89 Heater control/AC GMCTECH HELP

08-04-2004, 09:22 PM
if nothing works and fuses are good and all hoses are plugged in could the whole control be bad? Fix or replace Bought truck this way How do I check to see if the controls are getting vaccumn?

08-30-2004, 04:10 AM
I assume you mean the interior ventilation system. By moving the vent slider (engine on) , you should hear the slight hiss the vacuum motors make as they de/activate. If no hiss or change in air flow eg dash vents to floor vents, then there is no vacuum feed. The plastic feed line is behind distributor in wiring loom at bulkhead.

If you remove the the dash control unit do so CAREFULLY. It only comes out about 1/2", thereafter, you have to dismantle it. 1/4" AF spanner for the screws.

08-30-2004, 06:51 PM
thanks i got vaccumn but the accuator only move a little im going to try and take it apart and repair

08-30-2004, 11:23 PM
The vacuum motors are not repairable. You can test the motors manually using some vacuum pipe. Wrap some pvc tape around one end of the pipe so you can suck on it without getting black lips. Remove the vac feed to each motor and fit the pipe. Suck on the other end and listen. If the motor partially activates then returns to its start position hissing it's failed. Loss of "suck" as this happens will confirm. If it moves and stays put under the applied vacuum, but doesn't move fully, the vent flap may be jamming, or any adjuster may have come loose. Either way, I think you'll end up having to take the vent box out to do the repairs.

The other possibility is that the vent position selector lever has jumped out of its track. Thus even though you move it, the other end of the lever is not moving much.

Removing the lever assembly needs to be done CAREFULLY - it won't stand much pulling. It will come out from the dash about 1/2" or so. Thereafter you have to dismantle it where it is. Remove the lever knobs and undo the 1/4" screws to release the lever frame from the facia panel. Then the heater cable needs to be released along with the fan speed connector. You should then be able to examine the selector lever mechanism.

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