FredR

Had some fun today- the local Porsche agents arranged a 75th Anniversary event and central to it was a time trial type of event so I entered the Classic section. My 928 has been out of action during the hot season but I had to do my annual registration irrespective of anything else so got the car ready, it was detailed at my friend's shop yesterday and then today's event.

For a while I have felt there was some kind of problem on the rear axle,a kind of snatching feeling as though the diff was fully locked. During today's event the rear axle was making all kinds of strange noises but I did not see or notice the PSD lamp coming on. Upon completion of my run the Porsche chaps visiting from Europe, although pleased to see the 928 performing well, commented on how they felt my diff as locked, possibly all the time.

This made me wonder what may be happening- my first thought was whether the PSD slave cylinder was sticking in the locked position. tomorrow I will jack the car up and try rotating one wheel- as I am aware the other wheel should turn in the other direction. If it turns in the same direction that would suggest lock up. Given I was not seeing the PSD lam illuminating I am assuming that the issue is not being caused by hydraulic pressure being falsely applied.

Much appreciate thoughts/suggestions regarding similar experiences.

JET951

Hi Fred , we have seen several 928S4/GT/GTS ( 1990-1995 year models ) where the PSD has leaked the PSD fluid ( brake fluid ) into the diff itself , this "brake fluid " will cause issues with the LSD friction plates , the early " minor " issues is a shuddering of the LSD , if its attended to quickly ( Diff apart ) and cleaning out what looks like a black goo in-between the plates ( that's the remains of the old brake fluid ) and once'd cleaned and reassembled with a new PSD slave cylinder and the important 928 332 051 05 protective boot ( this helps to stop the PSD fluid getting into the diff ) it will be fine

Now onto the ones that we worked on where the owner of the said 928 ( 1990-1995 year models ) where the PSD / brake fluid was allowed to stay in the diff for a longer period of time , now this is where it gets interesting , the PSD / brake fluid effects the friction plates and the friction material de-laminates and the LSD "friction pack " literally swells up and the diff locks up & with de-laminated friction plates , well its all too late & it will need new plates ( if you can get them ? ) or a 2nd hand 928S4/GT/GTS PSD diff center that has not suffered that fate

Note } You may not have what I described above, it may be something else ? , but if you do have what i described , then , get it apart as soon as possible

Note 2 } the 928 332 051 05 boot I mentioned , when old , there will be little remaining of the old one , once you see a new one you will know what I mean ( its not just a simple boot )

Regards
Bruce & Sean Buchanan
Buchanan Automotive

FredR

Hi Sean,

Excellent post that fits my experience as it were.

For some time I have noted a very occasional "judder" at very low speed and very light cornering as in like when turning into my driveway. I am feeling somewhat "red faced" for not having figured out sooner what was developing given my knowledge base and engineering background but "C'est la vie"! Yesterday was a completely different ball game and I very nearly pulled out of the event before going on track. The symptoms during the event were very concerning - a lot of clatter on a level I have never seen or heard of before. Upon completing the 3 lap event the Porsche chaps managing the event told me it sounded as though the diff was locked solid and then it dawned on me that the minor events I had noted earlier had by now developed into something more significant. Maybe having sat for the last few months has not helped matters?

I have noted the occasional PSD system "green warning light" come on and go out but not in relation to the occasional judder I experienced. That there is a potential migration path is what it is and by the sound of it pre-emptive replacement of that rubber thing sounds as though that might have been the order of the day so anyone who has a PSD system please take note of this before your diff experiences a similar fate.

As for recovery well for starters I have a complete transaxle unit from my late 90S4 sat on my garage floor that until the wreck ran perfectly but that was nigh on 20 years ago. A couple of years ago I pulled the pan just to take a look at the general condition and to my pleasant surprise in looked in great condition. To deploy this unit I would think as a minimum it would need new seals front and rear of the gearbox and new seals for the half shafts plus a change out of that rubber part you mentioned.

