W201 aux fans won't turn off
 

After I have my a/c running and I stop the car and turn the engine off, the aux fans continue to run on low speed, even with the key out. This only happens when I drive and I run the a/c. The fans seems to run for....I don't know how long, but it drained my battery today. The other day they ran for about a 1-3 mins before shutting off on their own. Anyone have any ideas? I checked the silver relay at the back of the fuse compartment and even swapped it out, but it continues to do it. I disconnected the wires at the engine block to get the high speed fans to turn on, then I reconnected and shut the engine off, but the low speeds kept running. Please help.

Low speed is activated by the high pressure switch in the AC system.

so is the high pressure switch faulty then?

Possibly, or the relay. Sorry it's been awhile. I though the relay was somewhere near the left shock tower, maybe just ahead of it. I think you can test this theory by disconnecting the high pressure switch.

PS possibly right near the resistor which drops the voltage down to acheive low speed for the fan, look for the ceramic block.

I swapped out resistors, still have same problem.

I never suggested changing resistors, since the fans run in low speed obviously they were fine. I would TEST the relay and also the high pressure switch. Actually if you simply disconnect the high pressure switch and the fan stops, then you can assume EITHER the AC is at high pressure OR the switch is bad. If they continue to run with the switch disconnected can "reasonably" assume (NO guarantee!) that the relay is stuck closed, beyond that could be a wiring issue.

Relays are switched, brand new. High pressure switch gets disconnected and problem goes away....but I am unsure on how to tell if the high pressure switch is the culprit or not.

There are two relays for the aux fans. One for high speed and one for low. They are in the relay box behind the fuses. You can swap them around to narrow down the problem.

Already tried that as mentioned in the first post.

Your first post stated you swapped "the silver relay". This means you only fiddled with one of the two relays.

There is another relay, different color, in the small plastic box in front of the fuse box. I forgot the position, I think it was "position C": A bit bigger than the ice cube relay, and may have the 40A fuse. Check fuse first if equipped. Had to replace this particular relay on mine a while back.

Ah yes, since the low speed fan is the only one giving me the problems I didn't think I needed to bother the high speed relay, but I did interchange the high speed and put it where the low speed was to no avail. I since have put 2 new relays in place just in case. only thing I have not replaced was the high pressure switch, because when I did it 2 or 3 years ago, I had to release the freon. could the fans coming on be related to the temperature switch on the engine block?

NO, the A/C high pressure switch runs the fan in low speed only, the engine temp sensor runs the fan in high speed only.
Since it is rather unlikely that you actually have high pressure in the A/C system it is reasonable to assume at this point that it's the high pressure switch. Since the system will need to be hooked up to a recovery machine, the person doing the AC work can verify what the high side pressure is before evacuating the system to make sure the fans shouldn't be running. If the pressure actually is too high then they can remedy the real problem.

If you are opposed to having this all done, then you could leave the high pressure switch unplugged or pull the low speed relay, and simply correct whatever you've done when the outside temps are quite hot. There is no real danger in leaving it unplugged (the high speed fan is though), the AC just won't cool as well on hot days.

I know some owners have just installed their own on/off switch for the low speed fan and will switch it on when they need it, like on hot days or sitting for a long period in slow moving traffic, another option.
Gilly

Yes, I just got another high pressure switch, but I really don't want to drain my system and pay to refill it, plus.....if it ain't broke.....I live in Palm Springs and our summer average temp is around 110*F.....its only March and we're already having 88*F days so I ratcher not open up that can of worms. Ill wait until around November when temps drop back down. For now, I think I'll just unplug the switch and leave the relay intact.

The switch is also there to protect the system in case the pressure is too high.

If I were you I'd just do it right the first time - replace the drier as well and both switches (pressure/temperature), aside from vacuuming the system before recharging. Do you still have R12 or R134a?