BMW E39 climate control blower and resistor pack (hedgehog)
The E39 blower does not run or works only on some speeds, what is wrong?
The most common cause is a burnt-out resistor pack, also called the hedgehog or the blower's output stage. The fault shows as intermittent airflow, weak output or a missing speed. Replacing the resistor pack usually fixes the symptom. It is a cheap part.
How do I recognise a burnt-out resistor in an E39 built before 9/98?
The resistors are in front of the blower. The casing of a burnt-out resistor turns copper-coloured, while a working one is black. The slowest speed is always in use when the climate control is on, so its resistor burns out easily and the lowest speed stops working.
How does the blower of an E39 after 9/98 differ from the older one?
After 9/98 the blower is PWM-controlled, i.e. the engine control regulates it with a pulse signal, and it cannot be made to run with simple wiring. The blower has three wires. If the blower does not run, measure the supply and ground at the connector. Often the whole blower is replaced.
What causes blower humming, squealing or seizing?
The blower should turn easily by hand. Stiff turning indicates a worn bearing, and a seized blower is replaced. Humming and uneven airflow disappeared after replacing the resistor pack. A squeal when cold that stops after the blower has run for a moment is not yet serious.