Question about 1991 GMC Sierra
Ac light on heater controls is flashing on and off
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Posted on Jan 02, 2017
this will happen if the system senses low charge. This doesnt mean that is the problem the low pressure switch or cycling switch on the accumulator/dryer on passenger side firewall will also keep in from coming on. this can be checked by jumping the system there.If you jump it and it comes on and air is cold on both sides of cab this could definately be the problem.
Posted on Jun 29, 2009
The follwing is a cut and paste from a service bulletin:
Bulletin No.: 461202
Date: May, 1994
A/C CONTROL HEAD FLASHES OR ERRATIC OPERATION OF A/C CONTROLS (POLISH
1988-94 CHEVROLET AND GMC TRUCK C/K MODELS
1990-94 CHEVROLET AND GMC TRUCK TOPKICK/KODIAK MODELS WITH C60 OR C69 AIR
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
Posted on Jun 30, 2009
your heat/cool and vent selection options are controlled by engine vacuum through small clear tubes attached to the rear of your climate control interface. Other vehicles have small steel pull wires that operate the desired changes. You might try removing the trim and any fasteners affiliated with the interface/console. GENTLY pull outward and look to see if any of these tubes are pulled off or cut/damaged. Did anyone recently install a new stereo in the vehicle? If so, the vacuum tubing and or pull wires that change your outlets could have been damaged when the stereo unit was pushed into place. Hope this helps
Posted on Sep 09, 2009
Testimonial: "Very good information. THANKS!"
Defrost is the default position. Is the control a autyomatic temperture control unit? ( you set the temp and select automatic) if it is the system is completely electronic. You push the buttons and the control still says defrost or can you select something else and the control shows it but it just keeps coming out the defrost? If the control changes and the air comes out the defrost anyway and the cheack engine light isn't on, then the problem can be blown fuses or an air door actoater not working. Does the dash emit a clicking sound? if it does with the covers off the bottom of the dash reach around touching the different pieces till you touch the idem that is clicking. this is an acutor motor, or blend door motor. they have gears in them that wear out and break the motors aren't repairable and you just replace the ofending motor. the door that is moves can be stuck. and may have to be freed up with blaster, WD40, or other rust buster till it moves easy. maybe after getting the door to move smoothly you can get the old motor to move the door wiithout having to install a new one.
Posted on Jan 06, 2010
Well this will tell you that the door that directs the air flow in not moving fully into position.Either the motor that moves the door is not working correctly or there is something obstructing the door.We had mice build a nest in the air ducts and would only do heat any other selection would make the arrow flash until we cleaned it out.Well hope this helped.
Posted on Mar 13, 2010
Testimonial: "Hey thanks! It wasn't mice but the spring had popped out and desengaged the arm from the servo motor, put it back together and she's fixed. Thanks!!!"
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