Bill Ball

Some of you may have followed my recent leaky transmission misadventures
https://rennlist.com/forums/928-foru...ewed-yeah.html

That was resolved a week or so ago with one small remaining problem - I ended up with no speedometer. This was the result of swapping a 83-85 LSD into my 89 transaxle. Although any 83-89 LSD can be directly swapped into any other differential case in that MY range, there was a change, unknown to me and perhaps many others, in the configuration of the magnetic disc portion of the diff that signals the speedometer sensor mounted outside the back cover starting in 86. Along with that the rear covers had a change in the location of the well cast for the speedo sensor.

This all came to light toward the end of the earlier transmission misadventure thread. This problem is now resolved thanks to the assistance and generosity of Mark Anderson of 928 International who found and sent me an 83-85 cover.

Below are pics showing the inside of the two year ranges noting the 1cm shift in the well cast for the sensor.

There are a few other unrelated differences in the covers. The earlier cover has a drain plug near the lower edge. Although the boss for this is cast into the later cover, the drain was moved to the bottom of the diff housing and this boss was left undrilled. Note also the two round bosses above that. The upper boss is drilled and tapped for the fill plug for automatic transmissions. The boss below that is for manual transmissions. The covers for manuals and automatics are the same casting with different fill plug locations drilled out and tapped.

Again, thanks to Mark Anderson and 928 International for helping me end my transmission nightmare.

There is one rather obvious question I can't answer. How do you tell whether a donor LSD is 83-85 or 86-89? There is a difference in the magnetic disc but I don't have the parts any more to show that difference.

Also, note that since you have to pull the end bearings off both the original and donor diff to swap the original ring gear to the donor diff, this allows you to swap the magnetic discs as well. My Mercedes-trained transmission mechanic was not aware of the change in these discs and covers, and should not have been expected to know this as the differential components are purely Porsche. Once the 83-85 diff was installed with its older style magnetic disc and the trans was back in the car, it was far easier to just swap the cover, as I did.

WallyP

Good info, Bill. Thanks to both you and Mark!

IcemanG17

interesting.....very good to know!

Mark Anderson

This was a learning experience for me as well. 20+ years of selling 928 parts and still finding new info.

Bill Ball

Sterling:

You did a 5-speed swap, then swapped an LSD diff into that 5-speed. If the 5-speed was 86+ and the diff was 83-85 (or vice versa), then you know have the same problem I had. I looked at pictures on your website and the magnetic disc does look pretty far to the side, which would be consistent with 83-85, although I can't be certain. Regardless, I would determine which cover you have and swap to the other MY range. You can measure the dimension I show in the first picture on the outside of the cover. I just checked the outside of my 89 cover and the distance from the sensor outer edge to the center of the fill plug is approximately 7cm (essentially the same as when I measured it inside the cover). Early cover would be 8cm.

Dave928S

Valuable information Bill .. thanks for documenting and posting.

Bill Ball

Sterling: The issue I encountered is not related to the VERY EARLY LSD that has 8 discs. Those will not fit the later case at all without the adapters Carl sells. Carl claims those were sold 78-84. http://www.928motorsports.com/parts/...ip_adapter.php

I think Carl has the MY range incorrect. The LSD was 928.332.052.04 from 1978-82, then changed to 928.332.052.20 for 83-89, then went to PSD. The magnetic disc and cover changes were as I describe. I think any cover 1978-85 are the same in terms of the speedo sensor location and should fit any case. My information is that the covers and magnetic disc changed in 86 and that primarily involved the speedo sensor well location. The diff cover changed again in 91 when the speedo sensor was moved to one of the front wheel ABS units.

Anyway, if you lost speedo function after the tranny/LSD swap, this is almost certainly the problem. Sure it could be the sensor went bad or there is a break in the wiring somewhere. That's why I made up a little disc with magnets on it to spin past the sensor to verify the problem had to be inside the diff case. It's so likely to be the cover, I'd determine its MY range by measuring the sensor well offset and put on a different one - you will need a new paper gasket.

Vilhuer

AFAIK cover casting remained same for '91 MY expect late '91 MY covers might not have speed sensor mounting bolt thread made into cover at factory. '92-95 5sp GTS cover is totally different again due to cooling pump. '92-95 automatic cover is possibly still same casting as '91 MY but uses lower fill plug hole as diff oil quantity was reduced to prevent foaming.

John Speake

The Tech Info book for MY86 mentions that the drain plug was moved under the tranny from the cover due to the newer design of exhaust system. It was at this MY that the ATF reservois was moved to a new, much more awkward location...

Also mentioned under the manual gearbox section is that the rear cover "...is a new design. The location of rthe electronic speed transmitter for the speedometer is new. this cover is not interchangeable with older versions". From what Bill has found, it seems the new cover was also used on Autos ?

Vilhuer

As usual, info is available but its hidden deep in docs. 5sp and auto diff use same part number.

Landseer

Thank you Bill.

Glad your transmission is shifting well again, btw, excellent documentation.

Bill Ball

91 shows a different PN for the cover from 86, but only differing in the last digit. So, a small difference like that is likely. The PN is cast into the cover without the last 2 digits. For all the covers I have seen the manual and auto cover are from the same casting only having different fill holes drilled and tapped. The casting has an indication (AUTOMAT->) of where the fill hole for the automatic would have been if the trans were not manual.
Well, there it is..the smoking gun. Yes, the manual and automatics share the same casting, as I mention above.

Although the PNs in PET differ between manual and auto in the last 2 digits starting in 83, only the first 9 digits are cast into the part. So they are the same until the drill and tap step for the fill plug.

danglerb

I think the switch from old to new LSD occurred with the AT 3 spd to 4 spd change.

Bill Ball

Yep, that would be consistent with the 83 MY as I read it. This was followed by the change in the magnetic disc and cover in 86, for whatever reason.

Bill Ball

I posted this thread yesterday BEFORE I went out for a ride to test whether the speedo operation had returned. Just to finalize my experience, I'll report that, sure enough, it's back. So much for that problem. Thanks to everyone here, especially Mark Anderson, for helping me to resolve this so easily.