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Problem ABS/PDAS on a 1991 964 Carrera 4

I own a 1991 964 Carrera 4 for over 16 years. The car is 60.000 Kms.

The car started perfect, about 20 seconds after start, all dash lights goes out, if I drive the car, after 700 meters (half a mile) at 10 Kms x hour (6 miles per hour) ABS/PDAS warning light goes on with an alarm.

If I press the console button, the alarm goes out, but light still on. (photo).

I just scan the car with a Durametric tool, and the program indicate no faults after scan.

I pressure bleed brakes + diff locks and I checked the ABS sensors as per Adrian Streather book, troubleshooting procedures, no results.

Any recommendation, highly appreciated.
Nice weekend to all. Francisco



Old 07-31-2020, 06:31 AM
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Have you checked the big multiconector on the PDAS unit in the trunk? Remove it, check for any water ingress and/or corroded pins. Possibly dry out , clean with WD 40 and refit. A bad connection here will cause warning lights.
Old 07-31-2020, 10:45 AM
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Thanks Paperboy I will check and clean the conector, this weekend.
Old 07-31-2020, 10:51 PM
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Once you cancel the alarm and the PDAS lights stay on, do the lights then go out of you blip the throttle past 3k rpm?
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Old 08-01-2020, 02:11 AM
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LM 964
I will check it tomorrow early morning. Right now we are close to 40 degrees centigrade in temperature.
Thanks for the suggestion, will let you know my finds.
Francisco
Old 08-01-2020, 04:59 AM
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Quote:
Originally Posted by LM964 View Post
Once you cancel the alarm and the PDAS lights stay on, do the lights then go out of you blip the throttle past 3k rpm?
I had a problem with general warning lights, not just the charging light when the alternator belt started failing and the electric system was occasinally undercharging.
Old 08-01-2020, 05:19 AM
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I had, or still have, the PDAS lights remaining on when starting the engine up. However, after a quick throttle blip to 3-4k the lights go out and stay out - until I stop / start the engine again.

Streather's book points out that for both the PDAS and ABS lights to remain on, a fairly common reason can be that a battery voltage of less than 9.9v being detected by the PDAS control unit. Maybe that's why when I blip the throttle the alternator charge increase is picked up by the control unit. I can't remember which pin, but a couple of years ago I traced each connector pin in the control unit using the circuit diagram to check the relevant battery voltage signal input. That pin was fine and so eliminated that potential issue (the connector pins having corrosion deposits).

What I have done the last few days (while my C4 is on the lift for other work) is to remove and clean each wheel speed sensor and replace the O ring seals - all sensors were all really bunged up with debris and dirt. Not sure, but this should help improve PDAS control unit wheel speed signal input consistency. I won't know for sure until I complete the other diff lock work I'm doing and get it back on the road.
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Old 08-02-2020, 02:02 AM
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Thanks to Paperboy and LM964 for the valuable advice. I cleaned with special " electronic contact cleaner" the main connector, checked the battery volts and tests at 3500 plus RPMs the problem still there. I feel, I need now to proceed and clean the speed sensors, to see if I can get rid of it.
Happy week end, and again thanks
Old 08-02-2020, 07:44 AM
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Good luck Turboman - one of those annoying issues when you get no fault readings.

The wheel speed sensor 6mm allen screws may be very corroded and hard to remove - very easy to round off the cheese head with the 5mm allen key. Something like Plusgas or similar soaked in for a couple of hours might help. I had to remove the brake disc dust plate to gain access to a rusted allen screw with a pair of mole (locking) grips. Once the sensor was out and cleaned up I checked the sensor ring gear with a light. Dirt and particles I vacuumed out while rotating the wheel hub. Replace old allen screws with new ones and Copper grease if you can.
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Old 08-03-2020, 12:45 AM
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As per LM964 recommendations I ordered O rings in order to service the "wheel speed sensors" Let you all know my results.
Thanks
Francisco

Old 08-03-2020, 04:55 AM
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