Porsche Enthusiasts Club Forum

Hi again,

Having been overcome by a momententarty lapse of reason I now own a rather nice looking 928 S4 (89). Now you're probably not surprised to hear it has a few faults. The ones i want to fix first (annoying me most) ar the ABS and tyre pressure warnings, both systems disable themselves after a few seconds of driving. I've checked/tested the ABS relay and fuses. I've looked at the connectors on each wheel and they seem fine too. The tyre pressure system does actually work for the few seconds before it goes off (I can change the pressure in a tyre and it will come up with an arrow and say PRESSURE LOSS, then a few seconds later TIREPRESSURE OFF. Are these system inter-related or does it look like two seperate faults. Would appreciate any suggestion on where to look next

Thanks

George

Hi Geo

That system was dogged by problems to the point were it was removed from the latter GTS models.
Silly question but does your car have standard wheels (slots).

Stephen

Hi,

It has the correct slot wheels with the pressure senders, and yes it does seem a rather complex system. It does appear to function correctly for the very short time before it deactivates.

Hi

The two systems are NOT connected...unfortunately for you....

Dont know much about the pressure sensors but do know a fair bit about the ABS system....

Tell me EXACTLY what it does or doesn't do...lights etc... ???

All the best Brett

That's a bit harsh Stephen!

The system was removed, but was generally reliable (I find mine very useful as a reminder to check pressures, and to identify any slow leak caused by a bad seal at wheel rim).

The main problem with it was to restrict choice of wheels to those with the (hugely expensive) sensors. If it had a functional problem it was that many owners found it was over-sensitive, and required pressures a couple of psi above normal to prevent an annoying warning. If they got fed up it takes a single wire link to disable the system.

I am curious about how the pressure sensors worked. Anyone?

Hi Adrian

Links here
http://en.wikipedia.org/wiki/Tire-press ... ing_system
http://www.tiresensor.info/
http://www.mobilecardoctorinc.com/tpms.html

Stephen

Thanks Stephen! Will read with interest - popping off to look at them now.

The most likely culprit is one of the ABS sensors - the ABS performs a self-check once the car is moving over a certain speed (which is why it doesn't give the warning immediately you start the car) and then switches itself off if it detects a problem.

The way to test the sensors is to measure the resistance between the wires for each sensor, either at the ABS unit itself or at the connectors, where the wiring enters the car (front of spare wheel well for the rear ones, never looked for the front myself but believe they're in the front wing) - if you get an infinite resistance, rather than an actual value, it suggests that the sensor has corroded and shorted to ground.

I'm sure you'll find a more comprehensive description if you search the usual 928 websites.

Fix the ABS problem first and you may find it also cures the tyre-pressure warning problem.

Hi

Thanks for the replies.

Brett it is as bmjp describes, no lights when the ignition is switched on, but after driving a very short distance (a matter of meters) the digi display says ANTILOCK OFF and the antilock warning light comes on and stays on.

Can I test the ABS sensors at the connector plug where it connects to the control unit in the footwell ?

Theoretically, yes you can. However you will need to know which pins on the (rather large) plug relate to which sensor, and I don't know if they're marked.

Hi

ALL the lights SHOULD be on with the ignition ONLY on (engine not started)

AND yes you can test at the ABS ECU ...you need to test between a PAIR of pins for each wheel....

The manual will tell you WHICH pair for WHICH wheel...they ARE numbered but the numbers have no relation to which wheel....

All the best Brett

Oh sorry I meant with the engine on, yes all the warning lights light up at position II, but the ANTI-LOCK goes out as soon as engine is started, then comes back on after I move off (after a few seconds)

I'm looking at this diagram and pin outs on page 44-34 of the 928 Workshop manual, I can only see one pin from each wheel speed sensor 8,26,28 and 29, I was going to use the ground on pin 13, is this no good ?

Sorry I don't know about abs pin-outs but I can point you to extra sources of info/advice. in addition to the valuable help on this forum.

You really should join the 928.org.uk list, as well as Rennlist. Both forums have many active members who make me feel very ignorant about the 928. I have a passing knowledge of some of the car's systems - you would have access to help from people who appear to know every nut and bolt on every model year!

Yes, I've been lurking on Rennlist for a bit, but hadn't joined, have now though.