stevedavies

Join Date: Jun 2009
Location: Wales, UK
Posts: 163 Anti lock warning

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Had the anti lock warning light appear after approx 100yards of driving yesterday. Digital display alternates between 'antilock off' and 'differential control off'. Checked wheel sensor resistance at ABS/PSD ECU and all 4 check out at approx 970 ohms.
Drove car this morning. At start up ( no movement of car) no anti lock warning but PSD light on. Also can hear noises from rear wheel well ( passenger side RHD). Pressing brake pedal, PSD light goes out, take foot off brake, PSD light returns. Drive car PSD light goes out, after 100 yards anti lock light comes on.
I was hoping it was going to be something simple like bad connection to wheel sensor, not so hopeful now!
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Steve D
Euro 1991 S4 Auto RHD
Mercedes SL280 1999 Auto
Saab 9-3 2007 Convertible Manual

stevedavies

Forgot to add, brake pedal felt different, 'wooden' at top of travel, but then ok

stevedavies

Just checked fuse and ABS relay in CEF and both ok. Anti lock light came on much earlier this time, after 20 yards or so.
PSD light behaved this time. May have been pressurising itself earlier, hence the noise? Same problem with Anti lock warning light

stevedavies

Took all 4 wheels off and checked sensor wires. All ok and reading very similar resistance. Topped up PSD reservoir while I was in there. Anti lock light still returns after small distance of travel?? Anybody have problems with the ABS/PSD ECU??

jon928se

re-check the resistance across each pair of sensor wires at the ABS ECU plug. Then check for resistance to ground between a good ground point and each individual sensor wire.
PSD uses the ABS sensors for wheel rotation information. ABS sensors most commonly fail with an internal short to ground. This doesn't change the resistance across a pair of sensor wires but it does prevent any output from the sensors.

I suspect youh ave both poor connectivity between sensor and ECU and a sensor failure. Solve the connectivity issue first. Then jack up the car, turn the ignition on and spin each wheel in turn - ign off then on again and try a different wheeel. The wheel that doesn't cause the ABS warning to light is the one with a dud sensor.

stevedavies

Jon, I think you nailed it. Just rechecked the ABS ECU. All 4 sensors show similar resitance, but one of the rear sensor wires is shorted to ground. I owe you a beer!

Now to try and get it off!

wg928

I followed this post, very good, very good indeed.........invaluable source of experience and knowledge here!

stevedavies

I agree, wouldn't be able to run this car without this forum!