The Stovebolt Site Forums The Shop Area General Truck Talk '72 GMC heater/ac diagnostics

Hopefully someone out there has some tips for tracking down a problem with my heater/ac unit. I have a '72 GMC 1/2-ton with a 402 and factory air conditioning. The ac is charged and works. The heater controls are all connected and appear to be operating as they should. The duct work appears to be fine and the duct work to the defrosters was replaced just a couple of years ago. The AC heater valve that mounts under the hood has been replaced. I have a shop manual and have tried to make heads or tails out of the description and the diagrams, but there is no troubleshooting guide. I'm not sure what would be the most likely culprit. I could start replacing actuators, relays, etc., but would rather get an idea as to what most likely is the cause of the problem. It almost seems that there is a door inside the unit that is failing to open or close as it should. The AC was not working for a number of years, but I was able to get that fixed although the AC volume isn't all that good. With all controls off, I get low volume hot air coming from both the right passenger dash vent and the center dash AC vent. This center dash vent gets too hot to touch. I get low to no air through the defroster with the defroster control on. The blower motor is working. Any help would be appreciated. By the way, the truck is currently in a paint shop getting a new paint job and will stay the factory green and white 2-tone. I'll post some pics when it is finished.

I'd take a look and make sure that the fresh air intakes are clear, the evaporator core is clean along with the heater core. Make sure they don't have a bunch of dirt or mouse nests in them. Then check the temperature control door, make sure its working when you move the lever in the dash. It make require taking out the heater/a/c box to see whats going on.