Howdy Everybody!

I recently got my AC system charged and it's blowing pretty cold - I've been enjoying it immensely. However, it's starting to cool down outside too, and fog on the windshield is becoming more of a problem.

In the default setup, the AC gets turned on in the defrost position (and the floor vents/defrost position)... this makes for incredibly fast defogging of the windshield, but leaves something to be desired for ordinary driving, when you'd like to use the defrost vents to heat the windshield so that fog doesn't build up in the first place. The AC slows the heating of the windshield down, and is also a bit of a gas hog, leading me to cycle back and forth between defrost and normal venting. Clearly, something had to be done.

I looked up the wiring diagrams in the FSM and found out that a light green wire running from the AC/Heater control panel to connector C100 grounds to tell the powertrain control module to turn the AC system on when the switch is in AC positions. Splicing in a switch to this wire is the best way to override the AC - if the wire doesn't ground, the computer never tries to turn the AC system on. I chose this method instead of installing a wire to the AC compressor itself (or its fuse) since simply turning off the compressor would still let the system cycle and depressurize, which isn't healthy for the AC system. Also, the wire here carries very little load and is a really short run of wire to install, wherever you plan to put the switch.

The easiest way to access the connector behind the atmospheric controls is to remove the center dash (just pull it straight out, it's only attached by clips) followed by removing the screws from the power/cigar lighter console and the atmospheric controls console. Removing the cigar lighter console lets the cable from the back of the atmospheric controls come forward instead of getting hung up. The connector itself has a nub on it to keep it from popping loose - just push it in with a screwdriver while wiggling the connector out. After that, cut wire, splice in switch, install connector and mount where you like!

Following are some pictures of my install, along with the finished switch next to my dome light override switch, mentioned in this thread. Please note that this was all done on a '98 Cherokee - wiring for '97 through '01 should be the same, but please doublecheck before cutting any wires! I hope this helps anyone else who may be annoyed at the XJ's lack of switches, and if anyone from previous model years can shed some light on if the connector is the same, that would be great!

 

Nice thread, I live in north western PA. and don't like the slow warm up in winter either. There's nothing wrong with your switch setup, but you can achieve the same result by unplugging your low pressure switch on your accumulator in the Fall then plug it back in in the Spring.

 

Very interesting.

 

System was designed specifically to run the compressor in the defrost (winter) mode to lubricate the compressor shaft seal. Any alleged mileage gain is purely imaginary in actual driving, check Mythbusters for that info. Number one cause of fog build up on the windshield in cold weather is a failing heater core. If you can't get heated conditioned air @ the defrost or any other vent for that matter check the thermostat(195°), cooling system, and finally the blend door for issues.

 

With respect, the Mythbusters quoted actually found the opposite to be true - the SUV they ran with the AC on showed that the computed gas mileage was higher, but in testing ran out of gas 15% quicker than the SUV with the windows down. Being that the windows will likely be up during cold weather driving, fuel mileage gained should be somewhat higher due to lower drag.

All that said, it is best for your AC's health to be run every now and then to lubricate the seals mentioned, and the windshield does defog quicker with the AC on. That's why I installed this switch - to give me the option to choose whether to use the AC or not, and to let me have non-AC'd air on the floor and face or defrost and floor vents if I so desire (which I do!). It would be best to run the AC every month or so in order to lubricate the seals and make sure the AC stays in good condition, but for my everyday driving I'll leave it off and have the controls in any position I like

 

I just wanted to drop back in and say that over the last several days of driving, this has been working great. I've been using the floor and face vents on heat or vent without AC, as well as defog without AC and I couldn't be happier. Switch works exactly as expected and has caused no issues.

Cheers!

 

I also have one of these on mine. I went a little bit of a different route with it. I used a SPDT relay to interrupt the compressor clutch power. Have it in the normally on position for when I want the clutch going. Then flick the switch to turn the compressor off. Love mine and use it quite often even in the summer on the interstate when going up hills. Ever since my 92 volvo let me have a compressor engagement option, I've hated not having the switch.