I still have the original vacuum operated heater/ac control system in my '77. Overall, my system worked reasonably well from the start. All my vacuum actuators were good, and I replaced the fresh air door setup to get that working while the front clip was off. The weak link was the cable operated heat setup, with that finicky vacuum switch that shuts off hot water flow to the heater core. I’ve read plenty of threads complaining about these switches and how aggravating they are.

I have replaced the heater control vacuum switch twice now. The first one came with the car and never worked. The replacement lasted a year but then quit working. When the latest switch failed, I decided to come up with a fix that would be more reliable than the original-- I didn’t want to spend money on another vacuum switch that didn’t really work well to begin with. Also, I’ve never liked how hard I had to push the heat control lever over towards the cold side in order to make the vacuum switch turn off the hot water to the heater core. I hoped to do something about that.

I’ve read on the forum a lot of guys dislike the vacuum operated OEM system so I guess the question of why I’d want to bother with this fix is fair. First, I don’t want a manual hot water shutoff valve in the engine compartment. Jumping out of the car and opening the hood to turn on the hot water seems kludgy to me. The intent of the OEM design is good – have the water turn on and off with the lever on the console – but the implementation of that idea was poor. Second, an electrically actuated Vintage Air type full replacement system wasn’t an option for me due to the cost and because my car’s a ’77. No plug and play setup for my one-year-only orphan!

The fix I came up with was pretty simple and cheap. Now that it's installed, I think it works better than the original. I wrote this up in case anyone else finds it useful.

First, I removed map pocket on the passenger side of the dash to get access to the heater box. I removed the two screws holding the vacuum switch to the bracket and pulled off the two vacuum lines.

Next, I removed the rubber boot, return spring and plunger head from the old vacuum switch and pulled out the leaky dogbone valve from the body of the switch (it comes out the back after the plunger head is off). I was left with a hollow vacuum switch body that I could use to mount an electric switch.

I found a momentary push button that was about the same diameter as the vacuum switch body in my box of miscellaneous electronics parts:



The key criteria for the switch was its size (slightly smaller than the body of the vacuum switch), and it's momentary – pressing the button turns it on, releasing it turns it off.

I soldered two wires onto the switch, ran the leads out the side of the vacuum switch body through one of the vacuum nipples, then epoxied the momentary switch in place. I mounted my new electric ‘vacuum switch’ back in the original location. This is what it looked like:




Although it’s a little blurry, you can see the tab that’s actuated by the heat control lever lines up with the nose of the switch, and will press the button at the end of its travel. You can adjust the cable travel with the turnbuckle on the control cable to get the tab to press the switch just as the control lever gets to ‘Cold’ on the far left.

Bonus: for the first time ever, my heat control lever moves all the way to the left and doesn’t spring back towards center. It also doesn’t feel like I’m going to rip the console out of the car trying to shove it over to cold. This is because the heavy spring that previously pushed the vacuum switch plunger back out is gone.




After confirming with a test meter that the switch is closed with the lever all the way at cold and open at all other lever positions, I needed to find a solenoid operated vacuum switch. This is what I came up with:



I gather these are designed to operate 4WD engagement on some Jeeps and trucks. I chose it because it’s 12v, has a handy mount bracket, isn’t very large and is normally closed (i.e., the source vacuum is capped off when there is no power). They're available on Amazon:

https://smile.amazon.com/gp/product/B07N2Y8T1Z

You can also find them cheaper on eBay.

I hooked up the vacuum source to the bottom fitting (closest to the solenoid body), and the heater control valve line to the to the top nipple:




I mounted the solenoid to the heat control cable bracket on top of the heater box using the existing screw. As you can see, it’s kinda wedged in there between the heater box and the underside of the cowl so it’s pretty secure. It also doesn’t interfere with the movement of the heater control cable.

Wiring is simple: You will need an ignition-keyed 12v power source running in to one of the switch leads. Wire the other switch lead to one of the electric solenoid terminals. Connect the other solenoid terminal to ground. I drilled a small hole in the cowl support to get a ground close to the solenoid, but you could run a ground wire to another location if you prefer. I wanted to keep it as simple as possible.

To test, turn on the ignition and move the heat lever all the way over to cold. You should hear the solenoid click. Moving the lever away from cold will turn off the solenoid. Start the car, and move the lever back to full cold. The solenoid will click, and vacuum will be applied to the heater control valve in the engine compartment. Move the lever to heat, and the heat control valve will lose vacuum allowing hot water into the heater core.

Final thought: I don’t know how many amps are required to trigger the solenoid, and it’s certainly possible that my little electronic push button switch will die a sudden and ignominious death if there’s too much power going through it. If I were to do it over, I would try to source a more robust switch, or implement a relay that protects the switch. For now, everything is working and I’m happy.