BMW E36 A/C Problems – Air Conditioning – 318i, 323i, 325i, 328i, M3
car year: 1996
car model: M3
We have a 96 M3- –about 3 months ago we took our car to an independent auto shop who has worked on our BMWs for 25 years. They replaced the crankshaft position sensor. and as a courtesy the mechanic also topped off the freon and turned out the annoying air bag dash light. We immediately noticed that the air conditioning was much colder. The next day the air was so cold it blew ice crystals out of the dash vents. Then it quit working correctly. Now it blows cold air when the car is at idle and hot air when you give it gas. We took it back to the mechanic and replaced the in dash control unit (the part with the buttons to change from hot to cold and set the temperature in the center of the dashboard). This did not fix it. The mechanic has disconnected the A/C and it still comes on in “pulses” when the gas pedal is pushed. Any ideas? Could there be a wire that may have been pinched when the crank shaft position sensor was replaced?
Let’s address the ice crystals first: This was not due to an issue with the charging of the system, directly. Overfilling the system can certainly cause damage, but not generally in the way that you’ve described. There is a temperature sensor in the evaporator (the unit inside the car that cools the air being blown through it) that will turn off the compressor if the surface temperature of the evaporator gets close to freezing. Therefore, you should never have experienced the ice blowing out. This would tend to indicate that either the evaporator temp sensor is faulty or that the circuit from the sensor, through the climate control unit, to the ECU (engine control unit) and through the compressor relay to the compressor is compromised ……. and the compressor is not turning off, when the evaporator is near freezing. Of all of this, the relay could be stuck closed, forcing the compressor to run all the time (see fuse box diagram below, for location of the relay). This should be a standard 4-pin relay. Swap it with either of the Aux Fan relays to see if the compressor turns off (assuming that it is always running, per your description).
Second: Just what did the mechanic do to “disconnect” the A/C? If the compressor has been unplugged or the compressor relay has been removed, the compressor cannot “turn-on” and the system cannot produce cold air. One exception would be if the compressor clutch were faulty and was always engaging the compressor, regardless of the electrical power (of lack of) to it. This would, more or less, make the A/C run all the time. Of course, turning your temp control up to a hotter setting would overcome the A/C and the air would be warm even though the compressor is running.
Since you do not know just what has been done, we should start with a full description of just what is, and is not, happening and what has been disconnected …. from the mechanic.
The Bentley repair manual has full wiring diagrams for the system, as well as a description of the system operation. This may help your mechanic diagnose the system properly.
E36 fuse box (click to enlarge):