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Discussion Starter #1
Hey Guys,
Maybe you can help with my heater problem. My truck is a GMC,
1500, 4.3 V6, 1990 standard cab.
The defrost button does not activate it and the on/off switch
doesn't turn off the heater motor. The heater motor works fine
in low, medium & high.I get good heat by using the vent button
to send heat up or down to the floor. Also, the air condition
doesn't respond. Should the heater control unit be replaced or
just the switches.
Thanks for any suggestions. George
 

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Doc here, :pimp:

Here are the controls for the heat and air..





On the heater / Defrost Vents..It appears this model has no Vacuum controlled doors..So the Vent door linkages may have slipped the cables and fallen off in the closed position..(Parts 13, 16, and 17 on the top picture)

It could also be the cable has slipped of the dash controller side. You need to check those.

Electrically on the Air Conditioner (if it is not a Multiplex problem with the heater/air doors from the top drawing..) It could be The Relay (part #4) or the switch, (part #19) Or the Thermal Fuse, (mounted on the Compressor ) that has opened due to low pressure..Check those.

If the Blower motor runs at low speed all the time, The blower switch is defective..and will need replacement.

Doc :pimp:
 

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There are no vacuum controls on this system. All doors are controlled by 3 wire, 5 volt reference motors. Also, these units are common for control head failure, usually indicated by flashing A/C lights, or door position indicator lights, or loss of control of several systems at once. The switches and the control head are the same unit, and are replaced as an assembly.
 
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