What does the vacuum heater control valve do ?

[garyfish340]

I have a 1975 Plymouth Duster that had a slant 6, automatic, w/air. In the photo is a vacuum actuated heater control valve that seems in good condition. My question is, what exactly does it do ? I'm fairly certain that the gen III hemi ( without air ) I've installed doesn't have a bypass hose ( so to speak), unless it's internal on the water pump/timing chain/accessory cover. My 1971 Cuda had a bypass valve that was cable operated, and had a tube that crossed over to the return hose. This is just a single hose with a vacuum valve, and the vacuum system. Also the vents inside the car hang from the bottom of the dash, and the cowl box is only on the passenger side end of the cowl. I'm guessing the vacuum hoses that go from the heater control valve, through the firewall probably operate the vent doors under the dash like a GM car. (When my dad picked up his 1973 Duster new, it had a totally different system than this one hanging below the dash, there were vents under the part of the dash by the headlight switch, and under the glove box, by the door jambs, the rest may have been the same. but it's all unfamiliar to me.) I need heat, and defrost. Any answers ? Thanx...

 

[TylerW]

The heater control valve either permits or restricts coolant flow through the heater core, depending on whether vacuum is applied to it. It is a normally OPEN valve which closes when vacuum is applied.

When the valve is open, coolant circulates through the heater core and you have heat. When it closes, take a guess..lol.

The heater control valve is in the FEED line to the heater core, so if your question that wasn't asked is, how to hook it to the Hemi then identify the connection the Hemi used to feed it's OEM vehicle heater and use that. The return goes to the other heater hose connection that is not on the water pump.

Yes, the vents hang off the dash.

Your vacuum circuit to get the necessary vacuum signal to the heat and air system so it function is this:

See the vacuum reservoir which is the black can on the fender? First of all you need to keep that or the vents will switch when you climb hills.

That plastic can has two ports on the back. One has a line that should be connected to a manifold vacuum port. The other goes through the firewall into the car. I can actually see it from here. There's actually two lines there..the other runs the heater control valve.

It's not hard to hook up. Hope this helps.