c4 pdas / abs unit

[l.chadburn]

just had the car serviced and diagnostics run to see why the spolier light is on, the abs and the pdas lights are on.

it looks like it isn't the wheel speed sensor, but its either the pdas / abs control unit or the abs pump. (going down to pick her up in the morning and so that they can show me what they think it is.)

been on the phone all afternoon trying to track a used control unit - no chance.

does anyone have one they would like to sell?

if it isn't this and its the other then I will update tommorow


and - apart from the speedo not working will i damage my car if i drive it without the pdas and abs?

regards lee

 

[Dave Wilkinson]

Lee,

It's really difficult to diagnose problems like this from afar - particularly without knowing the outcome of some very specific tests. It's very easy to assume that a specialist might know all there is to know about problems like this, but, rest assured, they don't!!

The spoiler light has, first of all, nothing to do with the PDAS or ABS system - other than all the warning lights are in the clock. And that subtle point should not be dismissed as there are a number of issues that arise from poor solder joints inside the clock. The spoiler position micro switches can impact the operation of the warning light too.

Does the spoiler light come on with the ignition (as it's supposed to)? Is the spoiler down at startup?? Does the light ever go off?? Does the spoiler go up??

On the PDAS/ABS front, does the high pressure pump run immediately ignition is on?? Does it stop?? Does it start again after pressing the brake a few times?? Is the brake fluid level correct (right up to the top)?? Is the battery voltage good?? If working as it should (which it sounds like it isn't) the PDAS & ABS lights should go out slightly before the pump stops from initial ignition on.

If you can come back with some answers of some of these points it might help to narrow the cause(s).

All I would advise is don't expect that buying a replacement PDAS/ABS unit or hydraulic pump will achieve any more than lightening your wallet by some extortionate amount without finding some irrefutable proof first!!

Regards

Dave

 

[l.chadburn]

cheers dave for your reply.

I will get the full story tommorow, and i'll be bringing it home so I will right down exactly what happens in what particular order.

i will also take your questions with me to make sure I can answer them all.

its driving me mad!

regards lee

 

[l.chadburn]

Dave sorry to come back.

i have also been reading the adrian streather porsche enthusiast's companion which to be fair has helped me immensley.

it has a number of suggestions.

when looking at trouble shooting for speedo under

speedo not moving, odemeter not functioning. rear spoiler warning lamp remains on after driving off. Also PDAS / ABS warning light comes on shortly after driving.

Which is what is happening!

it says check wheel speed sensor (computer shows no sensor faults)
or PDAS ? ABS control unit failure.

just wondered if you had seen this book and rated it.

anyway will still take your questions with me and get some answers. hopefully can get it sorted.

regards again lee

 

[Dave Wilkinson]

Lee,

I have seen the book - and rely on it considerably - but I have the impression that Adrian tends to document what is "supposed" to happen under fault conditions and thus gives you the official cause/solution. Unfortunately, there are a lot of problems than can be caused by completely extraneous events and conditions which Adrian didn't attempt to address - and it's often the weird ones that are the most difficult to solve.

A faulty wheel speed sensor MAY well give the PDAS/ABS a problem - but only when the car is moving. It will not prevent the warning lights going out when the pressure builds prior to driving off. As part of the testing you should be aware that it's important to NOT drive off before the PDAS/ABS lights and the pressure pump have reached their stable (all off) position. In everyday use it's no SO important, but as part of the testing it's critical in order to get a clear picture of the problem.

You didn't mention speedo not moving before!! Is that something that's also happening?? If so that would explain the spoiler issue - no speedo, no spoiler - end of!! In a 1992 car that can be a faulty frequency converter in the fuse box.

Regards

Dave

 

[l.chadburn]

sorry dave

Forgot that bit. yeah no speedo or mileage movement.

really appreciate your advice. will take all this with me tom and do some testing.

will update tommorow as soon as I have done.

cheers again lee

 

[Steve Brookes]

A chap on Rennlist had the same warning lights. He doesn't go into much detail as to how it was fixed (by his mechanic) save to say that they bled the hydraulic system and replaced a relay (the ABS relay I think he meant).

