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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Posted on Jan 02, 2017


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2002 Silverado wont change from defrost to floor

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Vacuum control motors are used to position these airflow control doors.
SECTION 412-01: Air Distribution and Filtering 2000 Expedition/Navigator Workshop Manual DESCRIPTION AND OPERATION Procedure revision date: 06/17/1999 Air Distribution There are two sources of air available to the air distribution system:
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expedition air flow hvac system-xvcmtr1jbqa4kfrdszeh0lpe-3-0.gif Item Part Number Description 1 18A318 Vacuum control motor - panel/defrost door 2 18A478 Panel/defrost door (full vacuum position) 3 - Defrost airflow 4 - Side window demister airflow 5 18B545 Temperature blend door (full heat position) 6 19860 A/C evaporator core 7 19A813 Air inlet duct door (full vacuum position) 8 - Outside air inlet 9 18A318 Vacuum control motor - air inlet duct door 10 - Recirculated air inlet 11 19805 Blower motor 12 - Vacuum to emission control 13 - Vacuum from the engine intake manifold 14 19A563 A/C vacuum check valve 15 19A566 A/C vacuum reservoir tank and bracket 16 19B888 A/C-heater function selector switch 17 18476 Heater core 18 18A559 Floor/panel door (full vacuum position) 19 - Floor airflow 20 18A318 Vacuum control motor - floor/panel door 21 - Panel vent airflow
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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.
Chances are you have a blend door that is stuck or a failed motor.
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Someone will need to look behind the dash to see if all of the doors will move on demand.

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These trucks use a small electric motor to control the blend doors. You may have a door that is sticking or a motor that is binding up.
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96 dodge 360 4x4 replaced motor 181000 miles because craked head ran fine other than that. now with the new motor(97, 117000 miles) idels good running down the road cuts out and some times dies. the ac...

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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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