Climate control failure

My son's 96 318i had a problem when he bought it that seems to be very common, where the fan and a/c kicks on and off at random, even when turned off. Rather than trying to fix it myself and risk goofing it up I had the control sent to a very reputable fellow who replaced the bad capacitor, bench tested it and sent it back to me. Failed as soon as it was turned on. He repaired it again and replaced one additional part, which supposedly would make it less likley to fail, but it worked for about a minute before doing it's same old act. What this fellow tells me is that on a rare occasion a large group of electronic diodes (a bank of them he says..) can goof up and puts out far more ohms than the '96 unit can handle. Would be very expensive to repair. Suggests a newer unit out of a 99 would most likely work as they are 64 ohm rated and not likley to fail. He would refund our money on it if it did not, but, I wonder if anyone here as had this happen before and if there might be another reason, say, a short that would cause this. Anyone know what he is talikng about as far as this group of electronics is concerned?

Well, been 3 months, I would take this as no one has the bad luck of having this much trouble. I hate to take this car to dealer, anyone have any suggestions at all? Is there some alternate way of controlling heat/ac besides this lousy pc of engineering? I am tempted to simply buy a used one and try it, in case there was another internal problem with this unit. It supposedly tested OK each time repaired but who knows? Any ideas appreciated!

Well, have part of the answer. the item I was trying to identify is called the final stage unit. On E36 M3's up tp 1995 they used a blower resistor, on 96 up they used final stage unit. A bank of diodes that commonly fail, cause fan to run high and low. Gonna try this, will advise.

The final stage unit was bad on our car, as well as the control unit. The fellow we sent it to did fix it (the control unit) however it took replacing the final stage unit, which is under the dash, to get the fan working properly. All in all, cost 113 bucks for final stage unit, cost apx 42-45 bucks for control repair, although some people can repair themselves OK. Even though I work with electrical items I just did not want to risk ruining a very expensive item. I started a separate thread, in attempts to get help with replacing the final stage unit. In that post is a detailed instruction (with pics) of another person who installed the final stage unit on their car.

I had the same problem and replaced the capicitor you spoke of myself. It worked like a charm, and my climate control has worked perfectly for the last 3 months. I'm sorry to hear you didn't have better luck... That is strange...

From what I have read on BMW forums there is a lot of problems on both the capacitor and final stage unit. It sould seem BOTH were a problem on this car as repairing the capacitor helped a bit, but the fan was still going crazy. Replacing the final stage unit fixed that.