The power stage that manages the power for the coolant fans in a
928S4, GT, and GTS is located on the front bumper, very close the
the refrigerant filter and high pressure port. So on the passenger
side (EU). The power stage 928.005.364.00
gets signals from the fan controller 928.618.418.00
The Fan controller has the task of taking signals from various
sensors and using that to send a signal to the Fan power stage. The
power stage has the task to efficiently manage the power that goes
to the fans without heating up too much, and sensing that a problem
with either fan is sensed and sent back tpo the fan controller box.
The controller box then turns one fan to max to compensate for the
(other) failed fan unit.
Well, the system is pretty well made and works perfectly, until
it breaks. It turns out that it is not at all easy to identify
broken components that make the fan power stage act crazy.
Typical failures are:
- Total lack of power to turn on the fans (presuming 12v is
available on the round connector of the power amp, and ground
connection is ok)
- One fan runs max as soon as the battery is connected
- Both fans run max as soon as the battery is connected
- One fan runs max and the other is silent. Sometimes it
suddenly runs on both, or the problem moves where the problem
reverses (so running fan gets silent and failed fan suddenly
runs full power.
We worked for almost two years to find proper solutions to all of
the problems that make the fan power stage fail. And we are
confident we can now repair these boxes so that the system works
The repair consists of finding the broken part, replacing and
testing. Since the repair is not a simple "swap-a-component" we need
to confirm proper operation by a test drive for a few days. So we
only offer a swap service with a core refund. Every repaired power
stage is repaired professionally, closed so that water can not enter
anymore, new rubber seals placed on the main connector and carries a
one year warranty.