Question: how to test aux fan switch

Does anyone know the operation of the auxilary fan switch?

I refer to the one in the lower radiator hose that controls the radiator cooling fan. 2000 328i, manual trans, a/c.

I'm assuming it's open when cold, closed when hot, can someone confirm, and does anyone know the temperature where it should close. Due to the term "switch," I'm assuming it's not a thermistor or similar analog part - confirmation of that assumption would also be appreciated!

My fan is not working. Problem is either with the fan controller (fan runs on 12vdc), the switch, or something in between. I'd rather test than shotgun parts at the car.

Alternatively, does anyone know of it's safe to short the plug that goes into the fan switch to test?


Thanks in advance,
--Soren

I'm kind of in the same boat. I was wondering if the sensor on the hose was a sensor/switch or just a sensor and/or how exactly it is related to the aux fan.

I jumped 12v on the fan from the batt posts in the hood. Fan works. I found that of the three pins, the lower left and the top right (bottom right is no pin) are the points to power the fan. If the polarity is reversed it doesn't matter since it'll just blow the opposite way. Make sure you use a heavier gauge wire so it doesn't melt in your hand. ha

The computer uses several inputs to decide when to turn the fan on, including the coolant temp (lower radiator hose sensor), calculated catalyst temp, speed, battery voltage, and calc'd a/c pressure.

Originally Posted by Sansho

The computer uses several inputs to decide when to turn the fan on, including the coolant temp (lower radiator hose sensor), calculated catalyst temp, speed, battery voltage, and calc'd a/c pressure.

I have no doubt you're correct.

I also have no doubt that if the radiator hose indicates too hot, the fan comes on; that's really the point where additional cooling is needed regardless of what's going on with the cat, a/c, etc. Not cooling at that point risks the engine.

As far as I can determine, the three pins going to the fan controller are: +12v, gnd, and a pwm signal that controls the fan speed.

Unfortunately, I've recently renewed my hate-hate relationship with Chilton repair manuals. Their "advice" is to remove the switch and have the dealer test it. That will probably only cost 2x the price of a new switch. Hardly the point of having a "shop" manual. I guess I'm supposed to be glad they didn't just say "take the broken car to the dealer." What can I say? I've been spoiled by having the factory porsche manual (all SIX binder) for my 911. I figured even a chilton manual would tell you how to test a switch.

The rad hose switch (assuming it's a switch) probably is open cold and closed hot, but I'd like to know before I start jumpering things.

Anyone know?



--Soren