ABS, PSD, RDK Warning
 

Hi Guys
My 90MY S4 Auto has had an intermittent ASB - OFF & Differential Control - OFF warning that is now permanent and is now joined by TIRE PRESSURE - OFF
The warning comes on at engine start but before the wheels turn.
I know that the PSD is disabled if the ABS fails but I can't find any relationship between these two and the RDK.
Does anybody know of any common component that could cause all three to go off simultaneously?
As it was previously intermittent I don't think it is a fuse and as the speedo is OK the left front wheel speed sensor functioning.
I have checked the relay, fuse, and earth and other connections in the location of the PSD and all looks good.
Any pointers as what to check next?
Regards
Stuart Waite
Shropshire - UK

I would check all of the wiring harnesses at each wheel and where they connect to the main harness. The harness at each wheel contains the wiring for the ABS wheel speed sensor, the RDK sensor, and the brake pad wear sensor. The two front wheels harnesses connect to the main car harness in the engine compartment on the inside of the fender inner panel where they can get plenty of heat from the exhaust manifolds. The connectors get really brittle. I have already had to replace one on my 1990. Also check the condition of the grounds for the wheel harness and where the harness passes through the body. There is supposed to be a rubber grommet made onto the front harnesses to protect their passage through the body. If these grommets have disintegrated the wiring could be worn through causing a signal short.

Post back with your findings.

Mike

The ABS and the RDK are separate systems (ECU's) but the harness is common at each wheel end. I like Mike's suggestion of checking each wheel's harness to see if there is any chaffing or evidence of bad wires.

I suppose they could be run by a common fuse. If the fues has blown, that would take both out, but I'm not sure which fuse it would be on a '90 or if they are even fused at the CE panel.

Will shavings on the ABS sensors also cause the RDK warning or could it just be coincidental to the ABS/PSD warnings?

No... RDK computer is seperate from the ABS/PSD ECU.

In addition to checking the sensors
 

Check your battery voltage at crank.

When a bunch of diagnostic warnings go off it may also be a symptom of a failing battery - too low of voltage at crank can cause the modules to see out of bound data that throws diagnostic warnings. Another way to tell when the voltage is too low is if your board computer "forgets" all the temporary data.

When this happens you may need to plug in a "hammer" (Bosch KTS300) diagnostic tool and clear all the warnings.

Even the RDK relies on the ABS wheel speed sensors to know if the wheels are turning.
Typically when the ABS sensors fail due to corrosion in the surrounding aluminium crushing them, they experience a short circuit to ground which causes an "earth loop" which causes electrical interference, which the ECUs see as a signal that the wheel is turning when it isn't.
In other words, an ABS sensor can cause all three faults.
Check each ABS sensor for continuity to ground, there shouldn't be any.

Stuart,

The ABS, PDS and RDK are interconnected and this can be confirmed by the Information Technick Booklet for the 89 S4 & GT IIRC. A very extensive explanation can be obtained from the 87 booklet and the 89 Information booklets. I have make a hard copy of each of these booklets from MY 87 through to 90 and I have all the information from the inception of the S4 in on file, very handy reference manual.

To easiest way to ascertain where the fault is to get the use of a Bosch "Hammer" diagnostic tester or Theo's DT928 diagnostic tool and run fault findings on the ABS, PSD and RDK systems to check the speed sensors for the ABS and PSD systems otherwise you will have to check all sensors etc manually using DMM etc., by using the test plan for the EZK-Ignition and LH-Jetronic '87 manual WKD 493 921 and the LH-EZK-Diagnostic \/ Troubleshooting booklet of the 928 S4 & GT contained in the WSMs.

Unfortunately the JDS Spanner does not cover the PSD, IIRC. You can check this out with John.

As a start I would check the speed sensors on each wheel and clean the heads, check for continunity and connections within the plugs. As previously said the sensors can corrode and fail. I would also clean the earth connections for these sensors. Each speed wheel will need to be cleaned as well. Brake cleaner will work here.

You can also then clean up the RDK HF sensors (transmittors and receivers) in situ, check for continunity and plug connections. As all these systems are interconnected on the later S4s and GTs I would recommend that you recommission your RDK system. There is a two stage setup procedure to ensure that the pressure transmittors are correctly setup and working correctly.

You can then also check the relay, fuses and harnesses to verify everything is in order. This will be a time consuming procedure compared to running a diagnostic set up which will pinpoint the problem/s.

Have you carried out a service on the PSD unit to verify it is working correctly, that is changing the brake fluid and bleed the accumulator and the slave cylinder?

All the information, booklets, manuals, WSMs and wiring diagrams are contained in Jim Moorehouse's Technical CD that are available from Roger (928sRus) at a very reasonable cost. If you have not already obtained these CD, they will be one of the best investments you can make as a DIYer on your 928.

Keep us informed and we can help you as you progress.

I have just done all the servicing on these systems and all are now working as designed with no faults registered.

Tails 1990 928S4 auto