Defrost without A/C

[Eric_77]

Anyone know which wire to cut or how to disable the A/C from the defrost/windshield selection. IE have a deice mode. I can always push the A/C button when I want defog.

 

[RustyJC]

The A/C compressor runs during defrost to dehumidify the air being sent to the windshield. It's not just a "defog" thing. If it's cold enough outside, the moisture (latent heat) in the defroster air will be chilled below its dew point by the cold windshield; when that happens, the inside of the windshield will fog or (if enough latent heat is removed) ice up.



The only way to keep this from happening is to keep the dew point of the air coming out of the defroster lower than the surface temperature of the windshield - that's why the A/C compressor is (and should be) running.



Rusty

 

[SRadke]

Thanks Rusty. We (at least I) understand why they do it but it isn't necessary in most conditions, at least not in our weather. The 70's rigs with AC didn't have it cycling with the defroster and I had no problem with them. To listen to that clutch locking and unlocking in the morning is, to say the least, annoying and I'd just as soon eliminate it as well as the aditional wear it may cause in the long run.



-Scott

 

[LandShark]

Another reason to leave the defrost/AC system alone is that it is good to run the AC occasionally during the winter to keep the seals lubed. AC's that sit unused for months at a time fail far more often than AC's that are in constant use.

 

[Bertram65]

I agree the defrost in not needed when it is cold and dry, I would think you could cut the ribbon on the circuit board for the HVAC controls that tells the AC to come on when the defrost is on. I would try to purchase a used HVAC circuit board to play with and figure it out. I could be wrong through and the computer may tell the AC to come on and the HVAC controls only relay the requests to the computer, if that is the case it might be more trouble than it is worth.

 

[Eric_77]

I know why you want AC with Windshield (defog) but I can turn the AC on independently with the AC button. But I want deice IE melt the ice on the windshield as fast as possible warm air with or without humidty is fastest way; with AC on the air is cooled plus when you switch to heat you have to turn AC on otherwise all the humidity on evaporator is deposited on windows.

 

[Bertram65]

My mistake I meant to say I do not need AC with the defrost when it is cold and dry.

 

[rscurtis]

You might want to verify this with your owner's manual, but i believe that

the compressor is disabled in the defrost mode at about 32 deg. F.

 

[WoodChuck]

A/C w/ Defrost

The easiest way would be to cut the wire that goes to the compressor and install a on off switch inside the cab or just disconnect it alltogether in the winter.

 

[rcgibson]

May I suggest that you just remove the AC clutch relay (#64 in the fuse and relay center). I do this every winter and the heater works as it should but I do not get the AC cycling on and off in defrost mode. I have used this method in both past Dodge's (since 1994) without a problem.

 

[Eric_77]

If it was a simple wire I would cut it, I want the abilty to use the stock functions here in Texas mornings we could have frost but by afternoon sitting in traffic you need AC so I was hoping to just disconnect the auto AC in windshield mode, so pulling a fuse or having a second switch is not what I would like I guess I will keep dealing with it as is.



Strange part of this is my 1966 GTO had both a defrost (windshield heat w/ AC) and a deice (windshield heat w/o AC) selection. I guess in the infinite wisdom of the car manf if you give people a choice they will use the wrong one so the choice was eliminated or would it be a cost thing? but it seems like it would be more difficult to wire up a second switch and have an auto selection as well. oh well I will leave it alone most likely I would end up with no AC if I fiddled with it it sounds like.

 

[CRobbins]

HI Eric, I felt the same way you do about the A/C on in defrost mode. I figured if I needed to "defog" the inside of the windsheild, I could make that decision myself. Besides, here in michigan, I really got tired of the snow blown into the truck! Especially after a very long nite of snowplowing. So, one day I removed the hvac control and did a circuit analysis on it and cut the appropraite trace. I would be happy to tell you how I did it, but..... , I did that back in 99 or 00 and dont remember how i did it. I have been enjoying the results though. I do remember that the first trace i cut was the wrong one, but easily fixed, and cut the right one. Hope this helps, just sorry i can't instruct you on exactly to do it.



good luck



97 4500hd C&C HotShot, cummins, 5 spd, DRW, 303k mi