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Well about a month ago my 99' M3's A/C stopped working and I researched it and checked the climate control unit(OK), which I guess is a big culprit, and then read about the final stage resistor for the blower motor. Well I just got the new one in today and yes my fan works great now, used to only work intermittant, but the f-ing A/C still will not come on. I can make it work if I jump the relay in the fuse box, and it blows cold (39C) but it will not come with the climate control A/C switch. I checked the relay and even swapped it for another identical one but no luck. The accessory fan comes on when the A/C is turned on but not the compressor. I was 99% sure it was the final stage blower motor resitor but I guess not. The charge level in the A/C is in the normal range according to my gauge and there does not appear to be any leaks either. Is there any other electrical component that may be suspect? So does anyone have any other ideas, I'm about 100 miles from any dealer and it's getting hot in lower Alabama.

The DME I believe controls the relay, you may want to start looking in that direction. Do you have the Electrical manual?

I started thinking about this more at work today if my low press switch has failed that too would prevent the A/C from cycling on. Does anyone have a test procedure for this switch? I hate to shotgun parts at the car without a good reason. It just seems weird that it was fine before the fan started acting up.

Does anyone know if I put a meter across the low pressure switch if it is good will it read "open" or "closed"? And can I remove it without venting the freon to the atmosphere?

If your aux fan is turning on then the circuit is getting signal from the control head and the relay. I think you are on the right track. If the compressor isn't engaging then you may have an electrical failure. My Bentley only goes up to 98, but should still be close if not the same as your 99.

First, double check fuse #39 to make sure its good. If the fans are on then the ECM should be sending power to the pressure switch and relays. The only thing that should cause an open is either that 7.5 amp fuse #39 which is specifically for the compressor or its relay. You didn't mention which relay you swapped out. But if the fuse is ok then check the compressor control relay (#14). Check back if all ok.

To answer your other questions. It looks like the pressure switch is closed when good. Then opens under pressure.

In the meatime, buy yourself a Bentley manual. Its a necessity for the DIY. ;)

this is how i understand ac things:
climate control sends out signal to turn on ac compressor.
signal is amplified by relay
low (and high) pressure switch is either:1) between relay and compressor or 2) sends signal directly to climate control
older models have (1)
low pressure switch (LPS) is normally connected- if low pressure is present then its open
high pressure switch (HPS) disengages compressor and turns on radiator fan in pos 2
some systems can have multiple lo and hi press switches ( expensive models)
if your LPS is between relay and compressor, its ok because your compressor turns on manually, if it sends signal directly, that could be your problem

btw i have installed many systems and repaired too so you can ask specific thins and get ansvers from me

Ferris, I swapped out the A/C relay and checked all fuses again, all are good. Heres a pic of the wiring I talked about in the PM. The colors are as follows from rear of the car to front, and passenger side to drivers: Blue w/black stripe, solid brown--black w/brown stripe, black w/white stripe. I hope it is something simple like a wiring screw up beacuse the switch is 111.00. Also for those who are curious heres the old final stage blower motor resistor, located in the right drivers footwell area in the heater duct.

Your wire colors are different from my 95. I'll check the wire schematics later and try to assist.

very simple. if you can jump relay contacts and engage ac compressor only when this connector is connected then leave it. if you can engage compressor when this connector is not connected it sends signal directly to control unit.
then measure if two contacts are closed and other open (LPS contacts and HPS contacts). if i remember right then two outer ones must be connected, but i am not sure, so get tester and find out

Could someone please take a look at their low pressure switch(located on the accumulator, lower left engine compartment by the washer fluid resovoir) on a '99 M3 and see if their plug wiring matches mine in the picture. I had the plug apart a few months back to fix a chaffed wire and I think maybe I put it back together wrong. Please note the direction of the little clip on the side of the plug for orientation as your switch may be turned at a different angle. Also if anyone has access to the wiring diagram for this plu/switch and could sent me a pic of it or diagram I would greatly appreciate it.

Well I fixed the car this morning, turns out I had the bottom two wires crossed! Well I should have written down the orientation when I took the plug apart. Thanks for the help guys.

glad you fixed the problem, allthough MANY people dont tell the whole story and afterwards remember some tiny details for them= important details to people that repair