Heat - A/C Control Vent Doors Trouble Shooting Info


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Sep 8, 2018
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Ross, Ohio
My Car
71 Sportsroof Bright Red.

During my rebuild I checked all the vacuum motors for the heat and a/c vents. All were working fine and seem to be ok.  After reinstalling nothing seem to work properly.  I figure this was somewhat due to the large cam I have causing low vacuum.  Reeving the engine up seem to help a little but not much.  I decided to dig into it a little deeper before going out and buying a electric vacuum pump to help boost the vacuum.  

Following the service manual procedure on how to test the system I skipped the part of starting the engine each time and just connected my "top dollar" hand held vacuum pump to the vacuum canister.   Upon running all the test I found out that most but not all of the vacuum motors were working.  Also those that were working were not opening all the way even when I was pulling 20" of vacuum with the pump.   If I removed a line from a motor that was supposed to be open I would only loose vacuum very slowly at the pump.

I decided to pull the control panel out and to see what I could figure out.   This was fairly easy.  I just removed the knobs and nuts holding the radio to the bezel.  I then removed the bezel exposing the heat - a/c control panel screws.  From there I was able to remove the electrical connecters and the vacuum block connector.  I also removed the glove box liner to be able to access the one control cable and to see the vacuum motors better during my testing.

Once I had the control unit out I was able to figure out that there was blockage in the vacuum mode switch manifold.  This unit is held on the climate control panel by a screw.  After removing it from the panel I determined that it does not come apart.   I was able to see that the selector plate is help down by a spring.  After removing the spring I was  able to pull the selector plate up from the manifold. I took my air compressor and blow each port out.  I then reassembled it and checked each port with my vacuum pump connected to the black main vacuum port and my vacuum gauge connected to the port I was testing.   The different from before to after I blew out the manifold ports was like night and day.  I went ahead and  reassembled everything and ran all the test again.  Passed with flying colors!  All controls are working great now.

I would highly recommend servicing this and also blowing out the vacuum lines themselves if you ever have the climate control out of the car or if you are experiencing similar problems. 


Mustang - Cougar test procedure.


Vacuum routing and motors.


Removed line from intake at vacuum canister and connected vacuum pump to it.


Pulling vacuum at reservoir canister. 


Vacuum Mode Switch with spring holding selector plate down on manifold.

Vacuum Control Spring.jpg

Spring removed


Selector plate pulled up.


     Testing the mode switch after blowing out the ports and reinstalling the spring.  Vacuum gauge moved instantly with the pump.

Last edited by a moderator:
Jun 17, 2020
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Homer, Alaska
My Car
1971 Mustang Mach 1
429CJ, 4 Speed, 3.25 Traction Lock N case, A/C, P/W, tilt, fold down seats, int. wipers, deluxe interior.
Love this write up. When I had mine all apart I not only checked the canisters by themselves but then after everything was hooked up I checked at the vacuum line connector all doors function good. 

however, I did not blow out that selector switch. 
I just got done hooking up my climate control (harness, vacuum block, lines, warm/cool cable etc. 

ive got a ways to go before I have the engine in but if something doesn’t work I’ll know where to check! Awesome tips 


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