Author Topic: Aux fan working with temp switch but not AC (Read 5468 times)

lunchkrispy · « **on:** June 27, 2012, 03:29:30 PM »

Stuck in some traffic yesterday I noticed my temp rising so I pulled over and found the aux fan not running and fuse 19 and 20 blown. I have a large spal fan installed in place of the stock aux fan and I'm not running an engine fan. Long story short, after messing with the position of my low speed resistor which looked like it might be shorting out, and after burning through a few more fuses, the fan now works in both high and low speed if I jump it at the temp sensor plug but it will not come on anymore with the AC. At this point I'm pretty sure the resistor shorting out against the body of the car caused the fuses to blow but I think it must have taken out something else at the same time. Can anyone help me track down what that might be?

Edit: Did some more searching that I should have done before posting and I now suspect the aux fan diode has gone bad.

DesktopDave · « **Reply #1 on:** June 27, 2012, 03:37:58 PM »

There's a diode in the fuse box that controls the A/C unit switching on the low speed fan. Check out the wiring diagrams. It's a little round plastic unit.

lunchkrispy · « **Reply #2 on:** June 28, 2012, 05:20:01 PM »

Found a new diode on Amazon for about $18 http://www.amazon.com/OES-Genuine-Auxiliary-select-models/dp/B001G7MB2Q/ref=sr_1_2?ie=UTF8&qid=1340919999&sr=8-2&keywords=bmw+diode

$18 seems spendy for a little diode but reasonable enough that I was just going to go ahead and get it, BUT...... My aux fan cycled on and off successfully a few times yesterday while driving in the heat but then ended up blowing my 7.5 amp fuse 19 again (30 amp #20 survived this time, AC was off the whole time, blower was on 2) Replaced it with the spare 10amp I had and it has behaved since but now I don't trust it. I guess my question is, could a bad diode blow that fuse or should I be looking for something else? I would also be bummed if I killed a new $18 diode. Is there an easy way to test the diode with my multimeter?

ps, When I jump my temp switch my fan sounds like it is running at the same speed on both settings, maybe a symptom of something? Or maybe just the way the aftermarket fan works with a stock resistor, can't remember if I ever really checked it when I first installed it.

monty23psk · « **Reply #3 on:** June 28, 2012, 07:43:54 PM »

Pelican has it for $15.50, I believe this is the part number 61-31-1-358-268 from looking at realoem. I should have extra from when I had this issue.

monty23psk · « **Reply #4 on:** June 28, 2012, 07:46:30 PM »

BTW, you can test your diode to see if it is good or bad. That way you will know if you really have a problem with it. Very simple test with a mulitmeter.

lunchkrispy · « **Reply #5 on:** June 29, 2012, 08:03:59 PM »

Yep, the diode tested bad. Pried open the case just for fun and sure enough, it was melted in half. Thanks guys. Monty23psk, sent you a pm.

monty23psk · « **Reply #6 on:** June 29, 2012, 08:15:07 PM »

replied

lunchkrispy · « **Reply #7 on:** June 29, 2012, 08:26:36 PM »

Real cheap local option occurred to me:
http://www.radioshack.com/family/index.jsp?categoryId=2032269&allCount=449&fbc=1&f=PAD%2FProduct+Type%2FDiodes&fbn=Type%2FDiodes&filterName=Type&filterValue=Diodes

I have a soldering iron. Only problem is that my diode is mostly in little pieces so I don't know what it was rated at or if a 1 amp or 3 amp would cover it. Can't find this thing shown on any of my Bentley wiring diagrams either. Do you know what kind of load is on this thing?

DesktopDave · « **Reply #8 on:** June 30, 2012, 08:38:24 AM »

I have no idea about the rating but it's only switching the fan relay. The full load goes through the black/green wire other side of the fan relay.

I hope you have better luck than I did - I tried soldering a diode on the old e24 - and had zero luck with the plated connecters.

lunchkrispy · « **Reply #9 on:** June 30, 2012, 05:49:32 PM »

Alright, this problem just got a hell of a lot more confusing. I have my fuse box open at this point with the diode removed and some electrical tape over the ends of the leads for it so I can drive the car around. Picked up a couple 1 amp diodes from radio shack. Got back, checked fuse 19 and it's blown again. I replace it, turn the car to on (with the engine off) and start testing around to see what is blowing it. Jumped the temp switch to high and low, messed with my electric mirrors, ran the blower, turned the ac on and off. No problems, everything works fine (except for the aux fan coming on with the ac obviously because I still have no diode installed).
I still have the temp switch jumped so the aux fan is running, I start the engine and it immediately blows the fuse. WTF? I have been studying the ETM for a couple hours now and I can not see how having the car running would blow the fuse but with just the ignition on it is fine.

Anyway, on to part 2 of my confusion:
Even though it makes no sense I decide to see if having the diode disconnected could be causing this problem so I moved on to that issue.

Hooked one up with test leads first before I bothered soldering it in properly. Turned car to on with engine off) hit the ac button and the diode immediately went up in smoke. I double checked that I had hooked it up the correct direction and it was. Next step I tried was leaving the ac button on and jumping together the leads for the diode. The fan did not come on like it should have and the test lead started getting warm like there was a short so I unhooked it right away. The aux fan temp switch was unplugged at the time so no complications there.

I'm assuming these 2 problems are related but I can't figure out how.

One more test: With the high speed relay pulled the aux fan comes on at low setting with either side of the temp switch jumped and with the low speed relay pulled it will run at high speed with either of the positions jumped. Something is not right here.

lunchkrispy · « **Reply #10 on:** June 30, 2012, 08:41:11 PM »

Solved!
High and low speed temp switch wires were shorted together somewhere. Pulling them around trying to find an obvious short took care of it for now so I'll wait and see if it happens again before I go cutting open that whole section of harness. After that was back to normal I soldered in the second radio shack diode (nice that they come as a two pack) and now low speed is coming on with the ac switch like it's supposed to. Here's a link to the diode I ended up using: http://www.radioshack.com/product/index.jsp?productId=2036268&filterName=Type&filterValue=Diodes Part # 1N4001 If it fails in the future I'll update here but I think it should be fine.

DesktopDave · « **Reply #11 on:** July 01, 2012, 08:33:04 AM »

Glad you got it solved...that's an odd one - BMW harnesses are usually pretty tough.

Thanks for the link as well. I'll bookmark that for future reference.

lunchkrispy · « **Reply #12 on:** July 01, 2012, 11:46:47 PM »

It's definitely weird to have a short between two wires instead of just something grounded out against the body or something. I was surprised, this car has been taken care of well and the parts of the wiring harness I could see all looked in perfect shape. A couple days earlier I had left it parked outside in the most torrential rain storm I have ever seen and maybe that did it.