Tom. M

I've not looked inside the PSD diff yet.. but my impression was that it was actuated by a release arm (much like the front clutch)...that is pushed by the PSD slave. How far the slave pushes the arm (and squeezes the discs together)...kind of determines the percentage of lock...from 0...to 100.

Reading what you posted above...makes it sound like the unit has a set "Lock" percentage and if the slave doesn't push far enough..it won't function?

I've got a psd diff tranny in my track car and have been thinking of ways to lock it at say 50 or 80 %.using a slave and handbrake style setup....My initial thoughts were to do this since I still drive the car to and from the track and didn't think that having the diff locked the whole time would be good for it..eg. cruise at 0 lock..and then depending on the track...set the slip...etc..

Rob Edwards

Bilal-

The filter is integral with the bottom of the fluid reservoir, just as it is in the power steering fluid reservoir. There was an "NLA scare" over PSD reservoirs a while back, dunno if that's still an issue.

Here's the pdf of the PSD bleed protocol, if I'm violating any 928OC or Ott/Veninger copyright, holler and I'll take it down.

https://webfiles.uci.edu/redwards/pu...0the%20PSD.pdf

ROG100

PSD Reservoir out of stock until Friday.

Price is $36.62.

Bilal928S4

Roger, How much?
Can a Motive brake system flusher? be used or this procedure has to be followed?
Bilal

Bilal928S4

What about the need to replace the line from the reservoir? Do these lines deteorate?
Bilal

Rob Edwards

The cap on the brake MC and that on the reservoir are different part #'s but I don't know if they're the same diameter. Nor am I sure that a Motive power bleeder cap would fit up in the fender even if it would screw on the res. You just end up pouring brake fluid repeatedly into the reservoir, I usually use the lid off a can of brake cleaner. To remove the reservoir you will have to cut off the oetiker clamps that hold on the blue hoses to the nipples on the reservoir. Given the rusty funk that is likely contaminating the 20 year old rubber hose's internal surface, I would replace it.

GregBBRD

Mark Anderson used the PSD unit, in his race car, for quite some time. Since he didn't have any of the hydraulics, in the car, we fabricated a plate where the slave sits, threaded this plate, inserted a big bolt and lock nut, and then "played" with the pressure on this bolt to change the amount of lock.

It never worked very well....

There were several problems. The first problem, which actually is part of the second problem, is that it takes very little "fork movement" (when the unit is properly set-up) to change the lock percentage. This little bit of movement was actually a part of the overall problem. We could set the lock percentage cold, but then it would change when the whole thing warmed up. Never could get it "balanced" and stay at the same setting, for more than a few laps. It is really a constantly changing target, apparently because of the temperature changes.

Your idea of using a handbrake affair to move the slave has merit...but you will need some way to tell how much pressure is being exerted on the discs to ever get this to work....and you will need to have a third hand to move the handle around to compensate. A "true" 80% limited slip absolutely sucks on any long corners...especially anything that has a decreasing radius (like most corners do). A true 40% limited slip is actually too much....20% is about as much as a good driver can "put up" with.

The PSD system, with some current technology, could be a really great addition to a track car. Imagine a limited slip that would increase the lock percentage according to how hard the car was slowing....that would be the ultimate limited slip! If the lock percentage went up as the car got deeper into a corner....that would keep both ends of the vehicle going in the desired direction. Talk about the ability to get "deep" into a corner under trail braking!

Yes, it is possible to have so much wear on the friction discs that the fork needs to travel a long way to "lock". I've seen this adjustment so far off that I'm certain the unit would never "lock".

When you "set-up" the initial pressure on these limited slips, you "turn" down the giant threaded "nut", while turning the internal discs until it takes 100NM to turn the unit. You then back off the giant nut and then retighten the nut until it takes 20NM to turn. The giant nut is then locked in that position.

Note that there are no "spring/tensioning" plates in the PSD limited slip, like there are. in "standard" Porsche limited slips. Thre is virtually no compensation for limited slip wear, because of the lack of these "tension" plates. This "lack" of automatic wear compensation was another problem when using this limited slip in a non-hydraulically controlled situation. The wear issue is a constantly changing variable.

I probably should go try and do some work and leave this "limited slip" stuff to you guys.

Tails

Tom,
The pressure is built up in the accumulator, so the pressure is stored from the outlet of the pump, in the accumulator and into the solenoid valve block, so when you work on the PSD system it is advisable to wear safety glasses to if you get a spray leak it can penetrate the skin, so extreme care is required when working on the unit.

When the signal is received to activate the PSD diff lock mechanism the solenoid valve is opened in the valve block and the high pressure is 'fired' into the slave cylinder activating the yoke and closes the friction pads and locks up the diff. The rear wheel speed sensors will activate and deactivate the PSD when it detects rear wheel slip or if there is excessive 'G' forces in a corner it will also activate the solenoid and activate the PSD diff lock mechanism.

+1 to GB description of operation, it is great.

Tails 1990 928S4 Auto.