A/c compressor does not disengage?

[jon schapiro]

Tony - good question. I had to physically look. My car has High, low, Vent, Off, Defog and Defrost.

[TonyC]

Aha, okay then! What I have to offer is based on the '60s ATC controls, but if my guess is right it should be relevant to the '70s as well. The compressor runs on the DEFOG setting but not on the DEICE setting. Assuming that DEFROST is just a different name for DEICE (which I believe is the case), then the compressor should be off in DEFROST but run on DEFOG. And of course it will also run on LOW and HIGH but not on VENT or OFF. The vent option was absent on the '60s ATC setup.

---Tony

[Jannelu]

On my 76 Tc, its different settings for the defrost and defog mode.
The a/c compressor runs though in both settings.
Its incredible that the compressor is supposed to run with the heat on..
Im pretty sure that on Cadillac, Chrysler etc, you have the option to switch the a/c off If you want to save some fuel...

[jon schapiro]

The automatic temperature control is designed to use heat and air conditioning to provide a constant temperature. Utilizing air conditioning with the heat prevents the windows from fogging up.

[action]

On my 76 Tc, its different settings for the defrost and defog mode.
The a/c compressor runs though in both settings.
Its incredible that the compressor is supposed to run with the heat on..
Im pretty sure that on Cadillac, Chrysler etc, you have the option to switch the a/c off If you want to save some fuel...

That may be true for a heat setting where the air flow is to the floor. (Or in an unlikely event to the vent openings)
Even today in any modern car where the temperature is selected for blast furnace heat and the air direction is to the windshield, meaning defrost the glass, the AC compressor will be on! (The assumption is the vehicle HAS AC and said AC works) This because AC does more than just chill the air. It is called Air Conditioning not Air Cooling or Air Chilling.

One of the other functions of AC is to remove moisture. Because hot moist air on a cold windshield is bad thing. Until the glass heats up, throwing hot moist air onto cold glass will cause the moisture to condense and turn into fog or worse ice depending on the temperature of the glass. Boil a pot of water on a cook top and place a piece of glass over the steam coming up. The glass is fogged because the moisture in the air is condensing on the glass.

In a car, after the glass windshield is heated up that function may not be needed. But pretty sure Cadillac and Chrysler of the day have the same feature and what ever modern car that you drive today absolutely has that function too. It would be unsafe to not to have that function. In some later vehicles this function was described in the owner's manual. I do not have an owner's manual from 1976 readily available. (I might however if I do it is in un-labeled box in an uncooled shed. Temps start out here at 80 in the dead of night for a low and today will get to the 105 range. And likely higher in that shed. The description in an owner's manual fascinates me. Because I have seen the description in an owners manual of the early 1980s. Just NOT that much to be digging through that shed)

And a by product of the dehumidifying process creates moisture that condenses out of the air and it has to go somewhere. That somewhere is out the bottom of the HVAC case out a condensate drain and on to the street. Ever see a car that left a puddle of liquid while waiting for a stop light? That puddle was the moisture that was removed from the air inside the vehicle done by the AC being on.

Action

[frasern]

Also, running the compressor occasionally over the winter, is recommended so as to keep it lubricated. My newer vehicles cycle it in defrost and this is part of the reason, according to the manuals.

[frasern]

Yes, living in a cold climate, I have done a lot of driving, looking through a small patch just above the defrost ducts. I'm glad the newer design heaters work better today!

[LithiumCobalt]

As Hank explained to me, and he’s definitely correct, the system was designed for maximum comfort, not efficiency. Therefore, the AC runs in order to dry out the air and is then heated, if necessary. Gas mileage wasn’t on anyone’s minds during the design of these cars. Most that could afford likely wouldn’t care anyways. I think by 68 or 69 model year, there was finally a vent option on the automatic temp control cars, which didn’t use the AC system. They must have finally figured out that AC isn’t always required.

[TonyC]

Afraid not; a "Vent" setting on A/Cs didn't occur until at least 1970. Before then, specifically '66 thru '69, the only "Vent" was in the non-A/C setup.

I have been nursing an idea on how to mod such a system, though. The system's controls still send air through the appropriate ducts even when the A/C components are factored out. So, I think a nice-looking rocker switch wired into the output of the icing switch could provide a "vent" operation to A/C-equipped cars of those years. One day I'll put that theory to a test.

---Tony

[LithiumCobalt]

The control head of the non AC car must be what I remember seeing. A kill switch would definitely accomplish.

[Jannelu]

Sorry, double post..