Computamedic

Back in January I posted that I had a problem with my ABS - in that it doesn't work. There are no warning lights - just no ABS.

I popped the car back to the garage today having established over the weekend that I also have no PDAS either. Operating the rotary switch gives me a single "clunk" from the rear but no amber light on the console. Switching off gives me several "clunks" from the rear but it's obvious the system isn't working - no impact on steering etc.

The PST2 tester indicated a fault on the system - but then listed no faults. A different ABS/PDAS ECU gave the same result and in neither case could the diff locks be cycled to bleed the cylinders.

I had convinced myself that the problem would be hydraulics because the car had been in for brake caliper refurb and had sat for a few days with no calipers but I'm beginning to wonder if ABS or PDAS were not working before that.

With no warning lights, the hydraulic pump runs and stops fine as you'd expect. The ABS system and calipers have been bled (using a power bleeder) but both ABS and PDAS refuse to work. Conclusions are coming down to an electrical problem but it just needs more time to diagnose.

Any suggestions???

Regards

Dave

Streetfighter

Have you looked at the ABS control unit or the speed sensors?

Computamedic

We bled the ABS unit today but have, thus far, done nothing else. The garage had no more time available today so they will get on to it after Easter.

I'm working on the basis that problems with the wheel speed sensors would normally generate a warning light, as would most problems with the electronics. Also, the wheel speed sensors would have no effect on the manual engagement of PDAS via the console switch. It can't be a coincidence that ABS AND PDAS go faulty at the same time.

I appreciate the suggestions.

Regards

Dave

dfinnegan

Check PDAS control unit connection
- unit is mounted wrong-side up so moisture sits in connector and rots it

Computamedic

We swapped it today with a known good one and it made no difference. That's not to say that the connector and/or wiring might not be at fault. That will be on the list to check next week.

Thanks.

Dave

dfinnegan

As long as you're making a list . . . here is my list of things to check in the ABS/PDAS department. Althought, I think most of these relate to spurious warning lights, rather than failure with no warning light . . . maybe it will be of some use to you.

Hope you get it sorted!

bobc964

Gentlemen,

I'm having a ABS/PDAS problem, also. Your suggestions would be most appreciated.

My mechanic informs me that while trying to bleed the differential lock slave cylinders, we encounter communications problems with the ABS/PDAS control unit, and found excessive corrosion and two broken pins on the control unit side connector.

After repairing the two pins and cleaning the corrosion from both connectors, we were able to retrieve fault code (34) and communicate with the control unit. We erased the fault code 34, rechecked fault codes with key on, and no fault codes were present. Upon cycling the key off and back on, and retesting for fault codes, code 34 immediately appeared (without having to start the car). With the car running, we were unable to communicate with PDAS (is this normal?). However, ABS and PDAS warning lights were still illuminated in the cluster, wihout the horn.

We were unable to bleed the PDAS system with the Hammer. On the selection screen to bleed, when selecting "begin bleeding", the Hammer would freeze, and bleeding was not possible.

The DTC code in the control unit is number 34. We performed the entire test procedure for code 34, per Porsche service manual. All components passed the test values, except for the 2.2 ohm resistor, built into ABS hydraulic unit, which tested at over 400 ohms.

We dissasembled the connector on the ABS hydraulic unit, to discover the resistor was burnt (the resistor is connected from pin 87a of the valve relay socket, to a ground connection on the ABS pump motor body). We replaced the resistor, and retested. We still have fault code 34, and ABS/PDAS warning lights (with no horn), which was the same as before replacing the resistor.

After the resistor replacement, upon start up, now the ABS hydraulic unit valve relay and pump relay can be heard clicking or engaging four times, when before resistor replacement, they were not clicking as frequently (We understand that this is the self-test mode).

At this point of the test (the last step), Porsche recommends installing another control as a possible solution, but we don't have one readily available, and we hesistate to purchase another control unit.

Is it possible that an output transistor on the control unit has failed and could be causing fault code 34? Would you recommend that we replace the faulty transistor? Your thoughts are very welcome!

Regards,
Bob

Indycam

What year is your car ?
What bay is it that you are in the area of ?

bobc964

Hi Indycam,

Its a 1990, and I'm in SF Bay Area.

