Adrian

Dear Carl,
Sorry I missed your post earlier. We must have crossed in the post.
You have a different problem. When you apply the brakes the PDAS is out of use. It is disabled. Therefore your problem lies with the ABS. Interestingly the problem is not only intermittant but also the warn lights go out.
So first thing to do is to check your alternator output at idle or during reduction in rpm. A check of your battery is in order as well.
If this does not reveal any problems then I still believe it is power/voltage related problem to or with the ABS hydraulic unit. Because the lights are going out the fault is not being logged. The only thing that can do this is power. Either overvoltage or undervoltage.
In these situations I actually have to guess because there is almost no way of troubleshooting this unless you have a Bosch Hammer hooked up whilst you are driving along and see if it shows up a fault code.
I suspect a relay problem.
ABS relay R34 would be worth having a look and I would also recommend you take the cover off the ABS hydraulic unit and check the condition of the two relays on top. I have found corrosion on these relay connectors.
Do not forget the brake switch itself. Might be worth checking that the brake switch/sensor is working correctly. It is this input which switches the control unit from PDAS to ABS.
Ciao,
Adrian
964C4

Carl Muckenhirn

Adrian,

Thanks, for the reply, yes we were independently writing within 4 minutes.

I've recently replaced the battery, as well as the relays on the ABS pump. I'll keep rooting around.

The Brake switch/sensor is the one in the foot well correct?

Thanks again.

carl.

Adrian

Dear Carl,
It is by the brake pedal.
Ciao,
Adrian
964C4

DaveK

Although it's pretty unlikely :

When I got my car, I found that the ABS / PDAS lights would stay on first thing in the morning until the revs reached about 3K rpm. Obviously on C4s the lights usually do stay on for a while when started from very cold as the system re-charges, but mine wasn't time related - I could sit there for several minutes and the warning lights would stay on until I revved the car to 3K.

Mine was fixed when I replaced the clock (yes, the one that tells the time). If you do rule out the battery / relays etc. it might be worth a try. Mine never caused problems apart from cold startup - but somehow the clock did seem to be sapping some voltage which caused those warning lights to stay on so you never know. If you run out of ideas and can find someone to swap with for a day, it might be worth trying.

HSB

Just a followup, I finally opened both accelerometers and found them to be stuck. There were a large no. of magnetic filings as well as greasy dirt. I cleaned and reassembled the units and the light has not come back on. I guess I am glad that I listened to Adrian before shelling out money to bleed the interlocks. Thanks All Harpreet

garrett376

Harpreet,
Did you use brake cleaner to clean those little buggers?? How did you do it? I am sure I'll need to do this sometime! Thanks for sharing everyone!

HSB

Garrett, Heres a brief step by step: !. Remove two screws on passenger side of tunnel and remove cover. 2.Remove single bolt holding wiring bracket. Pull bracket and wires out and let hang . 3. remove rear most accelerometer by removing the two sheet metal screws. I used a 90 degree phillips screwdriver.Now you are getting some room to get that forward unit. 4. There is a small relay above the forward unit-- wiggle it out. 5.remove the front screw from the forward accelerometer and the patiently loosen the rear screw. This will take time as you only can execute partial turns. I labeled the accelerometers them to avoid confusing them .

I removed the covers and tried to clean the surface between the pendulum and the magnet but did not have the room. I lremoved the two screws holding the unit in and then carefully lifted it up taking care not to bend the pins or circuit board.This gave me just enough room.
First I removed the large filings and "magnetized debris with a fine angled pick and then intermittently irrigated the area with electrical cleaner ( from Pep Boys). Finally I got a thin piece of card board between the pendulum and magnet and cleaned the last bit.Before closing the unit I cleaned with the electric cleaner. Make sure it is dry or you get to take it all apart and dry it. You can tell that it works because the pendulum comes back to center after you move it in either direction.

Prior to cleaning mine they would remain stuck in both directions and failed to come to center.
Well I hope this helps Good Luck, Harpreet

PS I cut a slot through the rear mounting hole for the front unit this made it easier to reassemble... I was able to partially screw in the back screw and then slide the accelerometer .....nothing like a dremel tool..

lunaboy

Hi guys
This is my first attempt to post anything. I have followed this topic with interest as I have the same troubles, although different !

1990 C4, only had for 6 months, first Porsche and it frightened the life out of me recently when the ABS/PDAS lights and alarm went off!

Turning power off and restarting reset and solved problem. Then it ahppened again and to cut along storry short the frequency of the intermittant alarm has shortened to minutes.

After car has stood for a week, you can dive off for up to 30 mins, no trouble, before the alarm comes on but once the alarm and lights come on, then the alarm repeats every few mins. Sometimes the lights flash off and then back on. Also noticed traction control light (to right of switch) flashing at normal road speed.

In early stage garage read a relay failure, but when it was substituted still had problem. Latest garage advise is that ECU is stuffed, as they replaced off another car and had no failure. I am not so sure as it may be just the initial delay period as mentioned above.
If I need a new ECU, where is best place to get "less expensive" replacement. Dealer wants NZD4000 for new one !!

Appreciate all and any advice

HSB

Lunaboy,
Adrian's book is very helpful . After reading the section on the PDAS I was better able to understand how to trouble shoot my problem. I took mine into a garage and based on their PS2 scanner told me that I had a transverse lock problem. Adrian suggested cleaning my accelerometers since my problem occurred above 61 mph. Prior to cleaning them the PDAS alarm and warning light would illuminate about 10 minutes after highway speed. So far no alarms for the past two days. Goodluck, Harpreet

PS : Just clean your units, it's cheap and simple. After all these years I'll bet they are dirty.

lunaboy

Thanks HSB

Adrians book is called ?, available from ?

I did forget one further detail, the clock has started to run fast. Has gained approx 20 mins in a few weeks.

Intrigued to see mention in forum (DaveK)of clock replacement fixing a light problem

HSB

Adrian's Book is "Porsche 911 Enthusiasts Companion" By Adrian Streather
It is published by Bentley Publishers Good Luck

schiles

HSB -
How in God's name did you get that rear screw out from the front unit??? I must have worked on that thing for two hours yesterday and I still havent gotten it out. I wonder if I try a different offset wrench - any recommendations?

btw...your step-by-step was very helpful!

thanks!

Steve

HSB

I believe I used my 90 degree screwdriver and lossened it a little bit at a time.

Fred C

are the acceleromters inside the car under the carpet?

Fred C.

Swallowtail

I just took the accelerometers out of my car. The whole thing was much easier than I expected. I used a 1/4" ratchet (see pics).

I was quite surprised when I opended the sensors. There was not the slightest sign of dust or metal filings inside. I suppose they have been replaced at some point.