G'day

At a driver training session yesterday cones were set up with time trials around a set course with completion of the course requiring a high speed stop within a set of cones to simulate a garage. At the completion of the third run the ABS failed leaving two impressive black marks down the front staight at the local track. The ABS warning light on the dash board does not indicate a fault.

Following this little hiccup we pulled off the side of the track and tried the brakes on the grass and sure enough they locked up. Any idea what the issue may be?

Cheers

964 Turbo 3.3 91

 

Dunno, but if you figure it out, let me know. My ABS does not work, either.

 

The same happened to my car this summer. The problem is still there and the brakes work like regular brakes. I have changed the ABS computer and this didn't solve the problem.

I also have a spare ABS pump, but have not got around to change it yet.

 

Charlie, bring the car by and we can figure it out. The system is pretty simple.

 

When mine stopped , I pulled the connector and cleaned all the parts and it started working again .

 

My guess is the ABS pump too. Upon start the system runs the ABS ventiles just to avoid them being stuck. But If the car has been standing still for a long they could get locked in open postion which is great for breaking but not so good for the ABS function... The pump is not that hard to change but it will get messy and BF ruins the laquer /paint. And the bad news is: The pump is also very-very expensive.


Regards, Daniel Saxlid

 

I will take you up on this! Let me know when works best. I desperately would like to have my ABS working before next season. I have a few tasks to do first (putting on livery, and general maintenance). What does your schedule look like in January?

I also have the car on my lift now, so you're welcome to come down to look at it anytime.

 

The ABS system for the Carrera 2 and the 964 Turbos is fairly simple. I don't know how long you have had the Turbo, but from your post you seem to believe that the ABS was working. The fact that you have no ABS warning light means one of three things.
1/. It has been disconnected.
2/. The ABS is not being activated.
3/. The ABS thinks it's doing its job even though it doesn't appear to be.
Let's hope that point no 1 is not the case and just look at 2 and 3.

ABS not being activated:
The Carrera 2 series and the Turbos are not fitted with any sensors which feed the ABS. It relies simply on the driver telling the ABS that the car is braking. The system uses the brake switch on the pedal assembly. This brake switch also activates the rear brake lights. Are the rear brake lights lights coming on? Is the connector on the switch intact?

ABS thinks it is doing its job:
The ABS warning system only monitors electrical power, inputs from the wheel speed sensors, internal computer operations and a limited amount of electrical checking of the components (3 solenoids and the return pump) in the ABS hydraulic unit.

Note: The hydraulic brake boost pump of the 964 Turbo has NOTHING to do with the ABS system.

Excessive use of the brakes will cause normal DOT4 brake fluid to overheat and boil. When this happens all kinds of strange things can occur. These problems are usually mechanical in nature so will not always fire off the warning light(s), ABS and or Brake fluid level.
If the brake fluid overheats (or for any other reason) and the rear brake bias valve fails and stops braking power to the rear wheels, all the braking power will go to the front wheels. This can overpower the ABS and cause a front wheel lock up regardless.
On the Turbo in particular the pedal may not give any indication of the ABS attempting to work because the braking power is somewhat greater than a normal normally aspirated 964 and the hold pressure phase never gets to the release pressure phase. Yes this has happened before.

Note: How old is the brake fluid you have in your Turbo? It should be flushed and changed every two years for normal road use and more often if you track the car.

Note: These cars are also getting older and like the DME/Fuel pump relay, the ABS relay R34 is always powered and protects the system against under and overvoltage. I have seen a few of these fail in recent times. If the relay is not activated the system is not powered and therefore it won't turn on. It should fire a warning light, but not always depending on the actual fault inside the relay.
Ciao,
Adrian.

 

The system also receives terminal 61 which is the alternator excite wire so the ABS system knows the engine is running. The system is easy to determine which of the components are not funcitoning properly, even without a Bosch Hammer, although that would make quick work of diagnosing the failure.

The ABS light should come on when the key is first turned to the 'On' position. I believe it stays on until the engine is running (mine does), but mine is ABS-5.

 

Thanks for all the input. We have only had the car for two weeks. The brake lights are working. The brakes locked up from a speed of approx 70kmh. The brakes hadnt really been worked hard at that stage, the car had just had a PSC dealer service before delivery to us but I dont recall the brake fluid being replaced. I know previous owner had had car on track for local Porsche club track day. Car was only on its third run through the cones. First run ABS not required, car pulled up without skid or ABS activation (slower entry speed) second run ABS seemed to activate, no lockup but pumping through the brake pedal. Third run full lock up of front wheels, not back wheels. Instructor was encouraging us to brake heavy to understand effects of ABS. Car is in the workshop tomorrow so will let you know the outcome. There was no sensation of the ABS working through the brake pedal on last run, but as Adrain pointed out this may be the case at times. ABS light on dashboard does light up on startup. Looking like may be old brake fluid resulting in brake valve failure as Adrian suggests.