Uncontrollable Heater Fan and/or Dead Battery

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Uncontrollable heater fan causes dead battery

Vehicles Affected

  • E39 5 Series
  • E46 3 Series
  • E53 X5


While the IHKA is turned on and AUTO mode deactivated, the blower fan operation is uncontrollable or will change speeds on its own. Occasionally no blower fan operation is encountered. The vehicle battery may also go dead intermittently. Also fan operation might be noticed when the vehicle is off and the key is removed from the ignition.


The final stage resistor for the blower fan motor is malfunctioning. The final stage resistor commonly overheats and will go open or internally short out. The final stage resistor is supplied battery voltage at all times. In some cases will cause a draw on the battery significant enough to drain the battery below starting capabilities.


The final stage resistor will require replacement. Typically it is easily accessible by removing the RH lower dash panel and is located in lower portion of the IHKA unit. On some vehicles for example, like the E46, the glovebox must be removed along with a stepper motor that controls a mixture flap. It is usually retained in the IHKA unit by a plastic tab, on some vehicles there are additional screws that are also used to hold the final stage resistor in the IHKA unit. Only the rear portion of the final stage resistor with the electrical connector is visible when it is installed in the vehicle.

A picture of the final stage resistor removed from the vehicle is shown below. The final stage resistor has been redesigned due to the high failure rate of the previous design. The new design is made with more smaller and shorter temperature sensing rods as compared to the old design with a few large temperature sensing rods with a tapered contour.

Replacement instructions for final stage unit here

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