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964 PDAS troubleshooting help

 
Old 04-04-2017, 11:13 PM
  #1  
cknaak
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Default 964 PDAS troubleshooting help

Hello all, I picked up a 1990 964 C4 a month or so ago. The car needed some TLC but overall its a pretty good little 964 to knock around in.

The folks I bought it from mentioned the AWD light was on but they were able to get it to go off by replacing one of the accelormeters. When I received the car it was back on. I also notice a bunch of the warning lights were not coming on. So after playing around with the bulbs it was lighting up like a Christmas tree. I was able to resolve everything pretty easy but I just cant seem to get the AWD/ABS lights to go off.

I have gone through the below list of trouble shooting so I'm hoping someone may have another idea of what to try next.

Some info about the driving characteristics:

Before I did anything to the car the warning lights would come on almost instantaneously once I got over about 3 mph. Now,after doing the below, I can almost get out of my neighborhood about a 100yards but staying under 20mph. When I stop at the stop sign I can always feel the brake pedal give a bit...still no warning lights. after accelerating once I get up to about 20-25mph the AWD warning light goes off followed by the ABS about 5 seconds later. The brakes only settle as mentioned above that one time. Hopefully that all gives some clues as to what the issue might be:

Things I've tried:

ABS/PDAS

*Soldered Clock Cold Solder Joints - did this which resolved airbag/seatbelt lights
*Front boot crash sensor connectors-Cleaned contacts
*Replaced both accelerometers with newer solid state version
*Cleaned all 4 wheel sensors
*Relays-Replaced ABS, Brake booster, and fuel pump relays whith new
* Alternator - tested alternator and it was fine
*Check battery voltage -battery tested over 12 volts
* Checked PDAS control unit connection and cleaned connections
* Check pressure switch seal-no leaks detected
*checked and cleaned Ground wire behind pressure switch plug-
*Adjusted brake fluid reservoir level
*Checked ABS power connection at battery and cleaned

From my reading I see that replacing coils possibly is a next step but before throwing more money at it I had a buddy pull the codes using his Durametric scanner and this is what I have. I've googled around on these and cant seem to find much.

44: Fault Condition 18 Front-rear differential lock control deviations
32:Fault Condition 11 Front right ABS valve
248: Unknown Fault code

Any advice is greatly appreciated!! Thanks

Here's a pic in her current state:

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Old 04-05-2017, 05:47 AM
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All the right checks and fixes. I would say it is the usual waxy brake fluid issues. Strip and clean the ABS unit and it sounds like the 4wd solenoid block as well. Flush through a load of fluid and see if that does the trick.
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Old 04-05-2017, 08:31 PM
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Maybe a dumb question, but does your spoiler lift/retract correctly? Wondering if the speed sensor information is being calculated correctly.
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Old 04-05-2017, 08:45 PM
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cknaak
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Yes, the rear spoiler operates correctly.
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Old 04-06-2017, 07:58 AM
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Kind of had the same issue but not so prevalent as yours where your two ABS/PDAS lights come on after movement. My two lights stay on after engine start while all other lights go out. This is not so unexpected as with the C4 the brake boost pressure needs to build up (doesn't mean you have to remain stationary though). New hydraulic boosters will build up pressure much quicker of course. Do you get any PDAS alarm going off (stationary or moving)? If only the PDAS light remains on then more likely to de diff lock related. Are you able to engage 4-wheel lock on the console head switch (adjacent flashing amber light)?

Additionally, if both lights remain on (even at low speed) this could indicate the PDAS detected a battery voltage of less than 9.9V. Covered in Streather's book. Like mine, blipping the throttle above 2k rpm removes the lights and they stay out even when back at idle rpm. Switching the engine off then back on again repeats the lights on again. My charging circuit and battery are both fine. Not too bothered in this case.

You've performed an extensive list of checks and have fault codes too. Have you stamped the brake pedal at speed (clear road) and felt the ABS thump back at you on the pedal? Not mentioned in your list much re a fluid change. I would go the whole fluid flush and refill route. Done properly by someone who gets 964 C4's, this should cover not just brakes and clutch but also the brake booster, solenoid valve block, accumulator and the PDAS transverse/longitudinal locks. Anything up to 4 litres of fluid for this (depending on how bleed happy you get). Just had a complete flush done on the C4 and I have a couple of anomalies I'm working through - some corrosion on the ABS ECU connector male/female pins, and since the battery disconnect and hammer reset I've lost some ROM adaptation (need to get some fast driving done!).

Good luck and keep the post going with any findings.
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Old 04-07-2017, 07:52 AM
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Old 04-07-2017, 07:54 AM
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Old 04-07-2017, 12:39 PM
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@LM964 The warning lights/buzzer only go off after driving the car and getting some speed. Once on they never go off again until I restart the car. Prior to running through my checklist of troubleshooting they would come on when I was stationary so I'm sure I resolved other issues. I am doing a track day in about a month so I need to flush my fluids anyways so I will use this time to get a complete flush done by my specialist.

@968workaholic. Thank you so much for your post. I will check these items.

