Auxiliary fan wont turn on when idling

  • 1995 BMW M3
  • 155,000 MILES
With no fan on at slow speeds eventually the temperature gauge passes the halfway mark. Performed the following: Inspection of cooling system components, added coolant and bled the system. Top radiator hose seems to heat up as the car reaches operating temperature. Measured electrical voltage, voltage drop and continuity with a digital multimeter on related switches, relays, sensors. Replaced the auxiliary fan switch and all fuses. The fan only turns on if the A/C is turned on as well. After 45 minutes of idling the temperature gauge needle slowly started to move up pass the halfway mark but never allow it to reach the "red" zone. Again this only happens when prolonged idling (sitting in traffic or moving at 10-20 mph). This does not happen when going more than 40 mph or at higher speeds. Also the temp gauge needle does go back down to "normal"/ the middle of the gauge when the traffic moves from a standstill to speeds of 40 mph and seems to cool down faster with higher speeds. Currently using aluminum radiator from BEHR and thermostat too. These parts were replaced and installed by a BMW dealership. I believe the pumps too were replaced with ones with metal impellers. Can this problem cause the car not to pass smog certification? The check engine warning light is not on but before I added coolant a bled the system, I do get a warning from my on-board computer to check coolant level after I turn off the ignition and take the keys off. This warning no longer showed after adding coolant.
I am hoping to repair this problem without using my regular BMW shop due to financial restriction. Any advice will be appreciated.
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Sunday, October 30th, 2011 AT 2:05 AM

1 Reply

If the cooling fans are not working, the temperature would definitely go up at idle. When moving, the moving air is able to cool the radiator therefore the temperature migt not go up, at least not when runnig at high speeds.

Cooling fan is controlled by the Digital Motor Electronics (DME) control unit, located at front of radiator, and if there are any fault with it, the fans would not be commanded to come on when required.

Our database does not have any information on the testings and diagnostics of the system so I my assistance would be limited.
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Sunday, October 30th, 2011 AT 7:18 AM

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