UrS4boy

All vehicles need differentials on the driven sets of wheels because on a turn, the inside tires travel a shorter distance than the outside tires.

The Audi quattro system is full time four-wheel drive. Our quattro-equipped 20Vt cars have three differentials: front, middle and rear. The UrS4/S6s have a Torque-Sensing (TorSen) center differential that automatically mechanically shifts the front-back torque split from the normal 50:50 to up to 75:25 or 25:75 as the traction situation dicates. The 92-94 UrS4s and 95 (to June 95 build date) UrS6s, the S2s and the RS2s are equipped with an open differential at the front, a mechanical torque-sensing ("Torsen"), non-locking, middle differential and a lockable rear differential. The lockable rear differential that is used for starting out in otherwise wheel spinning conditions (snow, ice, sand, etc). This system disengages at 15 mph (25 kph). The later (95.5, 96 and 97) UrS6s are equipped with an open differential at the front, a mechanical torque-sensing ("Torsen"), non-locking, middle differential and an open rear differential that is partially controlled by the ABS system, not the lockable rear differential. Although this later system is called "Electronic Differential Lock" or "EDL", it does NOT lock the rear differential. It uses a more advanced ABS system to literally put the brakes on the wheel that is spinning (I think at any speed). Most people prefer the real lockable differential to get them out of bad traction conditions.

This post is about the lockable rear differential that is activated by a "DIFF" button on the centre console, as shown here:



But what happens when that "DIFF" button is pushed down?

First thing is, you should only push that button down when you are stopped and are either stuck, or think you are going to get stuck and you want all the driving power to both the rear wheels, with little or no spinning. That being the case, you can push the "DIFF" button down to engage the rear differential lock mechanism. IF you push it down while the rear wheels are spinning you can easily damage some gears in the rear differential. Not a good thing. This is what Audi says about the use of the rear "DIFF" switch.



So back to the story, you are stuck, or the weather or road conditions are very slippery and you are not going to travel at more than 15 mph or 25 km/h. You push the "DIFF" switch down. This sends a signal to the 893919173 rear differential lock controller that is located under the rear seat.

Here is a schemematic of the connection between the "DIFF" switch and the differential lock controller:


Here is the wiring harness details:



Here is the 893919173 rear differential lock controller under a UrS4 rear seat (my Emerald on Ecru 93 UrS4).


Here is a close-up of the 893919173 Rear Diff control module. Note the three hoses and the 6 pin connector that are attached to it.



Assuming that the diff lock controller is happy that there are no differentially spinning rear wheels (and the speed is under the controller limit), it will send a signal to the ABS system that the ABS controller uses to turn off ABS. In addition, it will use the yellow/blue-striped vacuum lines to push/pull the rear diff lock actuator to engage/disengage the rear diff lock. Here are some of the mechanics involved:





Here is a shot of Deephouse's (Brian C's) rear diff from his 92 Spec UrS4 parts car, showing the vacuum lines (cut) and the push/pull diff lock actuator:



Here are the internal mechanical bits that make it work.



Once you have a look at the above, it is clear why the left side rear diff seal has to be SO big, it has to deal with the larger drive flange with the diff lock teeth, something the right side drive flange does NOT have. The later UrS6 (95.5, 96 and 97) rear diffs do not have this.

Here are some photos of the "guts" of the rear diff showing the Item 20 locking coupling and the Item 21 shift lock gears:


Photo courtesy of Andy10v on the S2Forum


Photo courtesy of Andy10v on the S2Forum

Here is the Item 16 left rear flange shaft with the integral diff lock gear. Note the non-uniformity of the length of the gear teeth in some sections:


Base photo courtesy of Yodasfro(Austin)

As I understand it, the shift coupling (item 20) is normally parked fully over the teeth on the left rear drive flange (Item 16). When the lock acutator pushes the Item 13 shift fork away from the centreline of the vehicle, the shift fork pivots, pulling the Item 20 shift coupling partially off the teeth on the Item 16 flange and onto the teeth of Item 21, the shift gear that is attached to the rear differential and thereby, locking the left drive flange to the rear differential housing, over-riding the internal rear differential gears, locking the two rear drive flanges (and axels) together. No limited slip clutches here folks, this is the real deal: locked rear diff (albeit, temporary and controlled).

