Lockable Rear Differential Info for the S2, RS2, 92-94 UrS4 and 95 UrS6.

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. Our quattro-equipped 20Vt cars have three differentials: front, middle and rear. The 92-94 UrS4s and 95 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 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. 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 here

Photo courtesy of Andy10v here

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.

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 4Wheeldrifting 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.

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