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1979 Mark V Constant Heat

Hello Bill,

I purchased a 79 Mark V about 4 months ago and it has been what you might call a light to moderate restoration. Most cosmetic and mechanical issues have been addressed, but one remains, and it will shortly be more of an issue than it has thus far - it blows heat out all the time. Here is what I can tell you:

If you turn the climate control off, you get just a trace of heat. With any other setting heat comes out of the vents on the top of the dash by the windshield. If you put it on the A/C setting the compressor does come on, and I checked today and it's fully (or adequately) charged, and the metal parts of the A/C line get ice cold - so it seems to be 'compressing.' It's been converted to 134A. I replaced the heater control valve. That is a side note in that what I received was not the same as the one that came on it; the one that came on it had an external spring-loaded mechanism, and the one I received must have the moving part inside only (both vacuum activated). Not very technical but the new one is shaped like a cactus with the input and output as the 'limbs' - I wonder if those types are interchangeable? The supplier says 'yes.' In any event, would all of that give you enough to speculate as to a likely cause? The climate control unit?

Many thanks in advance.

Holt in NC.


Greetings Holt -

The AC system in your Lincoln is of the "semi automatic" type that will default to the heat mode and defrost in the event of vacuum loss. You state that "in any setting heat comes out by vents at the windshield at the top of the dash". If this is so and you have no directional control of the air flow with maximum heat at the defrost ducts then I would be diagnosing for complete loss of vacuum to the a/c heat control system. However if the system was locked on heat with correct air directional control settings I would be looking at the Sensor unit behind the dash above the glove box. Are you working on this system without the necessary shop manuals? Correct manuals are needed in order to be accurate with diagnosis and to avoid wasting time and money on unnecessary parts replacement.

The water valve only closes with vacuum for maximum a/c requirements and therefore is known as a "normally open" type. It will therefore be open with no vacuum. I do not know what type you have now installed on your car but it may be o/k and work fine. We do recommend and can supply to you the correct FoMoCo unit that we have in stock at Lincoln Land . If you find that you need the correct Shop Manuals or any parts for the repair please do not hesitate to contact us further at any time.