The auxiliary fan resistor is in need of replacement when the auxiliary fan does not operate at low speed. The aux fan should activate: When the AC is on (by way of selecting the AC button in the interior) 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 … Continue reading E30 Aux fan resistor replacement and the two different E30 auxiliary fan types