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Roy F.
Roy F., A.S.E. Certified Master Technician
Category: GM
Satisfied Customers: 132
Experience:  General Motors World Class Certified, A.S.E. Master Certified, 23 yrs. Exp. in the auto repair field
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1996 GMC: heater..AC...defrost postion the ac compressor runs..airflow

Customer Question

I have a 1996 GMC Jimmy that is haveing troubles with the heater/defrost/AC. when the controls are in the defrost postion the AC compressor runs as it would in the A C postion. also there is no airflow from the defroster opening for the windsheild unless i turn the temp control switch from the hot postion to the cold postion.
Submitted: 7 years ago.
Category: GM
Expert:  Gary replied 7 years ago.
Hi, the a/c compressor is supposed to run when in defrost mode,(even full cold), to dry the cabin air, to prevent window fogging.It sounds like a defrost mode door is not working correctly, you may have a bad actuator, or heater control head.This is providing the other modes work. I would check the actuator for the defrost door, for vacuum when engaged. you have to chase the actuators, and doors. something may be misaligned, or binding as well. Good luck...Gary
Customer: replied 7 years ago.

Hi Gary thanks for responding, I know that the a/c compressor cycles on and off even in the heat mode. this is not the case it runs as if it is in full a/c mode. as far as the doors working it seems as if its working backwards. In full defrost mode no airflow to the windshield as i turn the temp control toward cold i get more airflow to the windshield.



Expert:  Gary replied 7 years ago.
Hi, if this is the case, I would check to make sure you have good vacuum to the selector switch, If so there is an issue with the switch, or the actuators. If you are sure all has been reassembled correctly, when you did the heater core, then I would chase a vacuum, or control issue. Good luck...Gary
Customer: replied 7 years ago.
Hi Gary, this problem started about 18 months ago long before i did the heater core. My main concern is why the defrost wont blow hot air only cold. Is there a sensor in this system that would cause this problem? as far as vacuums supply all the other doors open and close as they should just not the defrost one
Expert:  Gary replied 7 years ago.
<p>The defrost door is controlled by a vacuum actuator. If the actuator is inop, or the door is stuck, this may cause the issue. I have also seen item that have fallen into the vents, that cause this issue. If you have good vacuum to the defrost actuator, and the door has full travel, there must be something stuck insude keeping the door from shutting. The actuator, and door are the only 2 Items that control the air flow.Good luck...Gary</p>
Customer: replied 7 years ago.

this still dosent explain why I get no air flow from the defroster when the temp switch is in the hot postion I only get air flow when i move the temp control to the cold postion.


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