A/C Compressor “hot wire” switch for de-fogging

roadbox

roadbox

True Classic
All,

I’m considering something which I wonder if others have thought of or actually may have done. I want to be able to blow air-conditioned air through the de-frost/de-fog vent on the inside of the windshield in my late X. Seems like most modern cars have this provision built in and, as with my good woman’s Scion, it’s hardwired so the de-frost/de-fog position always gets the compressor. We had a particularly moist morning last week and in order to keep the windshield fog free, I had to toggle between the a/c and the defroster position, since both operate exclusively of each other.

Anyway, here’s what I was thinking of. Since I don’t want an always-on situation for the compressor as with modern cars, I am considering taking a spare rear de-froster switch and using it to make a “hot wire” over-ride (not unlike what some folks do for the rad fans) for the a/c system so that it will run when the flap selector is in the de-frost/de-fog position and I have the switch closed. This would let me use A/C air or heated air for de-frosting or de-fogging at will, depending on need or time of year. Has anyone done this? Can you foresee any problems I might have?

Thanks for any advice.

John O.
 
I'm a little confused by this...

because when my AC runs, it is definitely blowing air out of the vent at the base of the windshield. So I'm wondering why you would use the Defrost button at all (or even what it's for, since all my vents are labeled "close to defrost"). Why not just run the AC and let it blow cold (dry) air on the windshield?

Come to think of it, I'm not even sure my car has a "Defrost" button. But it's pouring rain here cuz of the tropical storm, so I'm not going to go out and check...

Pete
 
Mine works like this:

Press the Max A/C or A/C button (2nd or 3rd button from the left) and the heater box flap moves so that windshield vent is closed. All other vents work, but A/C compressor clutch is energized, and blower fan comes on automatically to #1 speed (other speeds are available).

Press the defrost botton (last one on the right) and the flap position selects for the windshield vent. Air will still blow from all the other vents. The other vents must be maually closed in order to get maxium defrosting/defogging effect from the windshield vent. A/C compressor clutch is not energized in this mode. Blower fan must be turned on with the switch.

All of my late Exxe's ventalation systems behaved this way. What year is yours, Pete?

John O.
 
It's an 87....

Hmmm, I wonder if something is wrong with my "flap" valve. I would rather not have the AC blow on the windshield, it would be better if it all came out the vents towards me.

Pete
 
Interesting!

It may actually be because yours is an 87. My A/C guy was talking to me about when that became a standard thing in cars and it was mid '86. Could this be another difference between the '87 and other late Exxes?

John O.
 
I have the same

With the vents on cold air blows on the windscreen and causes it to fog on the outside of the car. I now have to turn on the wipers to clear the fog away on the windscreen. Yes it is an 87,


TonyK.

Grimsby Ontario Canada.
 
I used this many mornings on the '79

Turn AC on, close all outlet vents. The only place left for air to come out is the defrost duct. Want warmer air? Slide the temp control to hot.
 
so..... sounds like if you follow the '87 AC wiring, you should get the desired result...

I'll have to check my '87 to see what happens with defrost
 
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