I have got to the point on my restoration where the engine is running ok - bit lumpy at idle but under revs it seems smoother. I have just had the injectors serviced and the engine now seems to be running on all cylinders.

I drove it back on forth on my drive - there is a clunk when i put it into drive and then into reverse, which i am hoping is normal. I then took it from the drive round a 90 corner onto tarmac up a slope, at about 5 mph. There was a lot of juddering until i was round the corner, but it then went up the rest of the hill ok, changing up a gear.
I do have the error message when i start the car about the PSD not working. I was planning to ignore this until after i had got the MOT, as i was assuming that I could live without the limited-slip diff.
Apart from changing the oil in the diff (and the transmission), i haven't done any work on the transmission.

So a few questions:
a) in general should the transmission work ok even when the PSD is not working?
b) does the juddering I describe above sound like it relates to the diff, or could it be something else?
c) is there a description on Rennlist of a standard set of actions to service the PSD/get it working?

 

If you haven't serviced the transmission mounts, that may be contributing to some of the harsher clunk you're experiencing. Of course, you won't be able to feel a difference until after the work is done but I find that there's a bit of clunking on gear engagement on my car. It got better after I replaced the mounts but it's never completely disappeared.

As to the juddering, I don't think that would be affected by the PSD but I don't really know. I'd be more likely to chalk it up to a CV or wheel bearing issue but, hopefully, better wrenches will chime in.

Good luck, it sounds like things are moving along nicely.

Cheers

 

When the PSD is not functioniung it behaves like any other open diff would so not your issue even though it needs attending to. There is a procedure for flushing the PSD unit and that should be implemented every couple of years or so just as one would do the brakes.

You should also check that engine vacuum is getting through to the vacuum modulator located on the left side of the tranny looking forwards.

 

On the late model 928's, the first thing to fail in the transmission, is the reverse clutches. 80,000 mile failures are not uncommon.
A sign that they are "thin" is that there will be a delay when engaging reverse and when it does engage, it will be harsh.
Service the transmission and inspect the filter (Change it when the transmission is serviced.)
If there are "chunks" of paper like material in the pan or inside the filter, rebuilding sooner is way better than later.
Clutches are cheap.
Damaged "hard parts" are not.

The PSD systems are a huge problem. Porsche provided no "service interval" for the fluid. It's brake fluid and is very hydroscopic. Things worked pretty well, until the system absorbed too much water...then things got ugly in a big hurry.
The general problem with the PSD is the water in the system.
Damage runs the entire spectrum, from needing to clean out the old fluid (and cleaning the filter), filling the system and then bleeding, to the entire pump assembly being junk.
Start by checking the fluid level. If the fluid level is ok and there isn't rusty water in the bottom of the reservoir, listen to the pump when the key is first turned on.
If the pump runs, but will not shut off....that's a good sign.
If the pump doesn't run, at all, that's a bad sign.

 

My 84 has a very slight clunk when putting it into D or R but otherwise works perfectly, so am thinking that's kind of normal with the autos.

 

I have progressed some of this - the PSD controller is now functioning OK (having been rebuilt and primed). I suspect I have an issue with the vacuum system (as the car heater pushes out hot air regardless of setting) which might explain some sluggishness in the gearbox if the vacuum issue extends to the gearbox.

However, i still have this juddering from the back of the car when i exit my drive - tight 90% corner up a slope at 5 mph. On the straight, there is no juddering.

Does this mean my diff is shot? Or is the juddering likely to ease once the diff has been exercised a bit? Or is it more likely to be issues with the transmission mounts? Is there any way to isolate the issue?

I did one simple test: jacked the car up, put the trans in neutral. If I turned one wheel by hand, the other turned itself the same way. If I held one wheel, I was not able to turn the other one (I would have had to use a tool to apply more force). I don’t know if this is normal behaviour for a diff or it implies an issue.

 

you have a limited slip diff,
this is indicated by the opposite wheel turning the same direction as the one you turn.
NOTE in an open diff, one wheel will turn opposite direction to the one your turning.

NOTE the judder is the worn clutches slipping in the diff ,
you need to have the clutch pack rebuilt ,
or just drive straight
IOW they all do that with worn parts.

 

Did you ever solve this issue?

 

I am not sure - i haven't driven the car that much - and it now off the road again for winter - and i need to sort out a fuel weep from around the tank.

 

Ian,

FYI- Tried to send you a personal message but it bounced saying you are not taking such