Has anybody tried to upgrade the ABS system?

[s2_wot]

My 95 S2 coupe has the same ABS valve system as most S2's which is a 4 Sensor / 3 Modulator (4S/3M) system with a pressure restriction to the rear axle.
There is 1S/1M for each of the front wheels and 2S/1M for the rear axle.

If either rear wheel locks up the ABS will release both rear wheels and then apply pressure again. The restriction must be there to prevent lockup at the rear because the 4S/3M system is not ideal and could cause instability under heavy cornering & braking.

The s2 Avant's are set up with the same restriction IIRC.
The urS4 I'm breaking at the moment has the same 4S/3M system but the restriction is not identical to the s2 setup. The restriction is a girling part located over the rear axle from what I can see.

My old urS6 had a 4S/4M system and from memory I don't think it had the restriction installed. This car had EDL to apply brakes to an individual wheel if required.

With the more "modern" cars like b5 a4/s4 they have a 4S/4M system installed. Has anybody tried to shoehorn this setup into an s2? Was it possible to rewrite the ABS ECU software?

The main benefit would be full force braking to both rear wheels irrespective of whether the either had gone light, causing it to skid and come under the control of the ABS valve.

All input welcome.

[Error404]

Can you not just install the URs6 set-up wholesale i.e. fit URs6 pump, loom & ecu and add separate brake lines to the rear axle?

[s2_wot]

I honestly don't know Error, has it been done previously?

[johne]

It is my understanding that our cars were fitted with a 3 channel system rather than a 4 channel one because of the Torsen permanent 4 wheel drive hence the single rear circuit.

I can't explain it, it's just what I've read.

John

[Mikes2]

But the Ur-S6 also had Torsen

[s2_wot]

The 3 channel system was cheaper 20 years ago. Now nothing has a 3 channel system. In any case the urS6 has the same drivetrain as does the b5 A4 / S4 an they got the 4 channel system.

Is the EDL system (urS6) better than the b5 A4 system or does the b5 A4 system also have the EDL?

I must take some photos tomorrow.

[Mikes2]

From 1995 onwards, no new model Audi was available with a driver controlled lockable rear diff - it was all done by EDL. The S2/RS2 were allowed to continue until the end of production with the lockable diff

[A1QSHIP]

Abs

Can you not just install the URs6 set-up wholesale i.e. fit URs6 pump, loom & ecu and add separate brake lines to the rear axle?

Physically it is possible. Use the harness for the URs6 & add an additional brake line. What you need to consider is that the ABS is calibrated for the vehicle dimensions, i.e. weight & size. The engineers that designed it made it for that sized vehicle with those inherent vehicle dynamics. You may have to calibrate it for whatever vehicle you install it on. How one would go about that is beyond my abilities. Then again it is probably close enough to work without issue. My .02 cents.
Chris

[s2_wot]

Would anybody like to give any reason why the 4 channel ABS system cannot be safely used with a lockable rear diff?

That's a valid point Chris, The software will certainly take account of centre of gravity, wheel base and axle loading. Anybody ever hacked into this software?

[s2_wot]

Photos of the ABS valves I have access to:

B5 A4 quattro (Manual without locking diff) (4S/4M)


B5 Passat (2wd) (4S/4M)


C4 S4 quattro (Manual with locking diff) (4S/3M)


ABY S2 Coupe (manual with locking diff) (4S/3M)


Images of the load sensing valve at the rear of the urS4. This valve is broken, it normally has an arm that connects it to the suspension.

[A1QSHIP]

[QUOTE=s2_wot;662726]Would anybody like to give any reason why the 4 channel ABS system cannot be safely used with a lockable rear diff?

With the lockable rear diff feature of the URs4 & Coupe Quattro Audi saw fit to turn off the ABS whenever the rear diff was locked. This was done because with the rear diff locked both rear wheels spin at the same rate. ABS intervention on the rear of the vehicle during this time could cause an instability problem i.e.,put you in to the ditch or wall. Audi also incorporated a feature to shut off the lockable rear diff when the vehicle reached a predetermined speed of about 15mph. Not sure of the exact speed. Again to ensure vehicle stability.
With the 4-channel of the URs6 ABS you have some control over rear wheel spin at low speeds via the Electronic Locking Differential feature of the ABS as fitted to this model. If you are going to fit the newer style ABS to a vehicle with a lockable rear differential consider turning it off when the diff is locked.
What I did on my URs6 was to add the torsen rear differential from a V8 & an ABS off switch from a URs4. Here in Kentucky we get a lot of freezing rain & ice on the roads & ABS is s***e for stopping on ice. In these situations I feel more in control during braking with the ABS off. Weather that is really true could be debatable.

[s2_wot]

Circuit diagram for the back axle. If anybody can enlighten me as to the purpose of the girling part I'd be obliged