC indicator light to cycle on and off and shortly go off. Should I suspect that I have a problem with the control panel or the ac compressor relay?

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The follwing is a cut and paste from a service bulletin:

Bulletin No.: 461202

Date: May, 1994




A/C Control Head flashes, performs erratically or malfunctions.


Light surface corrosion on the Control Head terminals.


Remove the A/C Control Head and while holding the unit with the terminal
cavity down (toward the floor) so no particles will fall inside on the
circuit boards, polish the terminals on both sides of the cavity with a lead
pencil pink eraser. Do not use an ink eraser or other abrasive such as a
small wire brush or sandpaper, etc. Doing so will remove the terminal
coating and degrade the contact surface.

Performing this procedure, in many cases, will make replacing the Control
Head unnecessary. Using this procedure in conjunction with the A/C circuit
diagnosis found in the Electrical Diagnosis Manual and especially before
replacing a Control Head for any reason is recommended.

After polishing the Control Head terminals, but before reconnecting, check
for proper terminal contact as described in the applicable electrical
diagnosis manual. If contact is good, apply dielectric grease (P/N 12345579)
to the harness connector terminals. Please do not apply grease to the
Control Head.

Posted on Jun 30, 2009

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The a/c relay is located behind the dask pockett in a series of 3 on a metal brackett, unpulg each, one one at a time, till the lights in the control unit goes off and then plug it back in,If this doesnt work then disconect the ground cable on your battery for 15 sec and reconect . After this your light will not flash and you can turn on your a/c and it will work,thats is if your freon pressures are correct so as to give the low pressure switch enough pressure to send 12.50 volts to the compressor so the clutch will kick in.I hope this helps you out..

Posted on Sep 06, 2010

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The AC compressor relay.should be under the hood.

Posted on Jun 29, 2009

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