C2 ac valve troubleshooting

[Brandon K.]

Does anyone know how to bench test the brass valve on C2 air cond systems that cuts off the coolant flow through the heater? I'm not sure which side is the intake, or what is supposed to happen when the plunger is in/out. Can this be tested using a mityvac? I know that Dr. Rebuild has a writeup on how to rebuild these things, but I cant figure out how to get the piece apart, much less would I likely get it back together. Thanks in advance!

Brandon

[Joe R.]

Re: C2 ac valve troubleshooting

Hi Brandon:

I have my original valve and a new replacement valve that I bought from Long Island Corvette. I compared them both using a Mityvac.

The new valve is closed when no vacuum is applied, and it seems to be fully open with only 5 inches of vacuum applied. The old valve operates the same way, except that the vacuum leaks down if I stop pumping the Mityvac. The new valve seems to hold vacuum indefinitely. My conclusion is that the original unit has a leaky diaphram.

You can test the open/close operation by blowing through either port, although there is a direction flow arrow in the casting. You can also look down the inlet port and see the valve move in response to vacuum.

[Brandon K.]

Re: C2 ac valve troubleshooting

Thanks Joe for the fast response. I see that I did not describe the part in question very well. I am referring to the brass "switch" that sends the "vacuum signal" to the heater valve in the engine compartment. The part I am concerned about is mounted on the inside heater box in the a/c cars. Thanks again for everyone's help and sorry for the confusion.

Brandon

[Joe R.]

Re: C2 ac valve troubleshooting

Hi Brandon:

I share your interest in the brass vacuum switch as well. In the following discussion I will try to refer to the unit behind the glove box as the "vacuum switch" to distinguish it from the vacuum-actuated "water valve" in the engine compartment.

When I bought my car, the vacuum switch had been replaced with a plastic unit and the original brass one was in the glove compartment. I didn't even know what the part was until I saw one for sale on ebay. Later, when I took out the glove box, I could see the plastic replacement switch mounted on the heater box.

I think the intended operation of the vacuum switch can be ascertained by looking at the water valve that I described earlier. The removal of vacuum to the water valve interrupts water flow to the heater.

When the brass vacuum switch is actuated by turning the heat off (pushing the plunger in), it should cut off the vacuum to the water valve. The inlet side (engine vacuum) is the large port on the vacuum switch, and the outlet side (to the water control valve) is the small one.

While perusing the Corvette Central catalog recently, I saw what looked like the correct brass valve for $55, so I bought one to have as a spare. It came with an instruction sheet that states the following:

"This vacuum switch works on LOW vacuum. When the plunger is in there is no vacuum to the the heater shutoff valve and it is closed, shutting off the heat. When the plunger is out there is LIGHT vacuum to the heater shutoff valve and it is open, turning on the heat. You will be able to feel the vacuum coming from the fat hose, but the vacuum from the small hose is very light. This is correct."

I tried testing the new valve with my Mityvac, but it is very difficult to tell whether the valve is operating as intended. Regardless of the position of the plunger, there is leakdown to the vacuum source. My guess is that the vacuum is getting redirected rather than turned on and off.

I think the best test would be to connect a live vacuum source from a running engine to the large port on the switch, and measure the actual vacuum that appears at the outlet port. There should be no vacuum at the output when the plunger is in, and there should be at least 5 inches of vacuum when the plunger is out. I have not yet tried this test on the new switch.

[Bruce Boatner]

Re: C2 ac valve troubleshooting

Brandon,

I replaced the heater vacuum switch and related vacuum lines on my 1964 recently. What really helped me understand everything was a vacuum line diagram in the Dr. Rebuild catalog for C-2's with a/c. Take a look at the diagram if can find it and all will be much clearer. In a nutshell, the switch is between a vacuum line on the intake (input side) and the heater control valve (ouput side). When the plunger on the switch is depressed (off position) the flow of vacuum to the heater control valve is cut-off. When the plunger is extended (on position) vacuum is allowed to flow to the heater control valve diaphram thus opening the valve.

To bench test the switch you could attach a Miti-vac to the inlet side and a separate vacuum gauge to the outlet side. With the plunger depressed you should have ZERO vacuum on the outlet side. (If there is any vacuum, then hot water will be allowed to enter the heater core.) With the plunger extended, you should have a comparable amount of vacuum at the inlet and outlet. If you do have leakage it should not be a big deal because the inlet side of an installed switch has constant vacuum from the running engine.

I bought a replacement switch because mine was completely missing and it works great. I was able to replace mine by just removing the glove box.

Hope this helps.

Bruce

[Brandon K.]

Thank you gentlemen !

That is exactly what I did. I used a mityVac on the inlet and a vac gauge on the other side. Like Joe said, there is a purposeful leakdown of some vacuum across the switch, so with constant vacuum, a smaller amount of vacuum is on the "output" side of the switch. I got a brand new one from Long Island (exactly like my old one) for 50 bucks, and it works perfectly. my old switch, by contrast didnt pass any vacuum across. I guess by default if the switch leaks, then no coolant goes thru the heater core.

thanks again!
Brandon

[Bruce Boatner]

Re: Thank you gentlemen !

In my case, Bubba pulled off the switch and just spliced the vacuum lines together. Consequently, I had "continuous heat" to the heater core. Helped explain part of the reason why my a/c never got very cold and the car was always hot. I figured out I could disconnect the vacuum line from the heater control valve in the summer and connect it in the winter (or just use a big clothes pin inside the car!)