High Speed Voltage (archive)

I know that the most common cause for a lack of all but high speed blower operation is a failure of the final stage amplifiers in the blower motor housing, but I my problem has a different twist. At the same time that the variable speed stopped working, the defrost function stopped working as well. From examining the wiring diagrams, it appears that the high speed operation of the fan from the thumbweel control is done through the blower relay via the in-dash control board, but the high-speed operation for the defrost mode is done through the high-speed relay via the A/C control unit in the driver's footwell area. I know that the A/C control unit also controls the final stages for variable operation. Is this true?? And if so, do you think that the A/C control unit is the likely suspect??
Thanks for any input,
Patrick
'84 733i
P.S. In the event that it is the final stages that have blown, does anyone know where to find a list of components needed to rebuild them?? I know I've seen directions before, but I haven't been able to find them in the archives.
Patrick, I have read your statement a few times and reviewed the electrical drawings and generally will have to agree with your statement. But before I would be suspect of the AC control unit I would find out if there is 12 volts at the final stage amps. You will have to remove the cowling and then unplug the wire to the final amp and measure to see if there is 12 volts at that location. If the final stage amps are working then you might have to look at the AC control module.

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