I suppose the diff unit could be unbolted and grafted onto the current tranny or maybe the clutch pack could be removed and redeployed. Presumably such actions would require re-shimming of the diff?

I remain puzzled as to how the diff could "occasionally" clatter the way it did. One imagines that the diff is either fine or it is not but it would seem that is just not the case.

FredR

So I just jacked the car up and put it on my home brew liftbars and upon rotating driver side wheel forwards the other wheel revolves in the same direction so diff system definitely screwed at the moment.

I changed the dif oil a couple of years ago with a high spec Mobil diff oil but it was not one formulated for PSD. There was nothing I could find in the documentation I have to specify any other PSD specific diff lube but we had some discussion about this. Whether this is a relevant factor opinions appreciated.

If hydraulic fluid has migrated into the diff then I would expect to see a level issue in the reservoir although how much has to leak to cause an issue remains to be seen. One also wonders if the rear seal on the tranny has failed allowing ATF to get into the diff but there is seemingly no "pressure differential" driver between the two cavities?

The only other test I can think of at the moment is to open the slave cylinder bleed nipple to test for hydraulic lock up however unlikely that may seem.

Mrmerlin

Fred with a LSD what you describe is exactly how the wheels should work,
turn one the opposite side will also turn in the same direction.

What you need to figure out is how much force is needed to break the diff free
Simply lifting both wheels off the ground wont tell you much besides that you have a LSD

FredR

Stan,

Thanks for the correction- fact is I have never had a problem with this part of the system- figured it was "bullett proof" providing the system is flushed every couple of years or so. Not even sure whether the clutch plates can be removed in-situ assuming that may be part of the issue. For sure something has suddenly got much worse and now I have to hope that there have been no secondary degradations as a consequence

At the moment what is puzzling me is if the brake fluid in the PSD system had migrated into the Diff then to cause a serious contamination issue quite an amount of fluid would have to pass into the diff [ or so I would think] but I have had no digi dash alarm warnings of any kind and even the green PSD advisory lamp has not been flashing up even during the time trial yesterday.

Tomorrow I hope to check if there is any residual pressure "trapped" and thus locking the PSD. I understand reasonably well how the system works but I know nothing about how the pressure is regulated. If the valve holding the pressure is not releasing it that could cause such an issue but whether such has ever been known to happen I know not.

FredR

Had a friend round today to give me a little assistance. I asked him to grab one of the rear wheels and as expected they were locked solid. Tommorow I intend to investigate whether they are locked because of the phenomena that Sean advised about as seems likely or whether hydraulic pressure is somehow trapped due to something in the system seizing.

Assuming the problem is swollen clutch plates presumably such takes up any slack that is available- is there any adjuster in that system that could be eased off to release the plates? I seem to remember there is a special tool that has to be used when setting the system up so if it is adjustable maybe the lock on the plates can be released- any thoughts anyone?

JET951

Hi Fred,
That was Bruce's advice up above. Unfortunately there is no adjuster to release the tension of the clutch packs when they swell per se as once they go that way they increase in size quite considerably and are have too much tension in the clutch basket area. We had one recently that had brake fluid entering via the PSD slave for some time and you can see by the pictures below that it also caused quite a bit of corrosion inside the diff housing, It was quite difficult to get the differential side bearing carriers off. Preload on a correctly measured clutch pack in set via the pressure plate finger housing (Pic 5)

On the differential below we were able to remove one friction material and one steel plate so that the car was still operational and without the lockup noise (with the PSD disconnected) while we waited for a good 2nd hand PSD unit to arrive.

You can see in pic 1 and 2, the delimitation of the friction material. pic 3 and 4 is the corrosion. Pic 6 was a second diff that we used good plates from to make a working unit
Regards
Sean

GregBBRD

Fred:
Amazingly enough, even you will miss me and my help!

FredR

Hi Sean,

Much appreciate the photos. Today I released the slave cylinder bleed nipple and nothing came out so "trapped pressure" goes out of the window. I suppose it is possible that the cylinder is jammed in the locked position but it is so cluttered in that location I do not fancy trying to test for thrust due to seized piston at the moment.

