1979 Mark V Heater Control Valve Question & Follow Up

Hi Bill,
Thought I needed a heater control valve I am in the processes of assessing the problem. I find I have no vacuum at the valve. Going through the manual I see I have a green hose, purple and a black, green going to heater valve, black has vacuum at the plastic manifold but doesn’t continue to flow thru green hose to heater valve. When I jump the black hose (Vacuum) to green it operates the valve. Could there be a problem with the plastic manifold that I may not be seeing? There is also a thin cable with a rubber part that moves in that plastic manifold with the cable, don’t know if that has an effect. Thanks so much.
Kenneth
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Hi Kenneth –
That green vacuum hose that routes to the water valve will only receive vacuum from the valve on the power servo when the system calls for full cooling and the power servo has pulled that thin wire on the servo valve to the extreme cooling position,. The water valve will receive no vacuum and be open in any other position. This operation is shown on page 36-74-3 of your Factory service manual. If this valve is receiving vacuum but cannot send vacuum to the water valve it could indeed be at fault. the rubber plunger inside of this valve is known to deteriorate and fail with age or if transmission fluid has intruded into the ATC vacuum system because of a vacuum shift valve leak at the transmission. The presence of transmission fluid at the servo valve under the dash will be evident.
If the complete power servo with the valve mounted properly on the side is sent to us we can test and evaluate it for you and possibly repair it for you. The mounting pins on that valve can be brittle and collapse so do not over tighten them.
Sincerely,
Bill
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Thanks Bill,
I may take you up on removing servo if nothing else pops up and forwarding it. I want to check the ATC switch if I can find it on the motor, also is there a test for the ATC temp sensor at glove box.
Thanks again, hope to hear back from you.
Kenny
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Hi Kenny –
Before you tear anything apart you need to be sure how that system operates. As I stated previously, that water valve does not close all the time for a/c, only in the maximum servo a/c position. You did not share the problem that you are experiencing with your ATC that prompted you to check this water valve. Read your manual carefully to know how each component operates. What issue prompted you to suspect a water valve problem ( no cooling, poor cooling etc. )?
Sincerely,
Bill