Red rooster

As a minor input to this discussion it might be worth checking what driveshafts have been fitted front and rear.The 993 speed ring on the CV joint has a different number of teeth to a 964.With 993 front shafts and 964 rears the front /rear speed signal differencescould be enough to flag an ABS fault .

EK

Adrian,

Is there any way to disable the ABS controller's ability to inactivate the system? For example, if a "fault" was detected, the light would go off but the system would still remain functional.

Also, is there any input from the speedometer or speedo into the ABS system or does the ABS system simply compare relative wheel speeds through the wheel sensors without regard to true ground speed? I was told that there is no other speed input besides the wheel sensors but that doesn't make sense. The reason I ask is that the speed sensing mechanism had to be changed in order to accomodate the 993 drivetrain.

Thanks.


Red Rooster:

According to the mechanic, all drive shafts are 993 driveshafts.

EK

DaveK

Surely that makes no sense. The reason the controller disables the system when it detects a fault is because it cannot work correctly with faulty parts. And if it really is getting incorrect readings from somewhere it's probably safer that it has been disabled by the controller.

Adrian mentioned earlier that you really didn't want to know what it felt like when you needed to brake hard and the ABS controller makes the pedal go hard leaving you with no brakes. I have to agree with him - I've had that situation due to a faulty accelerometer. Braking for red traffic lights only to find the brakes were released and the pedal went hard. It's not fun.

Adrian

Dear Edward,
You have answered the question why you have two rear wheel speed sensor failures. You have the 993 set up feeding a 964 PDAS/ABS control unit. The 964 is Bosch 2 and the 993 is Bosch 5.
You cannot separate PDAS and ABS because they share the same speed sensors and circuitry.
No you cannot disable the interlocks.
No the speedo has no input into the PDAS/ABS. In your case you need the ABS to supply the speedo with its signal. I know you have a 1991 modelor up because in a 1989 or 90 model the speedo would not work. No pulse sender in the 993 transmission and this is what the 89 and 90 use.
Ground speed is of no concequence to the ABS or the PDAS of the 964 or to the ABS/ABD of the 993 C4. They only care what the wheel speed is. They look at what the wheel is doing in realtion tot other wheels.
I refer you back to my first posts. You have a hybrid system mixing incompatible sensors and electronic systems.
The only real fix for this is to rip out the 964 electronics and replace them with 993 electronics.
Ciao,
Adrian.

PS: As I also said. Others claim to have carried out such conversions and have been able to make them work. However when I have asked for the details of how they dealt with the 964 C4 PDAS/ABS issues like you face I never hear from them again. Sad but true.

EK

Since I really do need a properly functioning ABS sytem, I guess I need to seriously consider changing the ABS electronics to that of a 993. Besides the ABS controller/brain, wiring harness, and wheel sensors would I need any other major parts?
EK

Adrian

The ABS hydraulic unit and that is really expensive.
Please also understand Edward that I really feel bad for you. So many like you purchase such cars in good faith and get lumbered with these situations. There is only so much any of us can do to help though.
If you do not have my book or any other comprehensive 964 buyers guide at purchase time, you will not be aware of the potential difficulties of such things or more seriously where they can send you.
Ciao,
Adrian.

EK

Adrian:
Thanks for all of your help. I think this may be my last question.

Since the C4 and C2 964s use the same ABS hydraulic unit and wheel sensors, can I purchase a C2 ABS brain and rewire the system ... in essence I would have a C2 ABS system which won't look for the PDAS system.

EK

Dunasso

Very interesting technical thread. This is valuable information that should be kept in some sort of archieve. Like other technical data that has been hashed out here; it seems as if there should be a way to organize threads like these and separate them into an area or category of their own for easy access.

Maybe tehere is and I'm just clueless.

Duncan

Adrian

Dear Edward,
You misunderstood. You have to install a 993 Carrera ABS system not a C2 ABS system. Forget the 964 now you have a 993 drivetrain.
Ciao,
Adrian.

kris

Duncan,
I had exactly the same idea. Should be possible in some way.

EK

Adrian

The current 964 ABS system appears to work even with 993 drivetrain as long as the ABS warning light is not activated. The problem is that the warning light intermittantly deactivates the system.

I realize that using the 964 ABS system with the 993 drivetrain is not optimal (I guess since the speed rings have a different number of teeth) and the best thing would be to convert to a 993 ABS system but this apparently would be very cost prohibitive (I have no idea how much an ABS hydraulic pump would cost). It would seem to me that since the current ABS system is "functional", that I need a system that will ignore the PDAS system ... therefore converting to a C2 964 ABS controller and rewiring the system. Do you think that this would be possible (and less costly than the 993 system)? Thanks.

EK

EK

Also, I have driven the car a fair bit over the past several days to determine exactly when the ABS light comes on. It comes on intermittantly about 10 to 15 seconds after I accelerate above 70 mph. It comes on about half the time after I enter the highway and am getting up to speed. Other times it has come on after I have passed a car on the highway. If I am simply try to drive at a constant speed (even up to 75 mph), the light does not seem to come on even after 40 miles of driving.

In addition to the PDAS issue, it sounds like the car may have an accelerometer issue. This would appear to make sense since it apparently had the intermittant ABS issue before the conversion to a 993 drivetrain.

Adrian

Dear Edward,
This is getting very complicated. If you remove the ABS/PDAS control unit and replace it with an ABS control unit you can also remove the accelerometers. They only work with the PDAS/ABS control unit.
By the way the warn light does not de-activate the system the fault detected does and you tell me that the hammer threw up 2 rear wheel speed sensors as a failure. How is changing the accelerometer going to cancel that fault?
I am sorry though I cannot waver on my advice.
To do the job properly you need to install the 993 system. Personally I think it would be worth installing the 993 ABD as well to make a proper C4 transmission and to get the best out of it.
I am sure there are wrecked 993s you can salvage such parts from.
However in the end it is your money and you must decide how to spend it.
Ciao,
Adrian.

EK

My logic for the wheel speed sensor faults is as follows:

I am assuming that the accelerometer works intermittantly. If the car accelerates, both the accelerometer and the wheel speed sensors will detect an increase in speed. If there is a discrepancy between the accelerometer and wheel speed sensor readings, then it will be unclear which is at fault -- faulty wheel speed sensor or faulty accelerometer. Perhaps the default will be to give a wheel sensor fault code.

Since the wheel sensors appear to working when tested with a multimeter, I am assuming that the wheel sensor code is being thrown for some other reason than a faulty wheel sensor.

EK