2008 GMC Truck mode door

Tiny
2008GMC
  • MEMBER
  • 2008 GMC TRUCK
  • 5.3L
  • V8
  • 4WD
  • AUTOMATIC
  • 71,000 MILES
When I start the truck, the a/c blows good from the dash vents. And I can change it to the floor or to the top vents, etc. But after a while, the doors close, the air is still cold and the fan control still works. But the front dash vents close. There is no time on this, sometimes it works for an hour or better, sometimes the doors close before I pull out of the parking lot. Also, when this happens, I cant change the mode anymore, to floor or anything else. I still have cold air and fan control. After looking at several videos, I believe it is either the mode door actuator, or the fan resistor. I cant seem to locate the mode door actuator
Monday, October 13th, 2014 AT 2:09 PM

4 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,700 POSTS
The blower motor resister has nothing to do with this. Tell me, are the vent doors actuated by engine vacuum? If so, have you checked for leaks?
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Monday, October 13th, 2014 AT 5:44 PM
Tiny
2008GMC
  • MEMBER
  • 3 POSTS
Thank you very much for the reply. To answer your question, no I have not checked the vacuum lines. I will be doing that first thing when I return home. I had suspected that the vacuum lines may be the culprit, But I work offshore and have only been looking for info onlinr hoping to isolate the possible problem. Thanks again.
The reason I had the blower resistor in the mix, is that when I pull over and re start the truck the vents open back up. Upon reflection though, that still wouldnt be the resistor.
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Tuesday, October 14th, 2014 AT 5:15 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,700 POSTS
Let me know what you find.
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Tuesday, October 14th, 2014 AT 4:21 PM
Tiny
2008GMC
  • MEMBER
  • 3 POSTS
I took the truck to a shop, They re calibrated the mode system with a hand held computer. It seems to have worked. No vacuum system is involved I guess. Thanks again for your input.
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Thursday, October 23rd, 2014 AT 9:12 AM

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