1974 SB Heater Control Valve (or lack thereof)

[Paul L.]

I was doing some work last weekend on the engine compartment of my 1974 350 in anticipation of getting (within a reasonable cost) the A/C working again. My local GM dealer has a grey-haired fellow who can do a diagnosis on this stock old R-12 system as an inexpensive start. (If taken further I will need an R-134 conversion according to Canadian law.) I asked a few questions on CF but the answers were vague. So here I am.

I disconnected the electrics a couple of years ago (but left intact taped to a wiring harness) and removed the drive belt. The belt is back on and the electrics can be done in five minutes.

I could not find the heater control valve. I have worked on C3s before and it is normally in the 5/8" hose near the coolant overflow tank and vacuum operated. Hmmm...a previous owner must have removed it. I checked the various catalogues and only Paragon lists one (#12466 for 1969-74). The others have no listings for 1972-74. That led me to the AIM and lo-and-behold the 1974 350 with C60 did NOT have a heater control valve!

This raises a host of questions. Why did GM design a system where a heater core with hot coolant flowing through is placed next to an A/C evaporator? This will surely cut down on cooling efficiency. I have installed a ball valve to cut the flow manually to the core but my Q. is: Why was the heater valve omitted 1972-74?

[Terry M.]

Re: 1974 SB Heater Control Valve (or lack thereof)

My guess, and it is strictly that, $$$$.
The A6 has enough capicity to cool several Corvette interiors, if one doesn't consider air infiltration. When our Corvettes were new there was substantially less air infiltration than today. Functioning seals and weatherstrips make a lot of difference.

[Edward J.]

Re: 1974 SB Heater Control Valve (or lack thereof)

Paul, GM used the temp door to close of air flow through the heater core.and vaccum to open another door to allow air flow through the evaporator core.

[Larry M.]

Re: 1974 SB Heater Control Valve (or lack thereof)

My guess, and it is strictly that, $$$$.
The A6 has enough capicity to cool several Corvette interiors, if one doesn't consider air infiltration. When our Corvettes were new there was substantially less air infiltration than today. Functioning seals and weatherstrips make a lot of difference.

This is what I remember also from previous research on early Corvette AC.

Larry

[Paul L.]

Re: 1974 SB Heater Control Valve (or lack thereof)

Paul, GM used the temp door to close of air flow through the heater core.and vaccum to open another door to allow air flow through the evaporator core.

I am familiar with that (see pic, wish I wasn't!). I guess that is the way it was designed. Just seems strange that there was a valve (for A/C cars) prior to 1972 and post 1974 for all. I suspect there were problems with radiant heat in the cabin from the core and the heater valve cut back on that.

[Edward J.]

Re: 1974 SB Heater Control Valve (or lack thereof)

Paul , I am sure there is a little heat that does by-pass around the temp door,and would cut don on the cooling, And if converting to the car to r-134 the cooling is not quite as good as r-12, you may want to install a manuel shut off valve in-line ,to help with the A/C cooling in the summer.

[Paul L.]

Re: 1974 SB Heater Control Valve (or lack thereof)

Paul , I am sure there is a little heat that does by-pass around the temp door,and would cut don on the cooling, And if converting to the car to r-134 the cooling is not quite as good as r-12, you may want to install a manuel shut off valve in-line ,to help with the A/C cooling in the summer.

Edward,

(My car is not NCRS-material so please excuse the incorrectness.)

I have already done so and it works very well. The "far" side of that valve is almost cool to the touch when the engine is up to operating temperature.

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