'79 with A/C

Problem: overly hot air from the lower (heater) ducts at any climate control position.

The factory service manual is almost hopelessly generic in the A/C section. Beyond that it has no description of function and almost ridiculously the only troubleshooting regarding heat is "not enough!"

Question 1: What is the purpose and function of the "Water Shut-Off Switch"? When I study the vacuum diagram it [seems] that it is involved only in the MAX A/C setting when both the outside air and water shut-off valves close. But what does the plunger on the water shut-off switch do?

Question 2: What happens when I move the temperature control? Obviously I know that it operates some sort of damper that progressively bypasses the heater core. Diagram? Are there internal seals that are expected to fail after 30+ years? Is there some sort of adjustment beyond the "turnbuckle" (for lack of a better term) that allows the temperature control to move fully in its track?

 

I'll answer this one at least.
When the temp control is all the way to the left ON cold, it activates the vac SWITCH on the heater box behind the glove box which closes the vac VALVE on the heater hose.
If you're not getting vacuum to the switch the valve will stay open letting hot coolant flow through the heater core causing hot air in the vents.
Often times vac switches and control panels losing vacuum is due to lack of lubricant on the rubber seals.

I had a problem with the vac switch on my black79 that caused COLD air from the correctly operating AC to go warm after the car warmed up. Lubed the switch and AC stayed cold.

edit..probably should have said "If you're not getting vacuum THROUGH the switch the (heater hose)valve will stay open........"

 

Perfect answer..

Here is the vacuum schematic that might help you too.

 

Thank you both. I've got it now!

The other end of the temperature control cable has a mechanism that presses the water shut-off switch when the control is set at full cold. This in turn supplies vacuum to the water cut-off valve which stops the flow of hot water to the heater core.

For some reason I was thinking the plunger on the water shut-off switch did something as a result of vacuum at the switch instead of it being the thing that operates the switch by pressing the plunger. In other words I thought it an actuator--not a switch.

 

KapsSA:

I have a replacement vacuum mode selector switch on order (it failed).

The hot air problem came before it failed however. From my study of the vacuum diagram (both from my FSM and the one by Willcox) it sure looks as if the water shut-off switch gets vacuum before the function selector switch. Thus the water shut-off valve should close (presuming the shut-off switch is functioning and being operated properly) even if the the mode selector switch is defective.

Hopefully I'll have time soon to do some deep troubleshooting.

 

The vac SWITCH does have vacuum all the time, but just TO the switch.
A CABLE from the heater/ac control pushes a lever against the vac switch plunger opening the switch to supply vac to the vacuum VALVE in the heater hose.
Make sure the plunger is getting pushed in by the lever when the control is on COLD. Even the lever 1/4" off cold will not activate the vac switch. Control must be ON cold.

 

In this pic you can see the vac switch/cable in the upper left.

 

Years ago I had this same problem with my 77, I replaced the vacuum switch on the heater box. The replacement switch back then had a slightly stronger spring installed and the cable from temp control to the switch would not hold the plunger in on the vacuum switch allowing the cable to back off the switch just a little not making the switch. There is an adjustment on the cable, but this is just to extent the cable to make the switch, not hold the switch. This is how I resolved the replacement switch problem. Not sure if this problem has been corrected with the newer replacement switches.

 

Thanks for the photo and spring fix suggestion! Much appreciated!

 

I have a new valve for that if you need it.

https://willcoxcorvette.com/corvette...rfect-function

And it will work unlike the imported version....

 

What's going on here?

This is a photo with the temperature control lever moved all the way to "cold"




Now set all the way to "hot"




Notice that the tab on the control arm isn't even close to the button on the valve at "cold". While I haven't loosened things to attempt any sort of adjustment I don't believe that the tab could ever contact the button on the valve.

The switch does not look anything like what I see available as a replacement. Is part of mine missing? Or is something supposed to slide onto the tab on the control arm?

I haven't yet tested the water cut-off switch for function but pressing the little button seem so indicate that it is working as it pushes in with little pressure and springs back out immediately.

