Bypass the AC when using defrost?

I want to install a switch which prevents the AC/compressor/etc from kicking on when Defrost mode (or any other AC mode, for that matter) is selected.

On most cars made today, there is a switch that lets you run the AC in any mode. I'd interested in the opposite.

Last winter, I could hear the compressor cycling on and off in a snow storm when it was 10 degrees, and it just irritated me. I understand that running the AC in defrost mode helps dry the air better, but I still want to be able to shut it off.

I haven't had the panel apart to see if there are separate wires coming off the selector knob assembly in which I could install a switch.

Anyone have any experience with this?

Sounds like your defroster will not defrost without the dryer air that the AC produces. Isnt defrosting your window basicaly getting off the moisture that has biult up on the window? without dry air I dont think it would work very well. Just my .02

jeepme said:

Sounds like your defroster will not defrost without the dryer air that the AC produces. Isnt defrosting your window basicaly getting off the moisture that has biult up on the window? without dry air I dont think it would work very well. Just my .02

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They've worked w/o the AC for decades. I agree with the over ride. I've scratched my head more than once ( leave it alone! ) over trying to defrost and heat the cab with the AC running on the trail where I want ALL of the engine's power.
It should only be a matter of putting a swith between the AC switch and the relay.

You could run a switch into the ground wire on the pump and switch it off during the times you aren't using the AC and just want to defrost without the pump.

My YJ's and other cars didn't have AC, and the defroster worked ok.

I understand that running the AC will work "better", but I'm thinking more along the lines of keeping the windshield warm to prevent crap from freezing to the outside of it without the added stress of running the compressor, etc.

If I put an in-line switch to the low-pressure cutoff switch (on top of the dryer), will that work, or is there anything else that kicks on I don't know about other than the compressor?

Have you looked at the wiring diagram in your Haynes, Chilton or FSM? You should have one of the three that you bought on your way home from buying your XJ. Yes?

I noticed it on my XJ too... I had the defroster on and was looking under the hood and noticed the A/C compressor clutch was engaged.... What I did was cut the wires that engage the clutch and install 2 quick disconnect ends... In the winter time I disconnect it so that the clutch doesn't engage... I have no idea what the reasoning is for having the A/C to produce cold air for the defroster... You would think just vent air from outside would be fine.

sigh, if you guys would have looked down the wiring you would have seen a quick disconnect already.

i know if you've ever had a compressor to lock up the easiest way to fix it so you can use the defrost setting is to unplug the connector from the compressor its self..
just patch in wires from a switch to either the ground or hot wire for the compressor, don't mess with any relays or such..
i'm sure there are probably wires under the dash you could patch a swith into but jeez there's a freakin ton of em there, more simple to pass 2 wires through the firewall into the engine bay to the compressor..

SPVFD47 said:

I noticed it on my XJ too... I had the defroster on and was looking under the hood and noticed the A/C compressor clutch was engaged.... What I did was cut the wires that engage the clutch and install 2 quick disconnect ends... In the winter time I disconnect it so that the clutch doesn't engage... I have no idea what the reasoning is for having the A/C to produce cold air for the defroster... You would think just vent air from outside would be fine.

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not a good idea i don't think..

mainly bc an AC system has O-ring seals and those seals require lubrication or otherwise they dry and crack, AC refrigirant contains a proper amount of seal conditioning lubricant..
every so often it is good to cycle the AC system for a few minutes, like once a week even if its freezing cold out. denying your ac system a chance to cycle my be denying yourself the luxury of cool air the next summer..

This bothers me to. There has to be a wire that can be cut somewhere from the defrost circuit that would eliminate the A/C turning on while in Defrost permanatly. I did't see it in the FSM diagrams.

Another reason it's a pain when running defrost, when the A/C kick's on it also triggers my electric cooling fans which is normal for A/C operation.

You guys fail to realize that the A/C is more than just a cooler. It's a dryer. Which is why it runs when the defrost runs. It dries the air that blows into the cabin, the air is still hot, but dry. The water on the windshield sees the disparity in the dry air and is attracted to it.

In super simple terms.

Part 2: On my '99 there is a (I BELIEVE) 2 wire (may be only single wire) plug going to the compressor. You could just find the pos lead and wire in a simple, heavy duty, switch, or switch to a relay.

So long as the compressor isn't running you don't have to worry about it getting oiled. It's no different than if it simply wasn't on.

I didn't read the whole post super closely, so I apologize if this is redundant.
As for the air being cooled in the winter, I don't know, there MAY be a trigger in the system that tells the A/C system to not cool it.. I don't know, I doubt it. But either way, it all get's heated before it comes out the vent, so it's not like you are blowing cold air in the compartment.

