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Subject: Aux fan troubleshooting Author: jlatker1 as Jeff '84 733i 5-Speed : member since April, 2004 : 289 posts Posted on: 2004-04-26 12:21:20

Here is what I've found out so far. Please offer corrections if you know I've made a mistake. My fix turned out to be the ceramic resistor.

The aux fan comes on "normal" speed when the engine operating temperature exceeds 196 degrees F (91 degrees C). The Aux fan runs at high speed when the engine operating temperature exceeds 210 degrees F(99 degrees C). The aux fan comes on high-speed automatically when the A/C is turned on. Whenever the aux fan is on high-speed it bypasses the normal speed blower resistor.

There are two sensors on the right side of the radiator. The normal speed temperature switch is the sensor on top. The high speed temperature sensor is on the bottom.

There is a .6 ohm normal speed blower ceramic resistor located behind the kidney grill on the frame of the aux fan.

If no aux fan at all:

Check the fuses.
fuse #3(15A)-Normal speed; fuse #18(30A)-High speed

Check ground sources. Those you'll have to look for by following the wiring harness.

Short the leads to the normal speed temperature sensor switch. If the fan starts to run then replace the sensor.

Short the leads on the high speed temperature sensor. If the fan starts to run, replace the high speed tempreature sensor.

With the engine cold, check Relay K1-Normal speed relay-by shorting the leads of the normal speed temperature switch and swapping Relay K1 with relay K7. They are the same type of relay. If the fan works then you've got a bad relay. If it doesn't then the normal speed blower resistor is bad.

With the engine cold, check Relay K6-high speed relay-by shorting the leads of the high speed temperature switch and swapping with relay K6 with relay K7. If the fan starts to work, then you've got a bad relay.

Normal speed temperature (91 C) switch P/N 61 31 1 364 272
High speed temperature (99 C) switch P/N 61 31 1 364 273
Relay K1/K6
Normal speed blower relay P/N 17 40 1 373 177

Hope this helps you guys out a bit.

• Aux fan troubleshooting - Jeff '84 733i 5-Speed 2004-04-26 12:21:20 (6737 views)
  • Good post! (nt) - DavidC 2004-04-26 13:11:32 (676 views)