 

Looks like you have a 1980-1982 switch installed in your 79, or maybe not even a Corvette switch.

 

bmotojo: I believe you are correct about it being an 80-82 switch.

I looked all around the workshop and basement for something to fabricate an extension to the little button and failed. Then I noticed and old kindergarten style pencil and thought hmmm. Drilled out the core and narrowed it a bit. It pushed on. I then cut it to the proper length, went to install and it fell.

Guess what happened when I went to retrieve with a probe--my big fingers won't fit in that slot. I found a soft plastic extension piece! But the end pressed by the metal tab was deformed and it didn't depress the button fully so...

I found a metal sleeve and pressed it around the end to extend it very slightly and protect it from damage.


I'm a big boy so no problem enduring the "Bubba fix" comments

Will test the switch function shortly and I'll be very pleased if my fix works for any reasonable length of time. At some point you do get tired of $50+ little replacement parts that very likely won't work as well as even a slightly rigged high quality (OEM) part. It amazes me that my very recent starter replacement ($54 for a NAPA remanufactured) was not only one of the least expensive parts I've had to buy for this car but one of the easiest to replace!!!

 

Update:

The found extension piece was obviously part of the problem as after a few operations during testing it disappeared and is nowhere to be found.

My replacement piece fabricated from the pencil is currently drying after epoxy reinforcement. I believe it will be far more durable than the "original".

Shut-off switch testing revealed a perfectly functioniong switch (when actuated) but a catastrophic leak in the white striped line leading from it to the shut-off valve. I found the leak in the line very near the distributor in a dry and brittle area no more than 3" in length. Every other vacuum line in this car is subtle. I'll find a suitable and permanent splice for the line tomorrow. I may even cut out a bit more of that line as it is truly too long. The water shut-off valve is about the grungiest looking device under my hood but it held vacuum so I hold my fingers that it actually functions after who knows how many years without use

 

My black79, very late production, had a switch that looked like that. No mushroom head(80 switch as mentioned?) My 78 and white79 have the big head.
In addition to lubing mine as I mentioned in a previous post I added a rubber cap to the plunger(a wire shelf end cap trim, fit perfect)that gave it contact with the lever that it probably never had from the factory.
As for the heater vac valve, after 30 years of non operation the plunger corroded in place open and eventually leaked water(why I ended up going over the entire system in the first place) Found an nos valve on ebay.

As for your switch, it looks a little crooked up in the pic. Can you loosen the bolt below it and adjust it down more in line with the lever?

 

My '79 is also very late production.

The little piece I made from a pencil works well. I soaked it in a restoration product called "liquid wood" and filled the hole left from the lead with epoxy so it's much sturdier than the original softish plastic

 

Sadly the new "modified to seal and work" replacement vacuum selector switch failed before I could even finish testing for vacuum leaks in every position. It would not hold vacuum in any selector position.

Lucky, I disassembled and lubricated my original selector switch using silicon grease and its minor leaks are gone and it holds vacuum perfectly. No leaks in the entire system so (save refrigeration which is being an absolute pain) I should have a fully functioning HVAC system upon reassembly. Well--I'm not holding my breath that the water shut-off valve will work.

 

You are aware that the cable (hot/cold) that goes to the vacuum switch on your heater box DOES have an adjustment feature to it...so you can lengthen it and shorten it by turning the turnbuckle in the cable...don't you??? I did not read anywhere in your comments that you tried to adjust teh cable...unless I missed it..and if I missed it...I apologize.

Also...the yellow switch you have which is correct which has been previously discussed...it does have a little bit of ability to be moved also....which you can see in the photo of the one on the GM box that was posted in post#12.

DUB

 

Yes, I am completely aware and have used the turnbuckle adjustment in the cable. It was utterly impossible for the lever to reach the shut-off switch without an extension to the button.

 

The hot water shut-off closed completely! Water line to and from it still cool after a 15-mile drive. Ambient temp air from the system for the first time since I've owned it!