Personally, I hate it too, but it is better, I just wish I had the option to turn it off, I wouldn't, but I wish I could.

I've done this on almost all my vehicles. During the winter, the A/C compressor may kick on even when not in an A/C mode.
The dryer air that is produced is not much help in defrosting the window, rather warm airflow works fine.
Anyway, you can disconnect the A/C compressor clutch electrical connector in winter or wire up a switch to the ground lead for manual control.
You still should cycle the A/C compressor from time to time to keep the lubes circulated.

JLane99XJ said:

not a good idea i don't think..

mainly bc an AC system has O-ring seals and those seals require lubrication or otherwise they dry and crack, AC refrigirant contains a proper amount of seal conditioning lubricant..
every so often it is good to cycle the AC system for a few minutes, like once a week even if its freezing cold out. denying your ac system a chance to cycle my be denying yourself the luxury of cool air the next summer..

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X2!

sweetjeep said:

You guys fail to realize that the A/C is more than just a cooler. It's a dryer. Which is why it runs when the defrost runs. It dries the air that blows into the cabin, the air is still hot, but dry. The water on the windshield sees the disparity in the dry air and is attracted to it.

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X2!
Sure it works w/o the a/c but it takes waaaaaay looonger. Especially when it's raining and the air is already saturated with moisture.

K

I would normally use the defroster as God, and Daimler Chrysler, intended. The windshield does indeed clear faster with the dehumidified air.

I want the OPTION of being able turn the AC system off. Such as when the windows are clear, but crap is freezing to the wipers, etc. Or, better yet, starting the car (w/my remote starter) and letting it warm the windows to make it easier to scrape. At that point, I don't care about a little fog on the INSIDE; I'm more interested in HEATING the windows to melt ice. I don't see any point to running my compressor (and aux fan cooling the already VERY cold engine) while this occurs.

I'm not all that familiar with AC systems. Is the compressor kicking on the only thing that's activated when you turn on the AC (other than the flaps being aimed, etc)?

Cutting to your intended point here: Go look at your a/c compressor. More than likely its has 1 wire going to it but may split into 2 just before attatching to the clutch or front part of the a/c compressor. If it has 2 wires going to it from the main harness of the vehicle, NO BIG DEAL!! All you have to do is pick one of the main wires. If its a single wire compressor cut the wire before it splits in 2. If its a 2 wire compressor cut either wire. Preferablly the ground wire. The reason I say the ground wire is because there will be no voltage running through the switch. You will just be completeing a circuit. Now, run both ends of the wire that you just cut into the interior and hook them to a switch. A 2 prong switch mounted in your favorite position. No fuses needed or anything like that. Simple, yes??? When you want the a/c to work....CLICK....a/c on.

Now, just an FYI. When ever the a/c compressor is turned on, it is cooling and there is nothing you can do about it. When you move the temperature selector to the "hot" position, it opens the water valve letting hot water into the heater core, which over rides the cold air. There is a micro switch on the back of the a/c controls that turns on when the selector is in the a/c or defrost position. When in "defrost" mode, the moisture in the warm air blows accross the evaporator core of the a/c and clings to it thus drying the air. That's why you see water dripping out on the ground. All that moisture has to go somewhere. Warm moist air, especially in the winter with the higher humidity, doesn't defrost the interior of the windshield very well. That's why the a/c is made to come on when in defrost mode.
Hope I didn't bore you to death...............sorry about that.

Good luck.......................Mikey

I'm going to yank the HVAC control panel and see if I can do the slice there.

Are any other components activated other than the compressor when you turn on the AC?

TacticalFats said:

I'm going to yank the HVAC control panel and see if I can do the slice there.

Are any other components activated other than the compressor when you turn on the AC?

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I would be willing to bet that you can't.

And, I don't think so, I think everything else is passive and works simply by having the oil passed through it.

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Sure it works w/o the a/c but it takes waaaaaay looonger. Especially when it's raining and the air is already saturated with moisture.
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The only time it's drying effect help is in the rain or high humidity. In Ohio this drying help for a short time in the spring. In the winter the air (below 32 deg F) in Ohio has less moisture in it then in the air of Death Valley. At this time running of the AC with the Defrost is a waste of time and gas. Heat is what you need then and the AC just slow things down. I have unplugged my compressor for decades after it gets good and cold.
Switch or unplug what ever works for you.

There is a side benefit to leaving the AC connected. Early on, they found out that letting the compressor sit without being activated for a long time allowed the lubrication to leave the parts. Many times in the spring, the compressor would be frozen and wouldn't turn. By activating it a few times each winter, it made the compressors last longer.