Mister

Hello,

The cold weather must be upon us when we have 2 heater-related posts in as many days.

Sometime over the last 6 months, the heat in my car (1966 Oldsmobile Starfire) stopped working. My Dad and I did a little poking around, and found that there was no vacuum signal getting to the heater control valve. Probably a vacuum leak somewhere under the dash. So we jury-rigged the thing by running a vacuum hose straight from the vacuum tank to the heater control valve. So now the heat works, which is nice.

However it doesn't work in the way I expected. I thought that making the H.C.V. always on would make the air always blow hot, regardless of the console temperature selector. But on my drive back home today, I found that selecting "cool" blows cold air, as well as "warm" blowing hot.

Apparently the temperature selector works independently from the heater control valve. So my questions are these:
1. What does the heater control valve do?
2. What controls it, if not the temperature selector?
3. Am I causing any harm by keeping the valve open 24/7?

Obviously, I value a warm tush, but not as much as a working automobile. So if I should be rethinking my winter driving strategy, somebody please let me know.

Mister

joe_padavano

The heater control valve is designed to TURN OFF the flow of hot water when the A/C is on MAX. Running full time vacuum to the valve keeps it off, meaning you're getting no hot water to the heater core. Loss of vacuum (ie, disconnecting the vac hose) will allow the valve to pass hot water to the heater core. If you're still not getting heat, either the valve is stuck or the heater core is blocked.

kevinkpk

I shouldn't (perhaps) question also, as Joe is a plethera of knowledge, but along with alot of things that didn't work on my 70 Delta was the heater. I plugged the vacuum line in from it, and viola it worked ( I had suspected something simple). Thats not to say I didn't have to replace the heater core as it was leaking.

There are, least on my car, vacuum/contol lines that operate both a/c, and heat. I also had to replace the vacuum actuator valve that operates the flapper/door. That thing had broken into two pieces. There (refering my 70) are red, and green striped vacuum hoses, and it is important to know what goes where.

I have the manual control assy, the heater is controlled (temp wise) by a sliding lever that mechanically operates a "temperature door".

J-(Chicago)

I had no heat, replaced the H.C. valve (and H.C. valve only) and have heat again.
Don't know if this helps you guys' discrepancy at all, cuz mine was on an 85.

68conv455

Very interesting discussion since I noticed today that the 455 I'm preparing for my 68 cutlass has the subject heater control valve (its a '71 455 from a delta with AC). I just yanked the inline 6 today and it is absent of any valve. Both the feed and return line go to the front of the engine directly from the firewall. My cutlass came from the factory without AC. Does the discussion above mean I can just install an elbow in place of the valve to clean up the intake? Sorry 'Mister' in advance for cluttering up your thread.

Mister

Thanks for the replies. Generally, my experience was consistent with Oldsfan's description. There seems to be some disagreement as to the actual operation of the H.C.V., though, so I decided to *gasp* consult the 1966 Oldsmobile Service Manual.

Here's what I gathered:

Ignoring the Toronados, there were 5 heating/cooling options for Oldsmobiles in 1966:
1. No heater (manually operated cowl doors only)
2. Heater with no A.C.
3. Heater with vacuum controlled A.C. (88-98-Starfire only)
4. Heater with cable controlled A.C. (F-85-Cutlass only)
5. Automatic temperature control (Comfortron) (88-98-Starfire only)

Oldsmobile calls the part a "water valve", and it only comes with certain configurations. Obviously, if you bought Option #1, you have no heater, and no heater valve.

Interestingly, Option #2 also has no water valve. The valve is simply not mentioned in those sections of the manual, nor does it appear in the vacuum routing diagrams. Vacuum is routed from the vacuum tank to the console, but never again passes through to the engine side of the firewall. So on those cars, coolant must always circulate through the heater core, but air from the blower motor bypasses the heater core when the "temperature door" is at the "cool" setting.

Options #3 and #4 both have a water valve, and they work similar to each other, as far as the valve is concerned. The temperature selector controls both the aforementioned "temperature door" and the water valve. My car has option #3, and that explains why, even when the water valve was set to always be open (hot), I could still feel cold air when the temperature selector was set to "cool" -- the air from the blower motor was bypassing the heater. I suppose some air was seeping past the heater core, and that may have increased the temperature of the "cool" air by a few degrees, but it is good enough for me.

Option #5 also has a water valve, but in addition to the normal vacuum connection, it has an additional 2 vacuum lines at the other end of it. The Service Manual labels these other connections "THERMOSTATIC VACUUM VALVE (OPENS AT 120`)" I don't know what that means, and probably never will. Comfortron makes my brain hurt.

So even though the water control valve is only present in cars with A.C., from what I read, it really acts independently of the A.C. Also, the manual mentions that the valve is not present on cars with L-6 engines. Since you could buy a car with a 6-cylinder engine and air conditioning, this shows that the valve is not required for A.C. There is also a single mention that the water valve is not present on the 400 ci engine. This is only a single instance though -- the rest of the chapter treats all V-8s the same.

So, to answer my own previously posted questions,
On cars that have it, it controls the flow of engine coolant through the heater core.
The temperature selector DOES control the heater control valve, but it also bypasses air around the heater core, so the H.C.V. plays only a secondary part in determining air temperature
Nope. Many H/C options didn't even have the valve at all, so they were essentially "open" 24/7 too.

And to respond to 68conv455's question, your non-A.C. equiped Cutlass wouldn't have had a water valve anyway, so it is safe to not use it (assuming the 1966 Service Manual also holds true for 1968 models).

J-(Chicago)

*Applause*
Just as I suspected.
Stupid cars

joe_padavano

Excellent. My 66 CSM is in a box in the barn and I was too lazy to go get it. I know for a fact that since the mid-1970s, the heater control valve is a normally open design that closes when vacuum is applied. None of my earlier cars have A/C and thus have no heater control valves. The heater-only systems are cable operated and have no vacuum requirements. The A/C systems use vacuum to operate flapper doors inside the heater box to control the flow of air through the heater core or A/C evaporator.

It's very simple to test what configuration is on a given car. Simply connect a vacuum gauge to the vac hose that normally attaches to the valve. If you get vacuum when the A/C control is on heat, the valve should be normally closed. If there is vac on MAX A/C, the valve is normally open.

Oldsguy

Good post guys, thanks for the research and sharing. I now know more than I did 10 minutes ago, and, it is about Oldsmobiles <smiles>

MrRocket

I've been asking this very question and I'm glad I found this post! My HCV went out and I want to put a restricted nipple in place of my HCV. My car came with A/C but I won't be hooking it back up for a long time. Wanting to avoid the HCV because of the cost mainly, 66 Toro. HCV is 80.00 dollars from O'reilly auto. Thanks for the information!

Just saw how old this post was...wow!

BlackGold

Just wanted to comment that the function of the heater control valve may have varied over the years. For example, on my 1970 442 I don't know off-hand if the MAX A/C setting closes the valve, but I do know that the temperature lever closes the valve when slid to the far left (COLD). You can feel the extra resistance on the lever and hear the vacuum engage and release. This operation is also described in the Chassis Service Manual.

The benefit, of course, is that you can apply heat while the A/C or defroster is on, but when you slide the lever all the way to the left you are guaranteed to not be mixing any heat with your cold.