Cabin Temperature Control

One more question. About 3 weeks ago, my 99 1500 5.9L must have been inhabited by a poltergeist. When running the interior heat on both top and bottom, after a period of time, the air will begin blowing out of the defrost vents instead. It will stay this way for several minutes and then change back to the settings that are indicated on the controls. If I change the setting to Off and then back, it works properly.

Hmmm...

the heat and ac controls are catuated by vacume. I remember seeing something about a TSB (Technical Service Bulleten) that had to do with upgrading the check valve in the vac line that goes from the engine to the controls, you might check into that.

also, check the vac lines, if one of them is dammaged, the check valve won't help any.

Thanks for the response. I've done some more looking on this and have found some places where they suggest the panel heat actuator under the dash. I drove about 80 miles on the interstate today and noticed that this only occurs while using the cruise control and usually while going up a long gentle incline. Once I crested the hill, it reverted back to normal. How would I go about replacing this and is this a good approach?

Before you replace, if you haven't done so already, the problem is a vacuum leak. I just fixed mine that was doing the same thing. It actually would happen on cruise because it uses vac, and when accellerating, drawing more vac. You may want to do some searching for a leak or line disconnected. Mine was disconnected at the cross member in front of the tranny. If the actuater ends up being the problem, I can tell you how to do it, the dash has to come out, but look for a vac leak first.

Thanks for the response, Tony. I'll do some investigating and see what I come up with. I'm not much at vaccuum issues, so we'll see.