Drive Train

PSD Flushing

PSD Flushing

PSD - Positiv Sperr Differential. A microcomputer controlled locking differential found on later 928's

I'm servicing my '90 GT PSD unit with new fluid. I replaced the fluid in the reservoir and intended to activate the pump, and flush the new fluid through the system and out the bleed valve on the cylinder located on the upper right side of the diff. The pump motor runs when I manually close the relay contacts on the relay located in the spare tire well, but no fluid comes out the bleed screw opening. I took out the bleed screw and it is clear. A few drops of old fluid came out. I observed the fluid in the reservoir while the pump motor was running thinking I may see some fluid motion if the pump was bypassing fluid back into the reservoir through a pressure relief valve. I saw no fluid motion in the reservoir.

Is there another circuit that needs activated to get the pressurized fluid out of the accumulator and routed to the cylinder? When I manually close the contacts of the relay in the spare tire well, I can hear a relay down by the PSD unit click too.

Any suggestions for the next step? I am aware that the shop manual says to do this service with a Bosch device, but there has to be a way around that, and this car will never see the inside of a "real" shop again.


Louis Ott

From memory the shop manual procedure is quite involved and requires the use of the Bosch Hammer to command the PSD's computer to move the fluid about the various parts of the PSD. I really do not think that there is a way around using the Hammer.

My suggestion is to take it to a shop that has a Hammer and simply tell them that you will watch the service be performed, tell them why, and give them a copy of the procedure.

That's what I did last Fall when I had my PSD flushed. I had read far too many stories about people paying for PSD flushes and not getting them. The records from my Beast's POs indicated two PSD flushes but,like you noticed with yours, my RR fender liner had never once been removed.

I made the appointment, arrived at the appointed time, made it firmly but politely clear to the service manager that I expected to have access to the shop floor and that I expected to speak with the technician who would perform the work. When the technician came out of the shop to meet me I handed him a copy of the PSD flush procedure. When he saw that I was armed with a copy of the procedure and the actual shop manual volume his attitude went from sour to pleased - believe it or not. Halfway through the procedure I went out onto the floor, saw that the Hammer was connected and watched as black watery liquid crap was flushed out of my PSD.

This was at a bonafide Porsche dealer by the way.

Good luck.

-- David Chamberland
91 928GT (Amazonagrunmetallic: Green. NO! Blue! Ahhhhhhhhhhh!)

I figured out how to flush and bleed the PSD unit without the Bosch Hammer. It's not difficult. You need to energize a solenoid to release the fluid from the accumulator as well as operate the motor to pump up the accumulator with new fluid. I got about 1.5 cups of very dirty brown fluid from mine until the new fluid came through.

Does anyone know where the fluid will leak to if/when the cylinder on the diff case leaks? Into the differential?

Louis Ott
'90 GT

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