found this at:
http://www.systemsc.com/technical.htm....looking for clues:
Fan speed control in most climate control systems utilizes some form of a servo amplifier. The output
of the amplifier provides a variable ground which varies the fan motor current, as the input voltage
from the control head is varied. The motor current is sensed and provides feedback to stabilize the
actual motor speed based on the selected motor speed in the control head. The fan speed control
unit is usually placed within the fan blower housing to provide cooling for it. In some applications,
the fan speed control unit may have a large heatsink and be attached to the body, e.g. the Porsche
964/993. Because of the fan motor current (about 10 amps or more) and the power (25 watts or
more) being dissipated by the amplifier, the fan motor amplifier may fail keeping the fan motor from
turning off.