hey guys, '99 GMC Jimmy SLT. I have the automatic climate control system, and the other day I was messing around with the truck trying to figure out why the heat would only blow out the front dash vents. It wouldnt switch over to defrost or any other vents. It always blows out hot air as well.

Anyways, I found a stray wire just hanging, and i connected it to the "bulb" looking thing that it was supposed to go to, and confirmed that the metal bar wasnt moving because of this vacuum line. I have defrost now, and it moves when i switch to floor, and everything else but the heat still blows out the front vents no matter what setting. I dont really care about that, its just whatever.

Now my question, when I roll the temperature dial all the way to cold, it still blows out hot air. My understanding was that the blend door actuator controls this? But I saw the linkage physically move and confirmed it was working...Or am I looking at the wrong thing? I'm a little new to this HVAC stuff and never worked on it before, so a little help would be great. Pictures/diagrams are a plus! I apologize if a thread like this has already been started

 

I had this issue as well (98 with climate control)... It was the blend door actuator... The plastic gears inside the old actuator were broken... I bet yours are as well... google 98 blend door actuator and check out what they look like... go get a new one from parts store... locate the old one on the drivers side under the dash and swap em out... its held in by like 3 screws... the back of the actuator is "slotted" so you have to pay a little attention to line it up when installing it... That will fix your problem..

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so what is the thing called that switches the vents, you know, from defrost, to floor, to front vents, etc....because that thing is moving and I thought that was the blend door. Apparently its working. Am I looking at a totally different part?