2008 GMC Truck mode door

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

4 Replies

JACOBANDNICKOLAS

MECHANIC
109,885 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

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.