Tom. M

The fluid for the differential gears are in a separated area in the AT's. Need heavy oil in the diff....AT fluid in the trans....

The manual has one fluid for all....(no separate area for the Diff fluid).

The PSD pump is a totally separate unit and uses brake fluid (IIRC)..and need a bosch hammer to properly flush (there is a manual method written up in the 928 OC Site)..

GregBBRD

Correct.

Which means, like the brake system, the fluid should have been changed every 2 years.

I routinely see vehicles where the fluid has never been touched.

Not too tough to figure out why the PSD system in those cars isn't working.

blown 87

I have also seen some pretty bad brake fluid.



This was out of one of the accumulator from one of two brake circuits on a Rolls. (System one or System two)

Bilal928S4

Can you point me to the 928OC section for manually changing the fluid in the PSD. I cannot find it.
Thanks
Bilal

Tom. M

Sorry...it was the old 928 OC publication.. 928 Journal... written by John Veninger and Louie Ott...

http://928oc.org/journal/psd1.pdf

JWise

Is this hose under pressure at all times, or is it unpressurized when the ignition is off? Just wondering how to safely change it. Would it also be a good idea to flush the system when changing this hose as to eliminate any air in the line?

ROG100

Greg & Mark,
If we are talking about the same hose it is readily available for less than $20.
I am probably going to get "flamed" - "banned" - excumunicated et al for asking.
Why should I buy your hose over the Porsche or OEM alternative??? If I change it every 10 years should it not be OK?
I will add that I will probably buy your hose anyway as it has GB or Mark next to its part number.
Just asking not being jerk.
Roger

Tails

Bilal,

No the PSD system is a total independent system with it own reservoir, pump, accumulator and solenoid valve that connects to the slave cylinder mounted on the side of the transaxle to lock the differential up to 100%. It receives signals from the ABS system wheel sensors, as well as from a centrifugal force sensor (G force) when cornering and it operating pressure is 18 bar and will lock up the diff intantenously and release. If you have Jim Moorehouse's Technical CD there is a full description of it operation with drawings, schematics etc.

The PSD system was introduced in MY90 and is probably the most neglected system on the late model cars, as there is nothing in the Owner Operational Manual describing where it is located and how you can check the brake fluid level in the reservoir. I took me quite a while to locate it when I first aquired my car. I do a complete fluid change and bleed when I do my brake system every 2 years. There is 2 write ups on how to bleed the system without using a Bosch Hammer or Theo's DT928. One is contained in Jim's technical CDs and the other is available on PCA web site. The system was initially only serviced by OPC (Official Porsche Centres) service departments, hence the reason it was neglected when the cars moved outside of the OPC servicing unless the technician was Porsche Trained on 928s.

Tails 1990 928S4 Auto.

Tails

To manually test the operation of the PDS diff lock, I would suggest that car is place on jack stands and the PSD system is pumped up to pressure and the solenoid valve is manually activated ( as described in the 2 manual bleed system mentioned in my previous post) and get someone to turn one wheel and the other wheel should turn in the opposite direction and when the diff lock is actuated by applying 12 volts to the solenoid valve the other wheel should turn in the same direction (IIRC). Do not keep the 12 volts to long to the solenoid valve.

If the hose is expanding under pressure the diff lock should operate as described in the previous paragraph, as it has no engine torque being applied to the wheels. If any doubt change of hose or better still, before changing the hose measure the outside diameter with no pressure and then again under full pressure to see whether the outside diameter changes (expands).

Tails 1990 928S4 Auto

Vilhuer

True that system do not know if slave piston moves or not. Still I think system will give error if it can't produce enough pressure.

Tom in Austin

Tails ... "operating pressure is 18 bar"? Really, is that significantly more than a brake line, doesn't sound like it. Where are the extreme/dangerous pressures in the PSD system, just in the accumulator?

GregBBRD

Actually, I've seen systems that have no fluid in them and the pump is running all the time, trying to build pressure. There is a pressure switch that turns on and off, when the system requires pressure, but that doesn't seem to "talk" to the electronics of the system.

GregBBRD

Great question....very valid.

The stock hose, in the PSD system, is just like the stock hoses, in the brake system. They all "expand" when pressure hits them. Install a high quality set of PTFE hoses, in the brake system, and anyone can immediately feel the difference in brake efficiency. The pedal is firmer and the "spongy" feel of the brake system is reduced. Stopping distances will be reduced....especially with any car that is equiped with ABS. (That's a whole different discussion, BTW.)

While there is no way to actually "feel" the PSD system with your foot, the same thing occurs there. The system responds to need and then sends pressure to the slave. The faster the slave can respond, the faster the "limited slip" can respond. If you remove the "rubber balloon" from the system, the slave immediately receives all the pressure that is sent to it, not a lower percentage of that pressure....just like what happens in the brake system.

This is actually very important for the PSD system, as the response time is very important. The faster the PSD can deliver the pressure, the faster the limited slip can respond, and that helps the car get through corners faster and more stable.

Many of the PSD systems are so slow, that the pressure needed to operate the slave (and thus the limited slip) doesn't arrive or arrives so late that the vehicle is completely through the corner before the limited slip can respond. Of course, the brain is then "seeing" reduced speed differential from the wheel speed sensors and is shutting off the "signal" to the pressure regulating valve.

The result of this is a limited slip that is virtually useless.

One way to get an idea on the efficiency of your PSD style limited slip is to drain the differential oil and see how black it is (although you can also be fooled by this test....again, another discussion). The black color is from the friction discs slipping excessively. While this can be caused from worn friction discs (there is an actual initial "preload" that is needed for the discs to properly function....and this "setting" can be so far off, from wear, that the "fork" that operates the limited slip has to travel too far to ever make the unit "lock"), many times the black color is caused by the discs slipping too much through corners...or having excessive lock time.

Hope this helps you understand this system, Roger, and how to improve it. Obviously, this system is a bit complex and requires many pieces to function correctly, in order to work near its potential. Unfortunately, many, if not most, of the PSD systems, have been one of those "maintance free" parts of these vehicles. Nothing could be farther from the truth. At this point in time, most of the PSD systems are so neglected, almost anything one does will improve the system. Flushing out the system, installing a new stock hose, and changing the filter is going to help. For those that drive their cars more aggressively through corners, a non-expanding pressure hose will reduce "lock time" and make the car faster and easier to drive. For those systems that are really neglected, it might be necessary to "remove" the "limited slip" unit, and disassemble it for inspection, cleaning, and set-up of the initial "preload" of the friction discs. Many times, the friction discs, especially in the vehicles that received the two "early" versions of friction discs, will be worn beyond use and require replacement.

ROG100

Greg,
Absolutely stunning answer and now makes total sense and I want two. Order on its way to Mark.
Thanks for taking the time to explain.
Best,
Roger

Bilal928S4

I am going to try and change the fluid manually this weekend. Where is the filter that requires changing? Roger, do you have this filter?