cambria

I need to replace the magnetic disk in the differential that drives the speedometer sensor. Can I remove the differential from the transmission on a AT without removing the transmission, or do I need to drop the transaxle to do this swap. What are the pit falls in doing this job? I am aware of the shims that are on each side at the axles and keeping them together but any insight to this project would be appreciated.
TIA, Terry

Paul D

Terrence - I'm curious why you need to replace the carrier disk with magnets? I don't think you will be able to remove anything from inside the diff. without dropping the trans.

Bill Ball

The amount of obstruction varies by model year. In later models, the battery box and/or gas tank are usually too much of an obstruction to removing the innards in situ.

cambria

Paul, I had a cat fire 3 yrs ago, it 2 yrs to gut and rebuid the car. After getting the car back together, I didn't have a speedo. Went through all of the checks including the typical problems with the connector in the spare tire well and no speedo. I did some research on the effect of heat on magnets and pulled the back of the differential cover and checked the strenght of the magnets in the disk and found most to be very weak. the magnets would not operate the reed switch even with the switch next to the disk. I now have a new disk from Porsche and need to install the thing.
Bill, it looks like the pinion gear protrudes approximatly 1-2" from the back of the transmission wich maybe about the same as from the back of the differential to the gas tank. If no one has done this befor I will plan on dropping the transaxle. In my search it looked like BrendanC had some differential experience.
Thank you for your responses.

borland

I've setup the AT differential. You can do it without the WSM special tools, but requires careful consideration. I've also setup the pinion/ring gear backlash with the tranny installed in the car.

As you can see here, I removed the pinion nut (over 400 ft-lbs to remove) in order to perform an overhaul of the transmission. But for what your doing, you shouldn't have to do that unless you change the pinion and ring as a set.

borland
90' S4
Slate Metallic

cambria

Borland, thank you for your imput. To summarize, I can remove the differential from the transmission with the unit still in the car. I would think that the clearances would be the same in a 87 as they are in a 90 model. By the way, great photo's.

Jim bailey - 928 International

Does the new aluminum ring with magnets trip the switch ?

ibkevin

Good point!

Vilhuer

I would do shim adjustment by measuring and only use gear paste to check result. It can be done without expensive factory tools to level of accuracy which is enough to know ring and pinion are in right places.

cambria

Jim, The answer is NO. Now what? When I pass a magnet across the sender the reed switch closes. When I pass the new disk passed the sender, no signal.
I have line continuity from the sender to the speedo. When I jack up the rear wheel and turn it, I have no speed indication at the speedo. I have a new sender mounted on the rear cover of the differential. Do I pull the speedo and take it to a speedo shop? I am now at a loss as what to look at next.
Sterling and Krkka, since the shims already match the gear sets, couldn't I just re install them on the same sides or once the ring gear is removed this changes the adjustment?
Kevin, ( edit ), your Dad.
Thanks to all for the responses.

ibkevin

Apologies,

I thought Big Jim's advice was an important diagnostic step, no offense intended.

cambria

Kevin, no apology needed, It was ment in fun. I agree Jim was right and I was the dumb a$$ for not testing it first.

borland

Sounds like a weak sensor and not the magnetic carrier. But if you take everything apart and reassemble it with the new magnetic carrier, keeping all shims the same positions. It should be fine.

You shouldn't have to do this, but you can remove and reshim both side plates (for gear backlash adjustment) without removing the differential housing from the transmission or the transmission from the car. But I have never removed the differential from the differential housing with the transmission in the car.

With the matched gear set (pinion & ring), the pairs are serialized. They also have been stamped with backlash and pinion distance settings which were determined at the time of manufacture for minimum wear and noise.

With these inscribed settings for a matched pair, it is unnecessary to inspect wear patterns using gear paste, but does provide an extra measure of quality, as Erkka does to confirm. The WSM procedure only shows setup using the inscribed markings.

With a unmatched set, you must use the gear paste in order to inspect the gear mesh wear pattern and make necessary adjustments. This type gear setup would be much easier performed with the tranny removed from the car.

jpitman2

Suspect is now the reed switch itself? I can imaging heat close to that could terminate it, but if a bare magnet trips it....
Easy and cheap fix? IIRC Porsche used similar on 924, 944,968 etc?
jp 83 Euro S AT 50k

ibkevin

Damnit Guy's, this is what makes for a great Dad!


Now back to our scheduled topic.