944 S2 ABS Fault

[Guest]

I have a 1989 S2, for the last 200 miles the ABS has been engaging at slow speed braking accompanied by the sound of a
pump running, although under heavy braking the brakes function as normal, today however the ABS light on the
instrument panel is illuminated constantly. I've spoken to several Mechanics(not Porsche specialists) who have no ideas
as to the problem.
Has anyone had a similar experience and can throw some light on things for me, or can advise me on the implications
of replacing the ABS unit...Like where is it? would be good start.
This is my first Porsche,I haven't had it long so we're still getting aquainted.
I'd be very Grateful for any advise.
Mark England

 

[944Turbo]

The ABS ECU is in the drivers footwell behind the carpet (near the boot release) the pump is in the front wheel arch behind the liner. I would start by cleaning up the sensors and checking the continuity of the wires - they have sockets near the hubs, and again on the top inner wings at the front, and near the torsion bars at the rear there are also earth points in these areas that will probably benefit from cleaning.
Tony

 

[Guest]

Tony, Many thanks for the info, I'll work my way through your suggestions. Just to throw a spanner into works, after I'd posted my question the ABS worked fine, no warning lights or weird noises, I'm hopeful that this is an indication of a bad connection. Thanks again, I'll let you know how I get on

Mark

 

[Guest]

Tony, I found and cleaned all four wheel hub sensors and the various earth points you suggested,
unfortunately it had no effect on the initial problem. Although the earth points looked grubby
once dismantled it became obvious they were in pretty good shape.The drivers side rear sensor has
obviously been removed at some point and is quite battered, how it got that way I'm not sure.
My thinking is perhaps I have a faulty sensor, telling the ECU that a wheel has locked up, this is a stab
in the dark as I've only a limited understanding of the system. before I look at the Pump or ECU, thought about
contacting a breaker and getting hold of a front and rear replacement, substituting them in rotation in the hope
it might resolve the problem. Any thoughts would again be most welcome

Cheers Mark

 

[944Turbo]

Cleaning them up certainly wont have done any harm, but it is frustrating it made no difference to this problem.

It would be worth checking the cable continuity with a multimeter between the plugs and the sensors - it could be a broken wire especially if its good/bad. The suspension movement /steering will flex those cables.

At the front under the bonnet behind the struts there is a round assembly containing 2 plugs one is for ABS the other pad wear indicators. There is a similar one on the strut. I am not sure off the top of my head if one end is grounded or they make a circuit, some investigation with the multi meter on ohms should reveal what is going on.

At the rear the same style of connector is mounted on the back of the hub, a second one above the torqe tube.

So check you have continuity from the connector closest to ecu (above torque tube on inner wing) to the sensor and back to the connector or from the connector to earth dependant on if there are two wires from the sensor or one is grounded - sorry cant remember.

IIRC if you spin the wheel you will see the resistance change (but I could be talking rubbish) if you have access to an oscilloscope you might be able to establish further the quality of the signals.

Sorry it is over 2 years since I looked into this and on someone elses car. In that case it was a broken wire.

Good luck,
Tony

 

[Guest]

Tony, I'll enlist the help of my Dad(who knows his way around a multimeter) and go through the list you
suggested. I'll keep you informed, once again I very much appreciate your input.

Cheers Mark

 

[Guest]

Tony,
I've finally had the chance to get back to my ABS fault, I started by testing each sensor in turn with a multimeter,
three of them had a ohm reading of 1025 ish, the fourth,zero. I got hold of a used sensor from Porsch-apart in Ramsbottom(very impressed with the service) and although I've only been out for a quick spin, so far so good!

Hopefully that will have sorted it, so thanks again for your help

Cheers mark