2005 GMC Sierra 2500 6.0 Gas 4x4
ONLY blows hot.
Have replaced blend doors as a cheap and easy parts cannon type fix...no avail

Snap on scanner shows that the HVAC controls on the dash are being seen properly.
Mode/temp/recirc/etc all being properly recognized

Check the blend door positions and can see they aren't being commanded to move at all. Steps commanded "0" for driver and passenger side temp blend doors.
Mode door is commanded and works. fine

Any advice on diagnosing why the blend doors aren't being commanded to move? No matter where temp controls are set, both doors are just commanded to 0 steps and they do not move

 

Have you tried the relearn of the motors?

 

yes
tried relearn with the scanner

and also have tried the ol' "pull a fuse" method

no luck either way

they will move when doing the initial relearn...but never again. It's just not being commanded to move.
either side.

Not sure if BCM issue or HVAC module issue etc

 

Manual or auto A/C Controller? I had to replace my manual controller on my '04 1500 for what sounds like the same thing. I also replaced the servo motor around the same time... I can't remember what fixed the problem. The gears in the servos are just plastic and they brake. It could be the servo thinks it is already at the end and won't move any more.

 

manual AC controls

I have swapped the controls with known good controls out of another 05...no change
This was the expected result because with the scanner, I can see all the controls are doing what they're supposed to.
If I move the temp slider...that is recognized by the scanner.
If I move the mode door, or hit the recirc button, etc that is all properly recognized.

The blend door has been changed, and for 1 single cycle during a recalibration, I can see it move the blend door. So no broken gears or worn out motors etc. They move

They just aren't being COMMANDED to move when I watch the "blend door position" data on the scanner.

When I watch another 05 half ton with same AC controls...when you move the temp slider, I can watch on the scanner that the step counts on the blend doors are being commanded to change, and then the actual counted steps changes to match the commanded...and the door moves to the correct position

I just need to know how to diag WHY the things aren't being commanded so I can fix it!
Thanks

 

Just move to Alaska, put BLIZZAK snow tires on it. Problem solved.

 

I'm in iowa
so having heat this time of year is OK
But come July I won't even drive the truck if all I have is heat
Anybody ever seen this before and have some insight?

 

I would think It's the control module, But if you have tried that and no change then I guess it has something to do with the BCM if the signal is relayed through there.