None of this explains why the teeth on the Item 16 drive flange have those shorter teeth sections. Must help the engagement/disengagement.

I have combined the photos from Yodasfro and Andy10v into this new info:

"Here is an attempt to show how Items 16, 20 and 21 schematically work together(obviously Item 20 would have to be installed at the same time as Item 16 is installed into the differential housing gear set):"



Here are some great Shift Lock Coupling photos courtesy of 4WheelDrifter on this forum:

Unlocked Position (from the front of the car looking to the back):



Locked Position(from the front of the car looking to the back):




Not that I am an advocate of this, but, I am told that *IF* you cut the white wire with the blue stripe that goes into the controller, you can use the rear diff lock at straight line speeds more than 15 mph (25 kph) because the controller will not receive the speed-related disengage signal from the Pin T55/50 of the ECU. This can be useful when you are at a drag racing track. However, if you did cut that wire, you have to remember to manually disengage the DIFF switch afterwards otherwise you risk damage to the rear diff. People have different experiences with manually disengaging the diff lock when they have cut (or added a switch) to the white/blue speed wire. However, overall consensus is the diff lock will not disengage even with the DIFF switch OFF unless you are off the power and the car is slowing down (presumably taking the stress off the DIFF lock gears in the rear differential).

On a side note, starting with the 96 model year, i.e. cars built in July 1995 and later, including the US "95.5" UrS6s, the mechanical diff lock was dropped from the UrS6. Instead, "EDL" aka "electronic differential lock" system was introduced. EDL uses the next generation of ABS to apply the brake to the spinning rear wheel and thereby provide a form of differential control. However, it is NOT a lock. Some people like it, others prefer the mechanical rear diff lock systems.

YMMV.

Thefeek

Thanks Dave. That reminds me of when I rebuilt the rear diff last year. And those aren't pleasant memories, reference the home made seal installation tool that set the whole thing in motion. sigh,,,

UrS4boy

Did you happen to take any photos that you can share here or offline that I could add to that post? I am particularly interested how Item 20 Shift Coupling meshes with the Item 16 hub flange and the Item 21 shift gear.

Thefeek

Sorry Dave. I think out of shame I just plowed thru the job. I remember the gears though are exact to your diagram. Two gears splined to a shaft and a fork that slides one onto the other. My plastic manifold that actuates the diff lock was cracked, but Audi wanted $135 for it so I epoxied instead. Straight forward job, but a PITA.

UrS4boy

Shame??

I did get some photos from a fellow on the S2Forum and I have added them to the original post. (see above). I not sure what better photos I would have wanted. I guess the Item 16 hub. Anyway, thanks for the reply. (And good jerb with the epoxy)

UrS4boy

Got one from Austin (yodasfro):

Here is the Item 16 left rear flange shaft with the integral diff lock gear. Note the non-uniformity of the length of the gear teeth in some sections:


Base photo courtesy of Yodasfro(Austin)

Thefeek

The shame I mention is from sticking my home made seal installation tool into the diff and ending up taking the whole assembly out of the car to work on it on the bench.

Search the thread: Rear Diff,,,how screwed am I?

Things worked out appropriately. The reason for the seal failure was overheating from low oil level that also scarred a bearing race. The rebuild was called for.

UrS4boy

Combined the photos from Andy10v and Yodasfro into this diagram/photo and added the following text to the post above:

"Here is an attempt to show how Items 16, 20 and 21 schematically work together(obviously Item 20 would have to be installed at the same time as Item 16 is installed into the differential housing gear set):"

UrS4boy

Adding this:

"Here are some great Shift Lock Coupling photos courtesy of 4WheelDrifter on this forum:

Unlocked Position (from the front of the car looking to the back):



Locked Position(from the front of the car looking to the back):


"