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E30 Aux fan resistor replacement and the two different E30 auxiliary fan types

December 14, 2016

The auxiliary fan resistor is in need of replacement when the auxiliary fan does not operate at low speed.

The aux fan should activate:

  1. When the AC is on (by way of selecting the AC button in the interior)
  2. When the radiator coolant temperature sensor reaches sufficient temperature to trigger the aux fan

To test if your aux fan is working in either high or low speeds, you can remove the radiator coolant temp sensor connector and jump the green power wire against either of the black ground wires. One black wire will be for low speed, and the other will be for high speed.

On E30’s with older style radiators (1987 and prior), the sensor and coolant temp sensor is on the drivers side of the radiator. On 1988+ the sensor and plug is on the passenger side of the radiator.


As illustrated by the picture below, you jump the green wire against either of the two black/brown wires to manually activate the low and high speeds for the aux fan:

If your low speed does not work you will need to order a replacement new auxiliary fan resistor.

What most people do not know is that there were two different brands/types of auxiliary fans that came on E30’s. One type was made by Bosch and the other was made by Magna.

These two types use different types of resistors. So it is important to find out your auxiliary resistor type before ordering a replacement.

Even the BMW ETK/Realoem cannot tell you which auxiliary fan you have. The aux fan subpage will suggest both part numbers.


This is the Bosch style resistor (PN 17401373177):


This is the Magna style resistor now made by Siemens (PN 64121388069):


Because the resistors look drastically different, the best way to figure out what resistor type you have is to find the resistor on your E30 and visually confirm.

The resistor is accessed through the holes found on your front valance:


Replacement is as simple as it looks. The Bosch resistor is replaced by way of removing the screws on each terminal and reconnecting in the same fashion as removal.

The underside terminals can be seen here:


The Magna resistor on the other hand has to be soldered/crimped inline after cutting off the old one. The resistor is held on to the fan body by 1 philips screw. Cut the old resistor wires on both sides and crimp the new wires on:

2 Responses to E30 Aux fan resistor replacement and the two different E30 auxiliary fan types

  1. Hussam Jamil on August 21, 2019 at 1:28 pm

    I know this is an old post, but thank you. Just converted my E30 325i AC system from r12 to r134a (new parralell flow condensor, all hoses removed and cleaned, new o-rings, new dryer/receiver, new pressure switch and expansions valve).

    It blows super cold, but the aux fan never kicked in (though the compressor clutch does). After reading your guide, i quickly tested the resistor. Indeed it kicks in at high speed only, no low speed. New one will be purchased and installed, thank you.

  2. Jason on August 12, 2020 at 9:16 am

    i’m also late to this post but my auxiliary fan only comes on when commanded by the AC switch. New temp sensor, resistor yet still no fan activation when it comes to temp. Any advice? I’m about to start checking all the coolant temp sensor in the head too. Thanks!

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