Aux cooling fan temperature switch

[skidpast]

I'm having a problem with my 1997 BMW Z3 6cyl overheating on hot days, been fighting this for nearly a year. Seems like the typical problem with the aux cooling fan just like many other folks have posted about, and I believe it started after draining the cooling system.


I've followed the instructions to bleed the cooling system, replaced the thermostat and replaced the relays. The fan runs in slow and fast speeds when jumpering pins on the temperature switch connector, and it runs when I turn on the AC. There's 14v at the temperature switch connector.


The aux cooling fan just never turns on. It seems like the thermostat isn't working properly.


Specs match on the old and new thermostats but the parts are stamped slightly different. Could I still have a problem with the thermostat or have the wrong part? Is there a way I can test the thermostat with an ohmmeter?


New thermostat is stamped "5617 18J MADE ITALY 91deg 99deg", the original is "978 073 BM/ 91deg/99deg"


Thanks!

[BimmerBreaker]

So a few things, you seem to be mixing up terminologies a bit. The thermostat is on the engine block, in the thermostat housing (the plastic thing the two radiator hoses feed into). The stock thermostat is 92 degrees C. It sounds like you are having issues with your fan switch - the 91/99 degree C piece mounted on the upper passenger side of the radiator.

The first thing to check on a Z3 at this age is the aux fan wiring. Some of them were wired incorrectly, which prevents the low speed mode on the fan from activating. There are ways to test this, many threads in the past about it. There are also relays to check for the modes of the fan, but check the wiring first.

You also said this started after draining the cooling system, so if thats true and you never had overheating issues before then, it is more likely to be something like insufficient bleeding of the cooling system (air trapped in the system) that is causing this issue. If this is the case, bleed the cooling system again. Fan on low, heat on max heat, start the car cold and crack the bleeder screw every minute or so as it warms up. At operating temp, rev the engine a bit, as soon as you let off the revs, crack the bleeder again until just coolant comes out. Once just coolant comes out, no air, close the bleed screw and leave it until the next morning. Check the coolant level the next morning when the engine is cold, make sure it is filled above the cold or "Kalt" fill line, but NOT all the way to the top.

If this doesnt fix it there are other steps to look at but I would start there first

[rf900rkw]

I suspect the answer will be found in "I followed the instructions to bleed". Any air in the system and the fan will not come on, because the hot coolant doesn't make it to the radiator.

[skidpast]

Thanks @bimmerbreaker. I tried bleeding again--more than once--no luck. I pulled the fan switch and tried it in hot water, I can tell it's working fine, it closes when expected. Also shorted the wires on the fan switch to make sure the fan works, that's good too, in both high and low speed. But when I have the switch in the radiator, it simply never switches on. I've tried measuring it with an ohmmeter when the temp is high and the switch simply doesn't close. Here's a look at the temp climbing as it was sitting idling in my driveway.

IMG_4280.jpg

Is there another way other than bleeding that would keep the coolant from reaching the switch and causing it to turn on?

Thanks for listening!

[JayZ3]

Quick experiment to perform to determine with certainty whether it is the radiator and coolant or the fan and wiring and relays: With the thermostatic switch removed from the radiator but connected to the harness and the key on (not running), dip the switch in a cup of 100 C water. If the fan turns on, the problem is in your liquid loop. If it doesn't, it is in your electrical loop.

[skidpast]

Definitely in the liquid loop. Fan turns on in hot cup of water.

[BimmerBreaker]

I suspect Randy may be right then. Ensure the car is bled properly first and foremost