I've searched thread after thread, looking for a problem similar to mine, with no luck.

When the outlet selector is set to the dash vents - it will stay hot for a short while, then go cold and stay there. Sometimes it will stay hot for hours, but most of the time, it will go cold moments after setting it to the dash vents.

When the selector is set to the floor vents, it works normally, allowing adjustments through the entire blend range, without fluctuating - hot is hot, cold is cold, warm is warm.

I will double check tonight - but I'm pretty sure that once it goes cold in the dash position, it won't return to the hot side in any position, until I turn the truck off for several minutes.


random photo off the internet, of the same controls - I added the text

 

can you get hot and cold promptly on demand from the floor vents? I dunno if its possible but the blend door might be 'free wheeling' back and forth depending on where the air is headed

 

Update - no blend problems with the AC on, and on the chilly nights, the problem only surfaced, if the controls were on any of the AC settings, during and/or prior to starting the truck. If I put it on the setting circled in red above, before starting the truck, it would work fine, as long as I did not cycle through any of the AC settings, or switch it to the off position. As long as I stayed to the right of the AC settings, the blend door works without a hitch.

 

That third **** (the one that selects the various air outlet locations) has a very complex electromechanical switch behind it. I would suspect that there's some kind of electrical problem with that switch. You might see if you can locate a cheap HVAC head unit at a junk yard for testing purposes. Swap it out with the one that's currently installed and see if the problem goes away.

 

The thought had crossed my mind, to hack a junkyard piece and make a schematic. Tracing the whole circuit and creating valid test points being the goal. It's gonna be cold soon, so I should probably get something figured out... lol

 

Update: No problems all through the summer - in fact the blend seemed to default to the hot side at start up, blowing hot air down on my foot. Every once in a while, I would blast the A/C for a bit and it would be fine.

Now that it's getting cold out, it defaults to cold and will not budge until I've driven around for 15 to 30 minutes. I'm going to unplug the motor tonight, with it hot, and see what it does the next time I drive it.

Once it starts putting out heat, it will run you out - so I doubt it's the coolant.

A friend of mines Bravada was the same way, every season, I would set (rig) the door, either open or closed, for the duration of the season. I had to actually wire hers to the hot side, for it to stay there, as it would move to the cold side while driving.

Where we live, it's either hot or cold, with no in between - it goes straight from summer to winter and back around here.

EDIT: Well, that was a bust... short trip to the store, 40 degrees out, and I was cozy within a mile of leaving - same on the way back (there for about an hour). Dumped the contents of my glove box on the floor, and couldn't even get to the blend motor.