Re: This should help (archive)

Simple tip about blower systems on vehicles. If the blower knob clicks each time you move it, it usually means the system has a resistor that matches each step in the fan speed.Manual or semi-manual systems use this, requires that the passengers run the blower speed.

If the knob or lever moves freely with no resistance, it generally means that a transistor or several are used to allow variable speed from 0 to full, or close to it. Most automatic climate controls will use this set-up as a ECU can turn the blower on as needed. You couldn't do that with a resistor pack as easily.
BMW Sevens use the 2 packs, one in the top of each plastic blower housing on the old' E-23s to run all blower speeds except High, which is run by a relay mounted next to the blower motor.
High speed gets run for the "Defrost" mode by the "Control Unit"[which is at the lower left of the center console/heaterbox] or by you when you turn the thumbwheel all the way to full speed.

When you start to lose speed control or you only have High speed, then usually one or both of the transistors have gone out. Seems like one goes out to start with and takes the other one with it.
I've only had good repairs when these parts are replace in pairs. Would come back to haunt you if you try just one.

While you're in there, replace the A/C relay and the High Speed relay while you're at it. Fairly common 4-pin relays if I remember right. They're all getting old and worn, and you don't want to pull the front cowl too many times. Too easy to damage the vent vacuum motors mounted underneath or their plumbing.

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