On a more amusing note I removed the slave cylinder from my spare transaxle unit and as the thing came off the bolts there was a pile of white powder! I removed the circlip and there was no sign whatsoever of the aluminium piston- it had completely corroded away- why I know not- it is not as though it was exposed to brake fluid. On the positive side the inner walls of the body were spotless and it should be viable if a new piston is available!

I then took a look at the PSD unit and the fluid level was just above the minimum mark. I cannot remember where the level was after I chaged it out- probably at the full mark line so possible I have some fugitive brake fluid but the maximum it can be would not amount to much at all- circa 20cm3 there or thereabouts I suspect. I was impressed by the bellows and the escape path for any brake fluid that escapes the piston.

To do any meaningful work in-situ on the diff I concluded that the exhaust, the half shafts and the fuel tank need to come out- tasks I have completed previously albeit the fuel tank is nearly full at the moment!

For now I will be departing for the UK so time to study the WSM section 39 covering the PSD system. When I return I intend to disasemble the diff on the spare unit to harvest clutch parts and get a feel for working on it. It does not look too complicated as long as one does not screw up the shim packs.

928cs

Is it this tool that you were thinking about?

https://rennlist.com/forums/928-foru...n-reality.html

FredR

Yes that is the tool. I should check with the local agents if they have such gathering dust somewhere. Hopefully I will not need such.

FredR

A brief update of my findings to date as of Wednesday:
1. I opened the slave vale bleed nipple just in case there was trapped pressure but nothing was found. So unless the slave cylinder piston has jammed/seized then I am inclined to think the scenario described by Bruce is in play.
2. I removed the protective shield in the rear wheel well to expose the PSD unit. The level was at low point of the acceptable range. I cannot remember where the level was when I last flushed the hydraulics but knowing my modus operandi it was probably at the maximum level so I am assuming a small amount of fluid loss may have taken place that does not amount to an awful lot- whether is has leaked into the diff housing I have no idea at the moment- needs further investigation when I return back home. I did not see any signs of brake fluid emanating from the pinhole that vents the chamber encapsulated by the bellows for whatever that may be worth.
3. I removed the diff cover from my spare transaxle unit and everything in there that I could see looked to be in excellent condition. Hopefully the clutch plate set will be in stellar condition for potential re-deployment- but...?
4. I need to bone up on dismantling the unit once it is out of the diff housing. Nice looking piece of kit I must say and that before considering it is 33 years old. The clutch tensioning system is very simple even if the calibration procedure is a bit "tricky". I understand clutch plates are NLA - doubtless there will be facilities that can prepare such at a price if needs be.
5. Next step when i return will be to drop the exhaust and the half shaft to create some working room and then to check what happens if the slave housing bolts are loosened.

FredR

I returned home last week after spending 4 weeks in the dismal UK climate with my daughters. Today I decided to take a look at the above problem once more and my next step is to release the slave cylinder from its mount to see if that shows any signs of unlocking the diff. A month ago I opened the bleed valve to see if there was any trapped pressure but not the case.

Once one has access to the bolts on the slave cylinder getting them undone is simple enough but it seemed to me that the passenger side exhaust needs to come off followed by the half shaft to enable appropriate access. Before I do that I will take another look to see if I can possibly avoid such. However i am now planning for the worst case. Just wondered what others have found if and when trying to get the PSD slave undone.

Whilst in the UK I picked up a neat looking electric impact wrench supposedly rated for about 650 ft lbs. Not sure I believe that but am open minded. It had the wheels nuts off in a flash and given that is its prime role was pleased with the performance. With the wheel removed I wondered if it might get the stub axle nut that is torqued to 340 ftlbs or whatever. I let it hammer away for about 10 seconds but no signs of movement. This made me wonder if and when attempting something like this how long should it take to release the thing- any thoughts?

Kevin in Atlanta

My Ryobi removes them in seconds.