Question2003 GMC Sierra 2500 HD. Cold air blows from the defrost vents only. Placed in a new controller in dash and had same problems. Also when switching thru selection on the controller (floor, dash, defrost.. vent settings) the engine begins to idle roughly for about 10 to 15 secs. Any ideas? Thanks.
AnswerHi
Two common items I have run into that COULD be your problem
First would be the mode door actuator and cam assembly.
It is located on the drivers side of the HVAC above the gas pedal.
It is fairly diffucult to replace.
The other common problem I have run into is on the passenger side. If you remove the dash end close out panel
(the panel up against the passenger side door). Inside there there is about 6 electrical connectors.
You need to find the (12) wire brown colored one (C4), the 6 wire brown (C5) and the 6 wire gray (C8). All the terminals need to be checked for backed out terminals, bent pins, loose or poor terminal tension.
Also if you open the glove box, to the left you'll see a small wire harness taped, going down.
That wire MAY rub against the metal dash frame and be shorting out. It too can cause your problem.
IF POSSIBLE, I would take it to a good shop your familiar with and have them scan the HVAC system for Trouble codes.
If any of these codes are stored (B0229 B0263 B0268 B0408 B0414 B3770) You may have either of the 2 wiring issues.
As far as the engine running rough, I always hate to use the word coincidence, but as the HVAC system on these vehicles is 100% electrical, I do not see how the mode door could create a drivability problem. Not saying you do not have an idle problem, I think it may just be happening or happened at that particular time. Again.... It COULD make an issue, I just do not see how or where, and I have never seen it.
I hope all this helps in some way.
Just advice from experience.
Good Luck.
Pawl
