Diff lock to ABS connection?

[Glen Chamberlain]

Can anyone help me out by telling me how the ABS unit knows the diff lock is engaged? I.e what pins on the abs ECU receive a pos or neg?

I have an abs fault which I am struggling with and I want to rule out the diff lock situation.

The relays and pump do not run on self test when turning the ignition on but all the feeds seem to be present at the Relays and ABS ECU. I get the feeling it is being deliberately disabled somehow.

MY 1995 S2 Avant without the dash mounted ABS disable switch.

Thanks!
Glen

[UrS4boy]

Diff Lock 101

Diff Lock 101 (also on the S2 forum):

http://forums.quattroworld.com/s4s6/msgs/170940.phtml

[Glen Chamberlain]

Thanks for that. I've looked at that drawing but I still can't work out where the connection from the diff control connects into the J104 ABS control unit?

[UrS4boy]

Thanks for that. I've looked at that drawing but I still can't work out where the connection from the diff control connects into the J104 ABS control unit?

I think the answer is because it doesn't. It apparently it connects to the J156 ABS Combi relay which in turn connects to the J104 ABS control module I could only find partial C4 examples of this;

Combi relay: Driver's kick panel (c4):


A diagram that might help (I couldn't find an equivalent B4 diagram): Looks like the E83 to J156 connection is a Black/yellow wire (at least for a C4)

[Glen Chamberlain]

Thanks for your help so far

I cant seem to see the J156 Combi Relay on the car or on the drawings that I have, however Ive found this post from Rusty http://www.s2forum.com/forum/showpos...6&postcount=19 that suggest Pin 13 on the J104 receives and earth when the diff lock engages. When the rain subsides a little ill be checking that out!

After sleeping on this I am wondering if I am jumping the gun a little here so let me explain the problem and what checks ive already carried out to see if someone can see something that I'm missing.

Symptom;
ABS light comes on with ignition and stays on all the time, it only goes off with the ignition off.

Checks carried out on the N55 ABS unit :

1. Relays on the N55 ABS unit (under the bonnet) DO NOT click on self test (ignition on) and the pump does not run.
2. Pins 87 on the main relays of the N55 ABS unit are both live with good battery voltage.
3. The earth strap for the N55 ABS unit is good (I fitted an additional one anyway)
4. Pins 86 of both relays on the N55 ABS unit are receiving battery voltage from Pin 17 of the ABS ECU.
5. Pins 85 for both relays on the N55 ABS unit I think should receive a negative from the ABS ECU on pins 27 & 28 (on self test) BUT THIS DOES NOT HAPPEN.

Checks carried out on the J104 ABS ECU:

1. Pin 1 receives good battery voltage
2. Pins 20 & 34 are both receiving good earth connections
3. Pin 25 receives a good footbrake positive equal to battery voltage
4. Pins 26 & 24 G44 Wheel Speed Sensor (1.15 ohms / 1150 ohms)
5. Pins 11 & 21 G45 Wheel Speed Sensor (1.14 ohms / 1140 ohms)
6. Pins 9 & 8 G46 Wheel Speed Sensor (1.15 ohms / 1150 ohms)
7. Pins 4 & 22 G47 Wheel Speed Sensor (1.14 ohms / 1140 ohms)

(I cant remember which range setting I used on the DMM but they were all almost identical)

Ive checked the loom resistance between the N55 ABS unit and the J104 ABS ECU

1. Pin 27 ABS ECU - Pin 1 / 85 N55 ABS Unit = 0.01 Ohms
2. Pin 28 ABS ECU - Pin 5 / 85 N55 ABS Unit = 0.01 Ohms

I also cannot connect to ABS ECU using VAG-COM or Snap-On Verus Pro. Both cant reach the controller? That said I have had previous issues in connecting to OLD VAG cars especially to the ABS ECUs.

I am starting to wonder if the ABS ECU could be faulty but 9/10 times in my experience ECUs are usually quite reliable, however I'm no expert on these systems.

???

[UrS4boy]

Sorry - can't help with the detailed inner workings of the ABS system.

