Question about 1993 GMC Suburban

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Defrost vent door wont open

This 92 k 1500 I just bought is in excellent shape, with 125,000 and only 65,000 on the motor. Due to the low miles and under usage, the defrost door inside the venting system does not seem to open. I switched the controls from defrost to dash to floor, that worked fine. It switches to defrost but it seems like the air only hisses like some thing didn't open and the air seems blocked, only in the defrost position.

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Sounds like you have a vaccum leak or a blockage in the blend door area of the HVAC system most of the doors are controlled by vacuum if not by vacuum on of the servo motors that control the blend door is not functuning properly. Hopr this is helpfull

Posted on Jan 17, 2009

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Common question on this site. On late model cars, the vents are controlled by blend door motors. Small electric motors open and close doors to direct air flow.
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My heater is blowing cold air, and it wont blow hot air even if the engine is running hot... on vent it blows hot air like it should for half a second then it blows cold again, it wont blow hot air out the...

OK, a vacuum controlled blend door is the problem.


See Figure 1

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Fig. Fig. 1: Plenum door vacuum motors

  1. Disconnect the vacuum hose at the motor.
  2. Remove the 2 motor attaching screws.
  3. Rotate the assembly so that the slot in the bracket is parallel to the tee-shaped end of the door crank arm, and pull the motor and bracket off the crank arm.
  4. Installation is the reverse of removal.
--- Search for other vacuum motors under the dash to see if the vacuum hoses have somehow deteriorated or become detached. --- Try to look up your vehicle on for other vacuum motors under the 'heater' section of the 'chassis electrical' section of the 'repair guide' after selecting your 1996 Ford f250 crew cab vehicle.

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The vacuum motor is not operating right due to a loose vacuum tube, ore electrical connection or the logic is bad in the climate control electronics. The vacuum motor controls the door angle which controls whether the a/c blows out of the front vents or the defrost vents.

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