What controls the heater air vs normal cold air?

Discussion in 'General Chevy & GM Tech Questions' started by bluecow24, Jun 8, 2009.

  1. bluecow24

    bluecow24 New Member

    I am trying to figure out where to start looking. My '95 GMC Suburban will only shut the hot air off when it is cold enough outside to trigger it, my guess is that the heat is making something too hot for it to flip to cool air. Right now it doesn't matter whether the temp control is on red or blue, it is always hot. Also the lower heat will only shut off if you run the controls on face level and flip the speed to on. When it is cold outside you can hear the lower flip shut when you change the location. I am making a guess that the actuator is bad that controls the lower center vents / air distributor. Anyone have any idea though why I cannot get the heat too change to cool air? At this point if the A/C is working it can't get through the hot air. Many thanks!!
     
  2. SCAIR001

    SCAIR001 New Member

    Those have a electric actuator on the heater case that is more than likley gone bad. I belive that one is on top pasenger side
     
  3. bluecow24

    bluecow24 New Member

    Is the top actuator the same type as the other two? I can see one on the lower center and then one towards the center about midway up, but I am having trouble finding the other one unless it is a little smaller. I see a smaller black box with Bosch on it just atop the heater core towards the fan.
     
  4. SCAIR001

    SCAIR001 New Member

    No it looks the same as the other ones. almost has a triangle shape with lika a 5 or 6 pin flat connector
     
  5. daddytech

    daddytech Epic Member 5+ Years 500 Posts

    yeah that has the same set up that my 89 has and it's all electronically controlled, not one of Gm's finest designs by any stretch of the imagination. be sure to post your resolution on here so everyone can benefit from it. hell I may need it myself soon.
     
  6. KirkW

    KirkW Rockstar

    Another possibility is that the rubber gasket around the edge of the "blend" door has rotted away....

    On most (all?) GM vehicles, all the air flows through the A/C evaporator before arriving at a "Blend" door. The Blend door controls how much of the air then flows through the heater core. Max Heat = all the air flows through the heater core. Max Cool means the heater core is bypassed entirely.

    (There are other doors that control inside/outside air, and windshield/face/foot outlets.)

    On the '95 Suburban (and probably other years), all of these doors have a thick rubber edge around the perimeter to help make an airtight seal. Over time this rubber rots and falls apart. On my '95, bits of rubber would occasionally pop out of the floor vents (and like a punchline to an old joke, some of those rubber bits did indeed have part numbers on them).

    When the rubber edge breaks down, the door can no longer seal off the opening. When this happens to the Blend door it means some of the air is always heated. Adjusting the temp has only a small affect on the actual temp of the air coming out the vents.

    As the rubber ages it also gets sticky, especially when warm. This can cause the door to stick in one position or another, and the actuator motor is unable to move the door.

    Unfortunately, the only sure-fire cure is to disassemble the entire HVAC housing and replace the doors. Not a job for the faint-of-heart. At least when the dashboard is removed to get at the HVAC, it's a great time to really clean the base of the windshield :)

    This link has more info (and pictures) - scroll down to my post, the second one in the thread:

    http://www.gmtruckclub.com/forum/showthread.php?t=21319
     

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