Heater Control Valve
 

My '72 GMC has what appears to be a replacement heater control valve in the engine compartment. However, the vacuum fitting is not connected to anything, and I can't find any information on this in the service manual. Anyone know what it connects to?

Re: Heater Control Valve
 

Should be a small vacuum hose coming through the fire wall to a "T" then to the vacuum valve, passenger side cowl and the heater valve.

Re: Heater Control Valve
 

Thanks for the reply. Unfortunately the previous owner or owners did some fairly sloppy repairs. Lots of wiring and vacuum line errors. In this case, there's no vacuum line in the engine compartment that is unused. The reservoir is connected OK and there's a line to the outside air valve that works. I suspect the heater valve T's into something under the dash, as you say, just need to know what.

Re: Heater Control Valve
 

There should be a tee in that vacuum hose that goes to the air valve. One leg of that T will go to the heater valve.I can't figure out how to send a pic on this phone so I will send you a picture later
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Re: Heater Control Valve
 

Here's a pic. Also check vacuum. You should have 15+lbs.

Re: Heater Control Valve
 

This helped me:
https://farm1.staticflickr.com/304/2...0e5bf27f_z.jpg

Wow, that's great! Just what I needed to straighten it out. Can I ask where that diagram comes from?
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Re: Heater Control Valve
 

Note, the vacuum reservoir failed on my truck. I had too little vacuum to close the heater valve.