Regards,
Bob

Les Dawes

Dave
J Z Machtec fixed my ABS/PDAS problem, at an acceptable price.
Regards, Les.

Computamedic

Les,

The car is in the (hopefully) safe hands of Ollie Preston at RPM at the moment - it's just the waiting until they can get some time on it next week that's driving me mad!! The symptoms really look like the ABS/PDAS unit is getting no power as it (and a known good replacfement) won't respond to the PST2 tester. The PDAS light did blink once over the weekend so that leads me even more to think it's an intermittent power problem. Hopefully Ollie can track it down.

Regards

Dave

bobc964

Dear Fellow 964s,

I do hope that some of you can help direct me to solving my problem which I posted above (and repeat here). dfinnegan and Dave, can you please give me your thoughts? If Adrian still monitors this forum, I'd be very grateful for his insight. PDAS Fault Code 34 if "valve relay." Short of trying another control unit, do you have any suggestions?

Here's my earlier post:


I'm having a ABS/PDAS problem, also. Your suggestions would be most appreciated.

My mechanic informs me that while trying to bleed the differential lock slave cylinders, we encounter communications problems with the ABS/PDAS control unit, and found excessive corrosion and two broken pins on the control unit side connector.

After repairing the two pins and cleaning the corrosion from both connectors, we were able to retrieve fault code (34) and communicate with the control unit. We erased the fault code 34, rechecked fault codes with key on, and no fault codes were present. Upon cycling the key off and back on, and retesting for fault codes, code 34 immediately appeared (without having to start the car). With the car running, we were unable to communicate with PDAS (is this normal?). However, ABS and PDAS warning lights were still illuminated in the cluster, wihout the horn.

We were unable to bleed the PDAS system with the Hammer. On the selection screen to bleed, when selecting "begin bleeding", the Hammer would freeze, and bleeding was not possible.

The DTC code in the control unit is number 34. We performed the entire test procedure for code 34, per Porsche service manual. All components passed the test values, except for the 2.2 ohm resistor, built into ABS hydraulic unit, which tested at over 400 ohms.

We dissasembled the connector on the ABS hydraulic unit, to discover the resistor was burnt (the resistor is connected from pin 87a of the valve relay socket, to a ground connection on the ABS pump motor body). We replaced the resistor, and retested. We still have fault code 34, and ABS/PDAS warning lights (with no horn), which was the same as before replacing the resistor.

After the resistor replacement, upon start up, now the ABS hydraulic unit valve relay and pump relay can be heard clicking or engaging four times, when before resistor replacement, they were not clicking as frequently (We understand that this is the self-test mode).

At this point of the test (the last step), Porsche recommends installing another control as a possible solution, but we don't have one readily available, and we hesistate to purchase another control unit.

Is it possible that an output transistor on the control unit has failed and could be causing fault code 34? Would you recommend that we replace the faulty transistor? Your thoughts are very welcome!

dfinnegan

Sounds like you've tried the right things. It does seem from what I've read like a control problem.

Have you considered purchasing a unit from ebay as a test unit and then simply re-selling it again if that is not the problem?

That said, perhaps a more detailed discussion of the current state of the system would be helpful.

You've said that the warning lights are on. Which lights? What pattern? That is, all lights on at key on, some go off, some stay on, changes after time . .

Does the high pressure pump run at all? If so, for how long? Does it run every time you try to start the car?

Have you checked, and bled, the accumulator (the "bomb")? It is supposed to hold the pressure created by the pump. Apparently it has an internal bladder that can wear causing lowered pressure outputs. Perhaps this is an issue? There are test procedures in the Shop Manual.

Have you bled the valve block in the front boot?

Does the PDAS system engage if you attempt to operate it? Do the lights go on?

Hope you get it sorted.

mojorizing

According to Adrian's book and the schematics, pin 7 (valve relay) at the ABS control unit goes to a relay in the ABS Hyd. unit. I'm looking at the year 91 model. Code#34 is "valve relay" as per his book, but doesn't mention specifically this problem. Do you have the schematics for the ABS?

dfinnegan

I've been reviewing the Shop Manuals for the PDAS/ABS system. There are 55 pages of diagnostic/trouble shooting information there.

You indicated that you had access to the "service manuals". The coverage is pretty thorough. Seems like the best approach . . .