BMW Heat & AC Blower – High Speed Only
This answer is applicable for many BMWs in addition to the one listed below.
The fan blower only works on high and makes a sound as though it is hitting something. What’s the fix and how difficult will it be?
If your blower assembly sounds like it is hitting something, this may be due to worn blower shaft bearings and the blower fans are hitting the housing. You could also have a broken fan or a problem with the housing. The only way to know is to access the motor assembly. On your E30 chassis, this is done via an access panel at the top of the engine compartment firewall. Remove the panel and then you can remove the blower cover and see what may be hitting. The Bentley repair manual will detail this, if you need assistance.
When your heater / AC blower only works on the high speed, the fault is likely the blower resistor unit (blower final stage, on newer models). The blower resistor unit contains two or three resistive coils that lower the voltage to the blower motor, to give you the variable blower speeds (on later models that use the final stage units, these are electronic speed controls). These resistors can, and do, burn out (or the final stage units fail). When this happens, you will only have the high speed (which does not go through the resistors). The repair is to replace the resistor pack (or final stage unit).
NOTE FOR LATER MODEL FINAL STAGE UNITS:
BMW models that use the final stage units can also exhibit erratic blower operation … as if the blower motor had a mind of its own. This is actually more common than ending up with high speed only.
Some models can have different resistor packs and/or final stage units, such as:
E30 (3 series 84-91) – Can have two different types of resistior units. You must verify if the heater coolant tubes (that come through the firewall, into the engine compartment) are metal (silver-ish) or plastic (black). This holds true for any heater part on these models.
E36 (3 series 92-98) – Early models with the dial/analog controls use a resistor unit. Later modes with the push-button digital controls use a final stage unit.
E34 (5 series 89-96) & E32 (7 series 88-94) – may have either a resistor pack or a final stage unit. There were four different versions of heat/AC systems used in these models. Check our web-site or the Bentley repair manual for proper applications.
The Bentley repair manuals will detail the replacements of the various resistor and final stage units.
(click link and search for “blower resistor”):