Schocki

I was contacted by an independent Porsche shop from Germany about a very strange problem with the ABS. Here are the symptoms:

Ignition on, all warning lights come on as advertised. Start the engine and they all go out including ABS. Drive the car and everything is fine, BUT if you slam on the brakes ABS is not activated and no ABS light or also no fault code is set.
ABS und PSD share the same ECU housing and PSD works with no problems. Now to the things that have been checked and replaced:

1. all ABS related relais (including the two on the pump unit)
2. all power connections checked, all good
3. all ground connections checked, all good
4. all ABS-sensors checked with an oszi and also via KTS, all good
5. no fault codes in the ECU memory
6. power from the brake light switch to the ABS-ECU present (when pedal is depressed)
7. ABS can be activated via KTS
8. ABS-ECU and pump unit have been replaced
9. they even took the pump unit and ECU, removed them from the faulty car and installed it in another 928. They both work!?!?

Bottom line: for some reason the three valves that control the braking action are not activated. How does the system now when to work? I always thought that the signal from the brake light switch in combination with the ABS-sensors activates the system.
Obviously for some reason I'm wrong.....
I guess there is some sort of a wiring problem or bad contact.

My last recommendation was to remove one ABS-sensor at the front axle (keep it connected), secure it with a zip tie and drive. Just to see if the ECU turns on the ABS light and sets a fault code.
This way the PSD should be still working and without fault codes.

Any inputs on this are very welcome.... Below an overview of the signal inputs into the ECU.

RFJ

Not an expert here but perhaps there is an open wire somewhere on the feed to the brake sensor?

Schocki

What "brake sensor"????

Schocki

Update: some progress was made yesterday, but the problem still remains. They lifted the rear axle and put it into drive. PSD locked up 100% as it supposed to do. Then they slammed on the brakes, PSD was turned off and ABS started to work (pulsating brake pedal). This all happend with the front wheels securely on the ground and stationary.
Still if you drive it on the street: no ABS...

worf928

Could a previous technician have bypassed the front ABS sensors and spliced the signal from the rears to both front and rear ECU inputs?

Or in other words, I don't think you're looking at a 'normal' failure mode. Something has been 'done' to it.

I don't see - above - that contiuity from each sensor to the appropriate ECU pin has been checked?

Note also that the '89 has wiring that is unique to the '89.

ammonman

Perhaps the axles have been replaced with rebuilt early (IIRC) units with the incorrect number of rotor teeth on the sensing ring. Just spit-balling but the actuation with the rear wheels in the air and fronts stationary sounds like the system finally saw enough difference in wheel speed to activate. Just a guess.

Mike

Schocki

Hello Dave and thank you for your inputs,

the car has been at the shop for a couple of days now. You are absolutely right this is not a normal failure mode, just because the system fault code memory is clear, no errors.

The ABS-sensors are working, or better. providing the appropriate signals. Just because the RDK is still working.

As I mentioned before, the ECU and pump unit was tested in another car already and they work with no problem. I agree with you 100%, there is something wrong with the wiring.

I wrote another message to the shop and asked them to open the electric connector from the ABS wiring harness at the pump and to confirm that all contacts are present AND at the correct position.

This is a really tricky one....

Schocki

But if there would be a difference in teeth (90 and 45), wouldn't the system disconnect and set a fault code? The difference in pulses is quite large per wheel revolution in this case.

Found this here on the forum and the are very different, it just wouldn't work.

RFJ

I was thinking along these lines that a wire was broken or disconnected could be to the sensor or the abs brain or as was said wrong number of pluses .

mark kibort

what are those hubs? is the S4 '87ish on the right? I have that style

worf928

A completely random question: do the power side-view mirror adjustments work? With the engine running also? (I have a reason for asking this...)

When did this problem start?

What work was done to the S4 prior to observing this lack-of-ABS condition?

I would approach this by considering how I might deliberately cause this problem and see to which parts of the S4 it leads.

RDK working is insufficient. The RDK, ABS, and Brake Pad sensors are all discretely wired.

Theoretically, you could disconnect the front ABS sensors and split the signal from the rear ABS to go to both front and rear Control Unit pins thereby convincing the ABS Control Unit that all four wheels were turning.

In the absence of error codes from the Control Unit this seems to me where to start thinking about causes.

We assume - yes? - that the ABS computer will not throw a code if it thinks all four wheels are turning within 5%(?) of the same rate. Thus, if no codes are being thrown and ABS never engages with all four wheels on the ground even with the front tires skidding, then the computer must think all four wheels are turning at nearly the same speed?

I suggest removing the driver's side package tray, seat/mirror memory controller, whatever else is in the way and inspect the ABS/RDK/PSD harness from the firewall to the Control Units for damage or for 'funny' wiring.


I would also suggest disconnecting the ABS sensors (all four wheels) at the wheel hub-mounted connectors and checking continuity from each hub-side connector pin to the Control Unit pins.

EDIT: And also check for shorts across the 4 ABS circuits.

worf928

And one more: Are there any other non-working systems in this S4?

Schocki

Thank you very much for your inputs, I will go ahead and translate them into German and forward them to the shop.

IMHO I think somebody played wiring and messed with it.

Question: what has the mirror possibly to do with this?

These cars still amaze me, even after 14 years of 928 ownership and 24 years of Porsche experience....

worf928

On '89, and only '89, it is possible to connect the front ABS harness into the dash harness and the left-side door harness into the rest of the ABS harness. Nothing fries, but the side view mirror is in-op. ABS control unit stores no codes. (No idea if ABS works or not in this case; that '89 was brought to me in boxes and on a flatbed.)

I don't think this is the problem, but ...