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universeman's Avatar
 
Join Date: Oct 2014
Location: Lone Star State
Posts: 1,311
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ABS strange behavior

I know that I've got a problem with my ABS unit as it's either got a crusty ground or connection, or is failing - but I get an intermittent warning light over bumps in my 968 (92). Sometimes light flickers on and off, sometimes goes out entirely.

I'm planning to remove the RF wheel liner to check the unit (I think that's where it is, will double-check) but question I have for the team here is why would I get an electronic "assist/thump" sometimes when I hit the brake pedal under normal braking, appears the ABS unit is trying to do some correcting but isn't really doing anything smart. Any thoughts on what to check "while it's out?" Thanks!
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Jason - Austin, TX
82 911 SC targa (gone, but not forgotten)
92 968 coupe
Old 11-02-2019, 12:50 PM
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you might clean the tone rings and sensors. metallic dust on either can foul the signal generated.

tire pressures can effect rotational speed also
Old 11-03-2019, 04:56 AM
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You can measure the wheel sensor voltages at the ABS brain also. Most common issue IMO is sensor wiring getting flaky at the wheels.

It's usually in the front so maybe you can tell from the bumps what side it's on.
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1987 928S4
1992 968 cabrio
1994 968 cabrio - Supercharged
Old 11-04-2019, 04:18 AM
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the front sensor wires get twisted with every turn on the wheel and all four corners have the wire moving up and down with the wheel travel. I have gone through multiple wires over the years on various cars. Replacing.the lead is rather cheap
Old 11-04-2019, 01:25 PM
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Location: Lone Star State
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Pictures

Guys, got the wheels off to investigate. Looks like all my pad wear sensor leads are cut. I cleaned all the wheel-side grounds (was not too confident to do more) and pulled the RF wheelwell liner to assess the ABS module and clean its ground.

Still getting the intermittent light and power failure at speedo. Anybody have any ideas based on these images? Should I replace all these connectors with modern and try to reconnect brake wear sensors? What's the process for cleaning the ABS wheel speed sensors and rings? TIA

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Jason - Austin, TX
82 911 SC targa (gone, but not forgotten)
92 968 coupe
Old 11-13-2019, 05:05 PM
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