x50type

I have searched Rennlist but can't locate a method of testing the a/c air temperature mixing flap valve drive, located in the left and right foot wells.

I have pulled the part out and disconnected the drive unit. The flap is free to open and close so i want to test the drive unit. There is a 5 pin plug connected to it.

Any advice much appreciated.

4X4SCHE

There are two wires to the motor and three to the feedback potentiometer. See here.....

http://porsche964.co.uk/technical/servos.htm

ToreB

The easiest way to check the servo is to connect it to the car and turn the temperature **** up and down.
The flap should open and close accordingly.

You could test the motor inside by applying 12V to the correct pins, but the servo operation is also highly dependant on the potentiometer feedback. The latter is the main problem, the whiskers and PCB track wear out or burn. This is easy to see if you pop open the servo enclosure.
Details also here:
www.ccu.tore.bergvill.com
Cheers,
Tore

x50type

4X4SCHE - ToreB
Thanks for the excellent info and links.
Tested the mixing valve as suggested , clearly the flap is not moving - about to delve into the drive unit.
Will let you know the results - probably the springy contacts and/or the printed circuit have gone!

x50type

Pulled the Hella drive unit [Part # 993 624 325 00] apart - overheating evident on the spring contacts and the PCB contact area is damaged.

The motor and the rest of the mechanism is great and looks like it will last for ever. The tiny electric motor works great. The spring contacts can probably be cleaned up and used again, but the weak spot is the PCB.

A new control unit is about $300 - and of course, the separate parts are not available from Porsche. There are 5 similar drive units in the HVAC system; it might be a nice little side business for some one with the technical knowledge and skill to produce just the PCB - Rennlist Special $125........................?

davesnothere

I had all heat, all the time on the driver's side, and of course the culprit was a dead servo (with similarly fried electronics to the above). I couldn't revive the potentiometer circuitry with contact cleaner and a brushing, so I swapped the driver and passenger side assemblies, and moved the flap into the closed position on the broken unit.

So now I have no heat on the passenger side, but since I rarely drive the car in winter, no big deal.

I agree 100% that repairing these servo boards could be the next "odometergears" and a good side business for somebody.

- Dave

x50type

davesnothere

I tried to revive mine, as you mentioned but to no avail - the wear and grooves in the PCB being too bad.

Temporarily i have removed the drive unit, closed the flap valve by hand and re-installed the ducting; it's warm down here in the south, by the time it is cold enough to need the heater i will have saved my pennies [$326 + tax] for a new unit.

If only some one would could make the pcb.............................!!

mojorizing

I've looked at the same problem on the 964 servos. That's resistive paint on the PCB, and after years of rotating the shaft and moving the fingers across the surface, they just wear through. The paint has a particular ohms/length which is hard to duplicate. If the paint is in good shape along it's length, then shifting the fingers over (increase/decrease the radius where it touches) could make the fingers ride on good paint.

A lot of these servos will be NFG sooner then later, as these cars get older. I think I'm going to look at investing my 401K into owning a stockpile of new servos.....