 

[l.chadburn]

Hi Dave

Picked her up this morning, Managed to get the speedo working / spoiler, simply by changing the r24 relay with a spare I had.

right the abs / pdas problem.

there computer keeps flagging up PUMP RETURN.

when I got home I followed your checks and found the following.

When ignition is turned on all I heard was a kind of switching / clunking sound. I pressed the brake pedal a few times and the pump then activated.
When the car is started all lights are out. However when I set off both lights come on with the warning light and horn sound.

lee

 

[Dave Wilkinson]

Lee,

Good result on the speedo!! R24 is the frequency converter I was talking about - it's not a relay.

At the moment I don't have a clue what the "PUMP RETURN" error message is all about. The fact that the alarm activates when the car is on the move suggests that the problem is a dynamic one rather than static, so I would then discount issues like fluid level etc. Wheel speed sensors might be a good bet as are, maybe, the accelerometers. Adrian's book also suggests a potential problem with the diff locks too.

As long as the pump runs and stops as expected I'd guess that things are OK there.

Regards

Dave

 

[l.chadburn]

Hi Dave

thanks for your reply.

just looked at the book again, as you say it advises checking sensors or diff lock.

the guy said that their comnputer didn't pick up any errors with sensors, he just kept getting that message pump return.

how do I check the diff lock, is this something I can do or do I need another porsche specialist?

lee

 

[Dave Wilkinson]

Lee,

The diff locks are hydraulically operated by the PDAS/ABS unit. If you turn the rotary control on the console with the ignition on you should be able to hear them engage and then disengage when you rotate the other way. Even if you CAN hear them move that's no guarantee that they are operational although it's a positive sign.

You can try driving the car with the diff locks engaged and you should feel a distinct effect with a huge tendency to understeer even at walking pace (the orange light on the console should come on when you rotate the switch). Once the car reaches something like 30mph (I think Adrian notes the exact speed in his book) the PDAS will automatically disengage the locks (and the light will go off). If you feel no obvious effect with the locks engaged it may be that they are non-functional.

I'm not completely convinced that the computer diagnostic system would pick up any problems with the ABS/wheels speed sensors anyway. It's best to test them individually with a meter or oscilloscope.

Although not directly related to the PDAS/ABS problem, you should also make sure your distributor caps/rotors and coils are OK and not leaking voltage to ground. This can cause havoc with the PDAS/ABS controller and give completely false alarms.

Hope this all helps get there in the end!!!

Regards

Dave

 

[l.chadburn]

Sorry Dave posted this on a number of forums.
Anyway I think I have found an explanation - The book by Adrian Streather says that the pdas / abs unit part number for 89 and 90 is 964 618 126 00 and the part number for 91 upwards is 964 618 126 01.
The 00 version can be exchanged for a 01, but not vice versa. Therefore a 92 model cannot use a 00 version which mine has on it.
Let me know what your thoughts are or anyone else that has come across this.
Lee

 

[Alexander_Drobik]

yes that is the case - the later unit had a faster sampling frequency (that is what I was told some years back) and so an 01 unit will work with all (as it backwards support) but an 00 unit cannot foretell the extra needed in the later models. Never tried to mix so don't know if it half works or just fails - but you need a later model for the later car I suspect!

 

[l.chadburn]

Cheers alex - now the hard part, trying to find a second hand one, they are like gold dust.

lee

 

[Alexander_Drobik]

ah - the curse of the C4 has hit me. My awd and abs light has come on and we can't find an issue - all the relays and pumps have checked out as have the accelerometers. We are thinking it is a really transient wheel sensor, wire break or the abs unit.

Has anyone got a C4 near Maidenhead , 1991 model year or later, ie fitted with the -01 unit who could let me try it for 5 mins to see if I have a control unit issue?

 

[l.chadburn]

Alex

Mine appears to be the abs pump that has ceased. managed to get one for £87 off ebay which I thought was a real bargin.

Should be in next week for it sorting.

lee

 

[Alexander_Drobik]

Lee - good luck with that. I am still trying to sort what the problem is - sensor, relay/sync unit, or controller. I hate the process of elimination bit when these faults hit.