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78 GMC Heater Control Issue (New Member)

Moby Vandura

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Moby Vandura New Member

Moby Vandura

Posted
Posted

Greetings all, nice site...
.
First off let me say that I didn't realize that this site seems to be exclusive to GM pickups when I signed up, hope that won't matter...because my vehicle is actually a '78 GMC G25 Vandura 3/4 ton van. Different body for sure, but built out of the same stuff, right? Especially concerning the system in question, which is the vacuum actuated A/C-Heater control...hope there might be some "old timers" like me (sarcasm) who can lend some info / insight to my plight. Here we go...

First, I am a better than average shade tree mechanic, 66 in August and have serviced nearly all of my own vehicles for like 50 years now. Pretty fearless when it comes to attacking problems, with a good bit of intuition when it comes to mechanical systems. The heater control in the van has defaulted to defrost, which I understand is normal when there is no vacuum present. The fan operates properly at all speeds. A/C compressor kicks on / off as it should. I had to replace the heater core a few years ago, so pulling the dash cover off is a "done that". I did of course first check the vacuum reservoir under the hood...pulled the large hose and found that there was indeed pull at the inlet. I checked for vacuum at the outlet, which goes to the heater control in the dash, by simply holding the flame of a Bic lighter close to it and it sucked the flame in pretty strongly. I then pulled off the dash cover (lotsa fun) and started checking all of the hoses, each was properly terminated. I did find one that was pinched flat...cut and spliced it with a piece of chainsaw fuel line ( I have a bit of Macgyver Engineering experience as well)...but no improvement in the situation. 
Next I wrestled out the heater control itself (even way more fun) and looked it over, all seemed correct. The vac lines all attach by means of a gang connector, easy to remove. I went back out to the reservoir and blew through the small line leading to the control unit, not clogged. Inspected the ports on the control unit, no signs of clogging there. There are like four separate vac actuators on the core box / cover. Moved each connecting arm and they all felt the same. I would think that even if one was bad, it wouldn't make the entire system default to defrost. So that is where I hit my "brick wall"...

Reviewing my results...I now am hesitant to proceed. Certainly don't want to buy one of the used control units I have seen on Ebay, go through all the redundant work to install it if that's not the problem...they look so simple, but having never disassembled one before I'm clueless as to what's inside it that could fail. And even though I have verified the presence of vacuum going to the unit, could it be that it's too weak? Could there be an issue with the reservoir internally? If so, how would I measure this and what would be a proper level of vacuum?  I assume this could be done at the vacuum reservoir but lacking the tool and knowledge of specs I am stumped. If anyone can suggest a means of proceeding from here...I would greatly appreciate it. Thanks very much for reading...

  • 1 month later...
GoToGuy Enthusiast

GoToGuy

Posted
Posted

Any luck in your vaccum actuated problem?

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