Happy Sunday Folks,

Can anyone tell me how the heater valve is suppose to work on a '70 cutlass supreme (with a/c). Does the valve open WITH or WITHOUT vacuum? I can simply plug the vac line and see if it opens but I'd rather know for sure than just assume.

I'm about to embark on a "road trip" across town for a small coffee/cars meet this morning and remembered I don't get heat in the cab. The blower works fine just no heat so it's either a clogged heater core or the heater control valve isn't operating. The thermostat is definately working as the rad hoses get HOT HOT HOT.

I just picked up this car three weeks ago and well life gets in the way sometimes. I just haven't had the time to fully diagnose the situation. I spent the last half-hour searching to no avail.

Thanks for any input and sorry for what I'm going to assume is a very common question.

 

Heres a previous thread:
https://classicoldsmobile.com/forums...t-cutlass.html

 

On the 1972-earlier A-body cars the heater valve is normally closed and opens when vacuum is applied. Starting with the redesigned 1973 cars, this was reversed, with the valve normally open and closing when vacuum is applied. Be sure the correct valve is installed.

Note also that these model year breaks only apply to the A-body cars. Full size cars switched with the redesigned 1971 cars.

 

Joe, we had this conversation a while back and I thought there was a difference between my 70 and 71 Cutlass on how the valves operated. It's cool enough now so I could go out to the garage and verify how my car operates.

I just checked my 1971 Cutlass Supreme by connecting a vacuum gauge to the heater control valve vacuum hose in the engine compartment.

Slide control set to COOL = vacuum
Slide control set to WARM - no vacuum

The replacement heater valve that I have in my hand right now is open with no vacuum and closes when vacuum is applied. Part number is H25-124.

It's been some years now, but I think I remember this was the opposite function on my 1970 Supreme.


edit: I looked up the Murray H25-124 heater valve and it crosses over to Four Seasons 74602. Rock Auto's catalog shows 74602 for a 1971 Cutlass and Four Seasons 74601 for a 1970 Cutlass.
The threaded sections are different between the two as well, with the 74601 having a smaller diameter than the 74602. I am guessing that this correlates to the difference in vacuum operation.

 

Stop relying on faulty memory or incorrect vendor catalogs and check the CSMs. As I wrote, the 1972-earlier A-body and 1970-early full size use a normally closed valve that opens when vacuum is applied. In each case, when the model line went through a major revision (1971 for the full size and 1973 for the A-body), the valves were switched to normally open. This was done so that the system was "fail safe" - you would still have heat even if the valve failed to operate. Same thing for the HVAC flapper controls - the default position without vacuum is defrost and heat. The changes to thread size are unrelated to whether the valve is NO or NC. Keep in mind that the thread size change also corresponds to the change in water pump inlet diameter. More likely there was a wholesale change to cooling system flow rates to match the hotter-running engines needed to meet emissions requirements.

 

Just so there's no question on this, here's a figure from the 1972 CSM, out of the A-body air conditioner section. Note the statement in the lower RH corner. Vacuum is applied to the heater control valve "when lever is moved from cool to warm". The valve is normally closed and opens when vacuum is applied, same as every prior year A-body heater control valve.

 

Joe, I am not relying on faulty memory for how my car operates. As I stated above, I just checked my 1971 Cutlass and got these results:

Slide control set to COOL = vacuum
Slide control set to WARM - no vacuum

 

I was referring to your memory of your 1970, which operated EXACTLY the same way. Again, 1972-earlier A-body, normally closed, opens with vacuum applied. 1973-later A-body, normally open, closes when vacuum applied.

This page is from the 1968 CSM, which also shows that vacuum is applied to the water valve in the warm positions.



This page is from the 1970 CSM. Again, same thing.

 

I can't dispute what is printed, but that is not how my car operates.

As long as the vacuum control signals match the heater valve operation, which mine do, it will work. I will get heat on the WARM setting and no heat on the COOL setting.

 

Thanks all for the input. Very good information.

My car does apply vacuum in the heat position and none in the cool position so the valve must be operated by vacuum. That being said regardless of the position the heater hoses are not getting hot. That leads me to a clogged heater core or failed valve.

Thanks again!