CentralLine

I have a '89 964 C4, with 78,000 miles. Currently, about 30 seconds after startup, the alert [!] beep goes off, and the antilock and drivetrain lights illuminate. Sure enough, the car no longer has functioning antilock. This used to be intermittent, but not comes on without fail, at every startup. The brakes are otherwise working fine.

Of note, when the antilock system was "working" (6 months ago) my braking distances were horrible, once the antilock engaged.... it was almost as if the brakes failed once antilock was triggered. With that in mind, I would much rather threshold brake (with no antilock at all -- which is the current configuration), and at least have functioning brakes.

So, what might be going on? Any suggestions on the correct fix? Is the problem also affecting the 4-wheel drive system, since the drivetrain light comes on?

Thank you in advance for your input.


Christopher

Arjan B.

As far as I know has a C4 a kind of Pressure pump that pumps up the brake pressure and I most important for the system. Failing could cause your problem. May be you should send Adrian a PM and he can help you.

Adrian

Dear Christopher,
Are you a DIY person? If you are I recommend you purchase a copy of my book Porsche 911 Enthusiasts Companion from www.bentleypublishers.com
This book explains in detail the system you have a problem with. Unless you have some knowledge of what you are dealing with trying to answer your questions will be very long and could end up being very expensive and very frustrating.
I will advise you that the failure that is appearing is what is known as a hard component failure. The system runs a test after the brake pressure reaches the required level and the PDAS and brake pressure lights go out.
This suggests a control unit failure, an accelerometer failure, the dual solenoid valve failure or an ABS hydrualic unit failure.
This system requires regular maintenance. When was the brake fluid last replaced?
When was the system last bled?
I recommend an electrical system check as well. A bad alternator or battery can cause the system to lock out with an under voltage or over voltage condition.
The AWD system is NOT affected by this failure. What you have lost is traction control not AWD.
I realise that this can be very daunting and confusing. This is why I wrote the book so 964 owners can learn the basic system and obtain an understanding of what they are dealing with.
Ciao,
Adrian
964C4

PS: Dear Arjan,
I would appreciate it if we could keep these things on the forum. I have too many private messages already to deal with.

DaveK

Christopher,
One of the things that Adrian mentioned (accelerometer) caused problems with my ABS. I didn't get warning lights, but my problem was the same as you describe - it felt like the brakes were released when the ABS kicked in and my braking distance doubled. Once it was fixed, I also realised that the ABS had been kicking in too early.

It seems to be caused by a build up of dirt / metal particles in the accelerometer, and cleaning it out cures it (an air pressure line should be good for it). The accelerometers are under the transmission tunnel - there is a flap you can remove from the side of the tunnel to give you access.

It may not be your problem - as I said, I didn't have warning lights - but it is a simple place to start, won't cost you any money and should only take you about 30 minutes.

CentralLine

Adrian,

Thanks for your quick reply. Some questions answered:

1. Yes, I am a DIY'er and will buy your book as soon as I finish this thread--Thanks for writing it.

2. The brakes were last flushed and bled 1 year ago, by a Porsche Technician in Santa Barbara, CA... only because my brother who previously owned the car thought they would use the Bosch "Hammer" (diagnostic tool) to open all the valves. This did not happen, and we would have been better off doing the bleed ourselves. The car has only seen 1500mi in the past year.

3.The battery is one month old. No improvement with replacing it.

4. Does the fact that the Anitlock brake system was not stopping the car when actuated (6 months ago when the system "worked") narrow down the search for the problem at all? It was a bit scarry actually, and caused my brother to launch through more than one red light when the brakes seemingly failed. It was not until I took ownership of the car that we realized the brakes only "fail" when the antilock is engaged during a hard stop.

5. What is the risk (other than total brake failure = bad) of not having the antilock system work. Should I ground the car? ( I only drive it 1 day a month on average, so it is not as though I depend on it).

6. I have no good technicians where I live in Tucson, AZ... other than the dealer who I refuse to use. I probably know more about this model than they do, as the technicians mostly only deal with pre'89 cars. Is there a cheap(er) source for the $3000 Bosch "hammer"? Do I need it to diagnose this problem? Sorry in advance if all this is already answered in the book.

Christopher

DaveK

Christopher,
Think our replies crossed - but your point 4 above does sound exactly like the problem I had due to my accelerometer. I also ran a red light or two as a result - was very scary.

garrett376

You can get a Hammer 9288 tool for under $2,000. And with Adrian's book, you don't absolutely need the Hammer to diagnose a lot of issues with the PDAS system - it sure helps, though, if you've got one.

Springer3 may rent his to ya' - PM him about this. You do need the Hammer to bleed the locks which are important since lots of dirty fluid sits in those things, and the lines leading to the locks.

Adrian

Dear Christopher,
In a C4 unlike the C2 an accelerometer is used in the ABS system. The longitudinal accelerometer. If this is not working correctly it can cause the ABS to relieve the brake pressure for too long because it is not registering a de-celeration.
The other cause can be the control unit or the ABS hydraulic unit itself with sticky solenoids.
Based on your instant fault before driving off this suggests a stuck accelerometer (inotherwards input is not matching other inputs), control unit or ABS hydraulic unit.
The brake system is fail safe. If the ABS system has failed you will have full brakes at all times assuming fault is not in the brake rotors which in your case I would eliminate.
Brake rotors below minimum thickness can cause ABS problems like you have described but you will not get warn lights or if you do it will be whilst braking only.
My starting point would be the longitudinal accelerometer.
Ciao,
Adrian
964C4

dutchcrunch

check your pm

jenskleis

Hi Christopher and fellow Rennlisters,

When starting up my car in the morning, I get the same indication as Christopher, i.e. the PDAS and Brake Pressure warning lights stay lit, and after about 30 seconds the beeper goes off. The lights normally turn off as soon as the beeper goes off, or a couple of seconds after.

I always assumed that this happens because the system takes a little while to build up pressure. Should I be worried?

(The car just whet through the Swiss MOT without any probllems)

schiles

Adrian...
I have the same problem, ABS and PDAS lights with audible alarm right after start up before the car moves. Both lights remain on indefinitely although shortly after I first encounterd the problem, I sent the car to my shop via flatbed and the problem disappeared once at the shop and no error codes showed up on the hammer.

They couldnt find anything so I took the car back and lights remained out until I entered a tight, constant radius off-ramp after a spirited stretch down the highway at which time I got the lights again except this time while moving. Since this time, theyve remained on and have reverted back to coming on immediately following startup everytime.

I have studied past posts and as a result have thoroughly cleaned both accelerometers with no positive results - same lights right after startup. The car had the ABS unit replaced last year and Ive checked the voltage at idle and 3k+RPM and Im getting good current.

Any other suggestions - very frustating!

Steve

schiles

Does anyone have any further recommendations of things I can look for here?

Thanks much!

Steve

Kahdmus

Adrian recently helped me fix the problem that I was having (after about 50 emails..thanks again Adrian). Initially it was intermittent and then would not go away. I checked my fuse which was blown, replaced and it blew instantly. My problem turned out to be a sharp screw that had dropped behind the tight strap holding the main wires to the PDAS/ABS unit on the left handside of the luggage compartment. It had over time pushed into the wiring and shorted out the system.

schiles

Thanks Howard - I'll look at that tonight. Hate these electrical gremlins!

Steve