I cannot find a good thread covering my specific issue and wanted to see if I could find some answers before I take it somewhere and pay the big bucks to get it fixed. This is a recent issue and it has been a pita with the 90 degree weather. My AC is going in and out and only blows cold when I am driving. If I sit at idle, the engine will rise in rpms and the air will go cold for a few seconds, rpms will drop and then it will start blowing warm air. Same happens while stopping and going at red lights. Would this be more of a fan issue or a compressor issue? Another thing to note is when at a red light and applying the brakes, when the rpms drop, the brake peddle will give slightly. I'm assuming that is possible due to the engine using less power and the compressor is no longer working. I would rather do the work myself if possible. Any help is appreciated!
2004 G35x

 

I'd say first make sure both of your fans are running at idle and with the A/C turned on.

The other thing could be the compressor clutch can't handle the extra heat when it's sitting at idle. The compressor clutch has shims that keep it at a certain gap and if the gap is too wide or small it could work intermittently or be engaged all the time. For example on my Volvo the A/C would work and then turn it off, I fixed it with some RockAuto magnets and zip ties to get the gap smaller, since it was worn out due to heat/time.

If you want to do the work yourself; I would have it evacuated by a professional, do the component replacement, and then see if a pro could vacuum the system and recharge it. You'd need to buy a vacuum pump and A/C manifold gauge set to DIY. If it's the compressor clutch sometimes you can replace it on the car without having to discharge the system. But, finding just a compressor clutch can sometimes be hard.

 

UPDATE--
So I forgot to mention that a couple weeks ago I had a new key made for my car, due to me losing my original (long story lol). During the first few times unlocking and starting the car, my alarm would go off, which resulted in me pulling my horn fuse so it wouldn't be loud and obnoxious. Yesterday when I got home I replaced the fuse and now I cannot replicate the above issues with the AC. Everything seems to be working fine now, even after idling for almost a half hour and still having ice cold AC. Thank you for the advice though! Hopefully this will help someone down the road.

 

If you removed fuse 71 for the horn located in the IPDM that messes up the CAN-H signal and yes your AC will malfunction.

 

Well..update..
I thought I had it fixed. The problems came back yesterday. So here is the run down--
-Both fans appear to be running at idle with the AC on.
-I checked the low pressure end of the AC, pressure is fine.
-AC is blowing "cool" air but not as cold as it should be.

I read somewhere that the thermostat may be shot, but the temps only rise with the AC on and I figured with a bad thermostat, it would start to heat up regardless if it was on or off. I'm puzzled. Could my radiator be the issue? The air gets significantly colder at highway speeds, and gets "cool" at stops. Only when idling for a period of time will I feel the compressor stop and that's when the air warms up. Could the compressor be disengaging when the temps rise and then coming back on when the temps level out? Car has 163k miles so I know things are bound to go wrong sooner or later but I am lost with this one. I dont trust any shops nearby to give me a fair diagnosis, I'll probably "need a new engine" if i take it somewhere.