I have a problem with the heat being stuck on all the time on my 1990 GMC Suburban. It doesn't matter where I set the temperature control or the mode selector. Several things seemed to fail all at once: Heat stuck full on A/C compressor will not come on HVAC controls don't seem to affect anything. I've replaced the mode switch and the vacuum switch, checked the vacuum lines and control cable, and replaced the vacuum source line up to the manifold. I've been seeing references to the Air Blend Door actuator in online searches, but my Chilton manual doesn't mention anything about it. I'd appreciate any direction folks could offer on this - it's too hot to have the heater on all the time!