Additionally, I'll report back with my results so this post has some finality once the issue is resolved. Thanks all for your input.
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Old 04-12-2017, 08:08 AM
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Changing brake fluid at least eliminates old/tired polluted fluid as a contributing factor.

Worked the 964 really hard for the best part of 100 miles over the weekend with time spent on the Alpine Circuit at Milbrook then fast road use to the Porsche Expereince Centre track at Silverstone for some more sprinting. ABS worked really hard throughout and performed as expected. Was able to dependantly reproduce your similar observation of ABS/PDAS lights remaining on until moving (in my case 3,000rpm dead), thereafter they remain off until engine off and the cycle repeated.

Prior to the weekend, when I returned the car back to the idie that performed the fluid/PDAS flush to get them to investigate further why their work seemed to have introduced additional warnings, they checked the PDAS/ABS ECU and found some corrosion - pics below;





Read around some and talked with a few other specialists, and apparently water/damp ingress into the connector is not uncommon. The question is has any pin corrosion affected the board inside ($$$)? Cauntionary note to others advsing they check too (after isolating the battery).

ECU out and on my bench for further checks. The pin number badly affected was #11 - "Voltage, terminal 30". Terminal 30 (assuming pin #11 carries voltage for terminal #30) is "L-lead from diagnosis". None the wiser to be honest and maybe, just maybe, related to why only at 3k rpm with a voltage charge increase (>8.8V), my warning lights go out and stay out.

Lead and ECU cleaned up and much better. Corrosion not too bad - below.



My final thought is that I have realised my battery is now @ 4+ years old and not holding/accepting charge as long as it used to (despite being hooked up to the CTEK every day and night). While OK for now it may be prudent to change this soon which might then eliminate this PDAS battery voltage check and subsequent warning.

Keep us informed on any progress...

Last edited by LM964; 04-12-2017 at 08:14 AM. Reason: grammar
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Old 04-12-2017, 08:46 AM
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That is exactly what I also encountered when unplugging the connector on my PDAS unit. Only in my case the pin 11 on the PDAS broke off! Seems the only way I can try to fix that is by soldering a wire on the inside of the unit, leading it out of the box and connecting to the cable wire. Not a pretty sight but do not know what else to do.. I assume Pin 11 is indeed required for a faultless operation of the PDAS?
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Old 04-12-2017, 09:04 AM
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Originally Posted by Beezztie View Post
That is exactly what I also encountered when unplugging the connector on my PDAS unit. Only in my case the pin 11 on the PDAS broke off! Seems the only way I can try to fix that is by soldering a wire on the inside of the unit, leading it out of the box and connecting to the cable wire. Not a pretty sight but do not know what else to do.. I assume Pin 11 is indeed required for a faultless operation of the PDAS?
Interesting that it's pin 11 too. My only concern that doing a self fix and solder may, and can, introduce resistance fluctuations (amount of solder etc). With the PDAS/ABS ECU being a critical safety component some rethought needed. Not sure where you are located but I was told that in the UK there are a couple of specialists who can repair, test and certify these units. Certainly much cheaper than a replacement.
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Old 04-12-2017, 09:29 AM
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I enquired with a company in the Netherlands who does a lot of diagnose and repairs on this kind op stuff (the DME for instance) but they told me they can only solder a wire and bring it out. Also cannot test that unit. Would you happen to have an address / name in the UK, may be worth trying?
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Old 04-12-2017, 09:31 AM
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Regarding the PDAS/ABS ECU corrosion:
Water follows the wiring from the front wheelwell into the trunk, into the (conveniently positionned) ECU connector, causing corrosion.
The PDAS/ABS ECU computer should be relocated head-down to prevent this. There is enough slack in the cable to relocate it to the front trunk wall, using tara tubes and bolts.
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Old 01-03-2019, 05:40 AM
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Did you ever get to the bottom of this problem? I have similar problems with mine. PDAS light on dash followed by ABS light and brake pedal drops and goes soft. I have swapped out the ABS pump and accumulator, fitted a new longitudinal solenoid, bled brakes and valves although not done the clutch as I couldn't see the benefit.
I have tested the speed sensors with a multimeter and cleaned 3 of them, the other is hard to remove so left it for now.
would it be worth swapping out the coils? I haven't tested if the lights go out with increases revs yet which I will do but alternator voltage is good.
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Old 01-03-2019, 06:44 AM
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Originally Posted by DC Customs View Post
Did you ever get to the bottom of this problem? I have similar problems with mine. PDAS light on dash followed by ABS light and brake pedal drops and goes soft. I have swapped out the ABS pump and accumulator, fitted a new longitudinal solenoid, bled brakes and valves although not done the clutch as I couldn't see the benefit.
I have tested the speed sensors with a multimeter and cleaned 3 of them, the other is hard to remove so left it for now.
would it be worth swapping out the coils? I haven't tested if the lights go out with increases revs yet which I will do but alternator voltage is good.
Personally, I have had no further problems re PDAS function or PDAS warning lights (beyond my usual low voltage detected until @ 3k rpm is reached). Like you I have normal as expected alternator voltage output and charge circuit voltage within correct range.
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