However, I did find another wiring diagram on page 10/53 of this C4 wiring diagram set:

http://www.ziomal.itl.pl/motoryzacja/download/audi/Audi_A6_96_schematy.pdf

I extracted and annotated a section. Based on previous understanding of the Rear Diff lock system, I think how it works is you press the diff lock button down on the E121 diff lock switch. This sends signals/completes groundson the J187 Differential lock control which uses its vacuum lines to activate the rear diff lock. When that activation is complete the F100 rear diff lock switch confirms enagement and sends a signal to Pin 13 on the J104 ABS control module which in turn turns off ABS and activates the ABS off indicator light.

[Glen Chamberlain]

Thanks for your help. Your explanation is the same as my understanding. It's nice when some else comes up with the same explanation [emoji106]

I'm hopefully going to try another ABS ECU in the coming days if my pal has one. Fingers crossed![emoji848]

[UrS4boy]

Thanks for your help. Your explanation is the same as my understanding. It's nice when some else comes up with the same explanation
I'm hopefully going to try another ABS ECU in the coming days if my pal has one. Fingers crossed!

ABS ECU has to be from the same era as your's i.e. either pre-Electronic DIff lock or with Electronic DIff Lock. I think that is ABS 3 and ABS 4.

[domas]

believe me, i have spent too much time analyzing the system.
Unitl now, I have diagnosed following failure modes resulting in constant light on
Burned fuse in relay 76 - can happen when jump starting the car/other car/voltage spike
cold joints in relay 76 - resodlering the internals saved my abs in my 200 20v AND in another audi 100 sport (there were other faults as well though). But this did also result in no or very weak voltage in pin 1, which is not the case in your car.
and I have once discovered an open circuit from abs ecu to soleinoid valve but did not check it further as there was no access in engine bay to the abs pump

please let me know the following:

what is the abs control unit number/picture, OR check if pin 31 is present in the plug. If not, your ecu is not diag frindly and you need VAG1710 / bosch kdas0003 - i have one and no it is not for sale

and you did not specify what car/year you are talking about.

p.s. and check alternator voltage with engine running at some pin at ecu, you will find it in the schematic, it must be 14.X V

[UrS4boy]

please let me know the following:

what is the abs control unit number/picture, OR check if pin 31 is present in the plug. If not, your ecu is not diagnostic friendly and you need VAG1710 / bosch kdas0003 - i have one and no it is not for sale

and you did not specify what car/year you are talking about.

p.s. and check alternator voltage with engine running at some pin at ecu, you will find it in the schematic, it must be 14.X V

He stated MY 1995 S2 Avant without the dash mounted ABS disable switch in the initial post.

[domas]

somebody is really blind

[UrS4boy]

somebody is really blind

Speaking of blind where is "relay" 76? I assume that it is a double wide or double tall control module with an external fuse?

[domas]

at least in my car which as you might already know is 1990 200 20vt it is over driver knee panel/under steering wheel
a single size fused relay. if you compare it to a simple 214 relay, so yes it is a double tall.
but not double wide which is pwr window / sunroof relay for examble

[Glen Chamberlain]

Thanks for all your help so far guys!

ABS ECU Part Number is:
BOSCH 0 265 103 050
56300140
VAG 4A0 907 379s

1. Pin 31 is present but still no communication with VAG COM or Solus Pro
2. Voltage at Pin 1 is 14.2volts It is also a genuine positive and checks out using a test light as well as a DMM.
3. The earths on Pins 20 + 34 are also good genuine earth connections

Pin 13 Diff Connection
This shows a faint connection to earth all the time using a DMM. The DMM registers a circuit to earth but not a strong enough one to activate the "beep" on the continuity setting.
Once the diff lock engages the circuit becomes a good earth and activated the "beep" on the continuity setting.

I am going to try to remove pin 13 from the ABS ECU plug and re-test just incase the faint connection is enough to disable the ABS system.

[Glen Chamberlain]

I've removed pin 13 from the ABS ECU and it has made no difference, the issue still remains.