Heater Core Valve

Last Updated 09/21/06

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Heater Core Shutoff Valve

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77's have a vacuum valve under the hood to shutoff the hot water flow to the core.  This valve is controlled by a vacuum switch that sits on the top the heater box inside the car.  It is near end of the cable that controls the temperature.  The valve is normally closed except in the cold position.

Great idea.  Probably worked fine 25 years ago.  But I found this vacuum switch to be the source of my vacuum leak in the AC/Heater.  Nothing was working properly.  Various vacuum valves were not reliably working.  Once traced to this switch, I ordered a replacement. 

The replacement is NOT suitable for this application.  What's being sold as a replacement is the valve used on the wiper door on the 72 and earlier.  The spring is too stiff to allow the control to be moved to cold.

I tried various ways to get around the problem.  My first fix was to put a slight N kink in the temperature control cable so it would help hold the switch in place.  This helped but I didn't find it to be the ultimate fix.

Another person used the original spring from the proper switch.  This is probably a good approach.  But my spring was no longer around.

I decided to use a microswitch to activate a 12V NC vacuum relay.  This works perfectly.  The pictures below show how I did it.  It was actually quite easy. 

The change is completely reversable.  Nothing is cut or modified.  Only two small holes are drilled and tapped into the existing bracket on the heater box.


I used what parts I had laying around.  The idea is to close a switch when the temperature cable is moved to the last few millimeters of the cold position.  This provides power to the vacuum solenoid which diverts vacuum to the underhood valve and turns off the hot water.  Simple.  You'll need these parts:

  • Switch - Normally Open

  • 1x Vacuum Solenoid Switch

  • 1 - 3' of hookup wire

  • Crimp Connectors

  • Insulation - tape, heat shrink, etc.

Switch Specification

Any normally open switch will work if it fits and you can mount it so it activates when it should. 

I was looking for something easy to mount with no custom fabrication.  I happened to have a long arm Microswitch laying around from 30 years ago that worked for me.  Mounting couldn't have been easier.  Just drill and tap two small #6 holes into the existing bracket for the mounting screws.

I also looked at using a plunger type but that would have required a little bit of fabrication. 

I suppose a magnetic switch could be used.  What you don't want is something that has a lot of mechanical resistance.  That's what we are trying to eliminate.  The Microswitch fit the bill perfectly.

Vacuum Solenoid Specification

The vacuum solenoid was something I located on eBay.  Not sure what vehicle it is actually designed for.  The person selling it was in the UK. 

Most anything should work if it meets these specifications:

  • 12 Volt Coil

  • 3 ports

  • Normally Closed.  That means between port 1 and port 2, there is no connection without power.  Port 2 will either vent to the outside or be connected to an unused port 3 which will be the vent.  When activated, Port 1 will connect to Port 2.  Port 2 will disconnect from Port 3.  Vacuum flows between Port 1 and Port 2.

  • It fits in the available space

  • Has vacuum connections compatible with the existing ones.


You'll need access to the top of the heater box.  That means removing the right dash panel and possibly some AC ducting.  It needs to be open.

Figure out what kind of switch you are going to use and how it will mount it in a location so it will activate (close) at the right location. 

Mount the solenoid.  Mine is top, slightly to the right.  Picked because grounds are easy to reach and there's plenty of room here. 

Extend the vacuum source (black) and vacuum line for the underhood shutoff (light color) to the vacuum solenoid.  I did not cut the lines.  I used an inline splice connector and simply added enough hose. 

Run a wire from the microswitch "common" to switched power.  I connected at the heater control because I had the console out.  This also connects the solenoid into the same protected circuit as the other heater components.

Run a wire from the other terminal on the microswitch (N.O.) to the vacuum solenoid. 

Run a wire from the second terminal on the vacuum solenoid to ground.

Adjust the microswitch so it activates at the right location - COLD.

Test.  Place the temp switch in the COLD position.  Start the car.  Open the hood and pull the vacuum hose from the hot water shutoff valve.  You'll find it inline on one of the heater hoses.  Sometimes it's plastic.  Sometimes it's metal.  You should feel a vacuum from the open line.  Move the control from COLD to HOT.  The vacuum should be gone.  This line should now be vented to the outside through the third port on the vacuum solenoid.  You should be able to blow through the disconnected line if this is true.  If it doesn't vent. the vacuum will remain and the shutoff valve won't open for a long time until the vacuum bleeds off.

A very COOL fix.  No pun intended, of course.




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This site was last updated 08/15/06