Problem ABS/PDAS on a 1991 964 Carrera 4

[Turboman]

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

[Paperboy]

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.

[Turboman]

Thanks Paperboy I will check and clean the conector, this weekend.

[LM964]

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?

[Turboman]

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

[Paperboy]

Originally Posted by LM964

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.

[LM964]

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.

[Turboman]

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

[LM964]

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.

[Turboman]

As per LM964 recommendations I ordered O rings in order to service the "wheel speed sensors" Let you all know my results.
Thanks
Francisco

[Turboman]

After a busy week end, cleaning all moving parts of the ABS system, I was quite disappointed, my problem still present. No solution so far.
Any more ideas.
Thanks
Francisco

[group911@aol.co]

Are you disconnecting the battery after each step? Sometimes it will help reset.
Also, are there any friends near you with another c4 that you can swap control units with to see if that's the problem?

[LM964]

Agreed, if you can swap over control units with another C4 fellow then that will help eliminate the unit itself and give some idea if this is a mechanical or electrical/signal issue.

[Turboman]

Thanks LM964 and group911@aol.co I am going to look if one of my friends will help with the control box. Many thanks.
Francisco.

[LM964]

I did this the other day with a friend who had ABS problems. I would suggest he tries your control unit in his car and tests his PDAS/ABS rather than you try his control unit in your car - you don't want any electrical/signal issues that may be occurring in your car to mess up his controller!

[Turboman]

LM 964, wise idea, yes I will ask a friend to try my unit. Thanks.

[v8 lover]

have you recently changed the sparkplugs, this problem happens when your sparkplugcap is not mounted correctly on the sparkplug , mostly cylinder 4-5 on the bottom side,

[Turboman]

Thanks V8 Lover, no, I did not change spark plugs, but thanks for the advice, I will also check this. Francisco

[Paperboy]

I chased this problem a few years ago. One cause in my case was a faulty brake pedal switch, which I belive sends a signal to both the brake lights and the PDAS unit.
Check the brake lights. If they are not working check the brake pedal switch in place with a multimeter. If faulty not very expensive to change, believe a VW replacement can be used.
Brake light switch is pin 29 on the PDAS unit. I dont think a faulty brake switch throws a separete fault code.

[ian c2]

Sorry I’m late to the party .
Maybe way out here but it should be something you research anyway to prove my memory wrong ..

Iirc , On the c4 the brakes and 4x4 system are interconnected due to sharing fluid pump ?
This means brakes can upset 4x4 warning light and vice versa .
The 4x4 system also has acc and yaw sensors I think , and these can get a dirty connection or can fail .
I think maybe even wheel spee sensors for abs can do the same thing .
Will flash lights and give whoever is troubleshooting a hard time ....

Should be something you look into .