I don't think it is a "switch" of any kind causing this effect. Your Aux Fax is controlled by the ECU. The ECU gets signals from several places, including ambient coolant temperature. When the AC unit is on, the ECU knows this, and activates the fan accordingly. When the AC unit is off, I don't believe any other action, other than an over-heat condition, turns on the fan. I believe your '97 had the 4 pin connector (Power, 3 speeds). Newer models only had power + one pin (variable speed). In either case, I don't believe you should be seeing any action from the Aux Fan under "lower" temp conditions, especially with the AC off, unless you actually have an over-heat condition or a bad temp sensor.

larrym3711
2001 540iA Sport

Sorry, my car is a 1999 528it, I thought I updated my info. My engine temp guage is reading normal, starts cold and goes to 12oclock and stays there. I have dont a recent overhaul of the cooling system and have filled it with BMW coolant. I checked the coolant this morning when the car was cold and the level was between the min and max. On my E36 there was a electric switch that plugged into the radiator that triggered the AUX fan. I know this because I used a lower temp switch when I did the fan delete mod.

Possible that there's still air in the system. Unlock the cluster and monitor the KTMP.

If you decide to bleed again, follow this from the TIS:

It's very difficult to remove the air in the cooling system of the six. Try this procedure from the TIS. It assumes you have already topped up the system. You may have to do this 2-3 times. Some even jack up the front of the car by a foot or so.

TIS M54 Bleed Procedure.

To ensure the cooling system is properly vented, it is absolutely essential to follow the steps below:

1. Undo cap but do not remove from expansion tank. This prevent excessive discharge of coolant from the expansion tank.

2. Turn on ignition.

3. Set heating controller to max. temperature.

4. Set blower to low level. This opens the heating valves and sets the auxiliary water pump in operation.

5. Run engine and briefly press accelerator pedal three to four times (approx. 4500 to 5000 rpm) to flush engine cooling circuit.

6. In doing so, do not run engine for longer than approx. 30 seconds; otherwise coolant will heat up and expand.

7. If the coolant level in the expansion tank drops in the process, top up the expansion tank to the max. cold fill level.

8. Screw on cap tightly and allow engine to warm up until thermostat opens.

9. Check coolant level. Pay attention to differing expansion tank designs.

Transparent expansion tank:
Allow engine to cool down before checking coolant level. Coolant temperature must not exceed 30 deg C. If ambient temperature exceeds 30 deg C, allow engine to cool down at least to ambient temperature.

Check coolant level and if necessary top up coolant to max. cold fill level (see marking on expansion tank).

Note:
The tank mark indicates the fluid level at approx. 20 deg C. Use recommended coolant only.

Black expansion tank:

Allow engine to cool down before checking coolant level. Coolant temperature must not exceed 30 deg C. If ambient temperature exceeds 30 deg C, allow engine to cool down at least to ambient temperature.

Check coolant level and if necessary top up coolant to max. cold fill level (the ball on the float needle is on the same level as the top edge of the expansion tank).

Note:
The tank mark indicates the fluid level at approx. 20 deg C. Use recommended coolant only.

I drove myself crazy trying to figure out the Aux. fan when I first bought my car. The fan ran all of the time. I first discovered the temp sensor on the bottom radiator hose was not plugged in. When I connected it my fan would not turn on at all. Finally, I took it to an indy who does BMWs. He plugged it into his super computer and immediately found a code missing that controls the fan in the ECU. He reset the codes and my fan now works as it should. Looking over the service record I learned that the PO had replaced the ECU. So, I'm guessing it came improperly coded. But, rather than deal with the problem as I did and spend the money on diagnosis, the PO simply unplugged the temp sensor. With that said, what I'm getting at is that a defective temp sensor might cause the fan to come on before it is needed to cool things. My aux fan does not come on as soon as I turn on the AC but only after it is hot enough to be needed.

I have the low cluster, can I still unlock it? I dont beleive that I have air trapped in the system, but will attempt to bleed the system again to be sure. When I sealed everything up fluid was oozing out of the bleeder screw and was no longer allowing air out. I did recently replace the top idler roller (water pump/ alternator) and had to remove the top hose from the radiator so I suppose air could have entered then, although I thought I did a good job of bleeding the system. The fan doesnt always run, it just turns on after a small amount of driving, but I never start to overheat. Thanks for the replys everyone

Mine seems to come on and off as the ktmp bounces back and fourth from 109 to 110.

This is my first and only observation
After recently bleeding the cooling system so don't know if it's normal.

V-8

I would bet the fan clutch starts failing.
The only way to know is if you unlock the OBC. Had same issue last year.

Will do, I will unlock it when I get home and take some readings. How can I tell if the Clutch fan is starting to go? it doesnt spin freely, and I can hear it wooshing after sitting at a red light on a hot day when I take off, and once I get moving it disengages again.

The way you describe it how it works, it's normal - it should work like that. However, do the OBC test #7 readings. If temps get to 98°C and stay there for extended periods of time, the fan clutch doesn't move enough air. If the temps creep even higher, the fan clutch is definetly toast.
I'm no expert in this, but that's how mine went.

Well at least I have a area to inspect now. I will report back with my findings.