Auxiliary fan, fuse 25...new resistor...still no go

[mancini_mark]

Hey guys,

I just replaced my aux fan resistor (Siemens/Magna) on my '90 525 M20, since the aux fan wouldn't run, and fuse 25 kept blowing. I tested the old one, and sure enough, no resistance at all. So, I got the new one in, and it still won't turn. fuse 25 blows again.

What's next on the list? Control switch on side of rad? Relays?

Thanks,

Mark

[Tiger]

Trace the wire... look for short. What is the resistance on the fan unit itself (disconnected from car)?

[DaveVoorhis]

Does the aux fan turn easily? If it's seized or tight, the fuse will blow instantly.

When you say you "tested the old one," do you mean the old resistor? If the resistor shorts out internally (unlikely), the fan will simply run at full speed and the fuse will be fine. If the resistor is open (i.e., infinite resistance), the fan won't run but the fuse won't blow, either. If you've got a cheap ohmmeter, a good resistor will show as almost 0 ohms, because it's a 0.5 ohm resistor. Of course, if the resistor shorts to ground, it could blow the fuse.

Your most likely culprit is the fan motor, but it's worth checking the wiring, control switch and relays too. Wiring diagrams can be found at http://shark.armchair.mb.ca/~dave/BMW

[mancini_mark]

So...is that fan supposed to spin free? I can't turn it by hand.

I didn't check the fan resistance, but sounds like I may not need to if it's ceased. I checked the resistance of the new and old resistors. My father has a pretty decent multi-meter. The new one showed resistance, the old one didn't, plus it looked like it was leaking an acid of some sort.

Thanks,

Mark

[Tiger]

Yes, the fan is supposed to spin freely.

[DaveVoorhis]

So...is that fan supposed to spin free? I can't turn it by hand.
The bearings are seized. You need a new fan. When it seizes, the fuse will blow and it sometimes takes out the resistor, too.


I checked the resistance of the new and old resistors. My father has a pretty decent multi-meter. The new one showed resistance, the old one didn't, plus it looked like it was leaking an acid of some sort.

By no resistance, I assume you mean infinite resistance, or infinity ohms. No resistance usually means unmeasurably close to zero ohms. Assuming you mean infinite resistance, the old resistor is burned out. There's no acid in it -- it probably melted before it popped. Those things get pretty hot under normal use; a seized fan would only make it get hotter.

[Jehu]

Is this fan inside the dash ? If it is what delivers cold air when you're at idle i think i have an issue too. As i was stuck in traffic recently for 40 minutes the ac didn't deliver as it does when driving.

[DaveVoorhis]

Is this fan inside the dash ?
No, he means the electric fan in front of the radiator.

[mikell]

You may have the same problem, even though the fan is in front of the radiator - if the aux fan doesn't work (it should be on slow speed any time the a/c is on) you don't get enough airflow over the condenser when stopped in traffic, so the a/c quits blowing cold. If you get cold air when cruising, but not in traffic, check the fan - you can see it through the nostrils. If it doesn't turn with the a/c on, the Bently tells how to check it.