DaveK

Hi,
Some of you will remember that last year I had problems with my ABS - that when it was activating it would sometimes release the brakes way too much. This would mean it took ages to stop - and sometimes even threw me back in the seat as the car suddenly launched forward.

A replacement ABS hydraulic unit didn't fix it, repeated brake bleeds didn't fix it - and neither did a temporary swap with another ABS control unit. The one thing that did fix it for a few months was bleeding the clutch.

However, the problem is back. If I stamp on the brakes, sometimes I stop very quickly and sometimes definitely not. It's often felt to me like the ABS was a bit too keen to cut in sometimes - the cases where it works, the ABS hardly pulses. The cases where it fails the ABS seems to cut in very early and the pedal stays pretty hard. This makes me suspect sensors (although I know they should log a fault if they fail).

But - the one other side effect - after a period of heavy braking, the clutch will not return fully. But this is the only time I have that problem. Which makes me suspect a pressure problem.

I am going to take the car to a different mechanic (JZM) to have the brake system flushed (again) and get a second opinion. If anyone else has thought of possible causes in the last few months, let me know.

DaveK

When I booked the car into JZM, I had an interesting conversation with Jonas.

He said that they had seen a similar problem recently - and it was the longitudinal accelerometer sticking. This can cause the ABS to cut in too early (which I've always felt mine does) and reduce the braking too much, even though the wheels are not locking.

Most of the work I've had done for this so far was hydraulic based so this is approaching the problem from the other end, and it sounds hopeful. It's booked for 3 weeks time - I'll post the results when I get the car back.

Johnny G Pipe

Dave, I still get the ABS overactivating, it (sometimes)pings and jumps under my foot usually as I coast to a stop at junctions. I await your results expectantly! My own problem I hope is going to be improved by a new battery - it doesn't happen to my car so much if the battery is charged up beforehand.
I have to say that your previous experience does seem to implicate the clutch circuit...and also again, if an accelerometer is down, the hammer would surely detect it? I guess this wouldn't be the case if it was an intermittent fault, tho.
Anyway, good luck!

Christer

Dave

Sorry to hear problem is back but hopefully JZM will get it sorted out. Can't believe I let you drive behind me when we met up with Randall G!

I'll be up at JZM on Sat 17th May for some 'tweaking' so if anyone is else is due in that day - see you there!

DaveK

Johnny,
Apparently, it depends where the accelerometer sticks. Jonas said that these things are basically pendulums and if they stick so that they are sending a signal then the computer is happy and doesn't log a fault.

John Boggiano

Very interesting.
Where is the accelerometer located?

DaveK

Ah, now that I can answer (but only 'cos I read Adrians article on John Miles site). It's under a flap on the right hand side of the transmission tunnel. But - I don't think the C2 had a longitudinal one (or at least, it doesn't affect the ABS on a C2).

Taj

C2 does not have them, the ABS is driven by the wheel sensors only.

Taj.

Adrian

Dear Dave,
The longitudinal accelermoter is only fitted to the C4 and it only informs the PDAS/ABS control unit that the C4 is decelerating. Its function is for logic control within the control unit. The traction control system is disarmed and the ABS is armed when the 964 start to decelerate. The input of the acelerometer is based on the rate of change of voltage as the moving mass moves. If it sticks then its output is ignored. As far as the system is concerned the C4 has ceased to decelerate and if the signal does not change is a specific time the fail lights will come on.
The primary source of all wheel speed control is the four wheel speed sensors.
The reason the accelerometers are not in the C2 (there is a lateral unit in the tiptronics but this has nothing to do with ABS) is because there is no traction control to turn off.
The ABS will activate when a wheel blockage is detected between 20% and 40% based on a wheel speed reduction of 0.8 kph front to front or front to both rears.
There are two possibilites;
If the brakes seem to stay off for a period of time it means the ABS solenoid valves are in the pressure reduction mode. Your problem means at least both fronts are in this mode. This means the wheel speed sensors are saying both front wheels are blocked, or
There is insufficient brake pressure or fluid in the brake cct when the ABS valves return to the pressure increasing or off position.
The questions I have to ask are:
Have larger calipers been installed?
The second question is, what the status of the brake pads?
This ABS problem normally occurs on 964s which have had larger brake callipers installed and the brake pads are worn sufficiently to cause fluid capacity problems.
The question I am asking is of course, do you actually have an ABS problem or do you have a brake fluid capacity problem.
Your problem is clearly in my opinion as it always has been mechanical in nature and not a system component problem.
Ciao,
Adrian
911C4

DaveK

Hi,
I'll have to hope JZM find the problem whatever it is - but I must confess I was a lot more hopeful before I saw Adrians reply.

Adrian - standard brake calipers, and pads are fine (fronts were renewed at the last service at the end of August).

I will ensure that JZM completely flush the system to start with. After that, they will simply have to check everything.

Adrian

Dear Dave,
Are these the same people who changed out your ABS hydraulic unit with the claimed return pump fault. Over current I believe.
Standard brake system, okay. I would be looking at the return lines to the brake fluid reservoir. Make sure one is not blocked. The clutch issue is interesting because the clutch has its own sub reservoir inside the main reservoir as does the other systems.
We know that the problem is not the main control units.
Now I have to say that I had this happen to me on the ice under heavy braking. The ABS system of the 964 is somewhat limited.
Sadly in these cases I would love to have the C4 in front of me.
What tyres do you have installed?
Has the master cylinder been checked. The front brake cct is push rod actuated. An internal leak to the return line might be the cause.
What about the rear brake bias valve or regulator. Is this working. Are you getting brake pressure to the rear wheels. Some people have removed this regulator and caused themselves huge problems.
I would also be checking the brake lines around the ABS hydraulic unit for damage.
Ciao,
Adrian
911C4

DaveK

Hi,
The people who suggested the accelerometer are different from the people who replaced the ABS unit - I am taking it somewhere else simply for a fresh perspective. Interestingly - when they told me about the car they'd seen with a similar fault and told me about the accelerometer, they said it had also had blocked brake lines which had also confused the control unit. It sounds like this was probably the real cause.

Neither has been able to explain the clutch issue because they both say the only shared thing is the reservoir and it makes no sense. Definitely happens though.

I know that there will be limitations with the ABS given the age - but the rate at which it occasionally shoots forwards is enough to convince me there is something wrong.

Tyres - Bridgestone SO2 (almost new).

I'm not convinced the bias valve has been checked - I did touch the discs the other day after getting home and the rears were warm so there is some brake force there. Fronts were warmer (and front right was noticeably the warmest, although the car brakes in a straight line).

I don't think master cylinder has been checked, but I will make sure it is. I've always wondered about it - when the ABS works normally and the car slows properly I don't hear anything unusual. But when the car does it's trick of shoot forwards / hard brake pedal for several seconds there is a noise. It sounds a bit like the pump that pressurises the system but I don't think it is - it always sounds like it's coming from the brake pedal area, whereas the pump noise first thing in the morning doesn't seem to come from anywhere in particular.

After your comments yesterday - and a bit of testing this morning - I'm convinced it is still a hydraulic issue. It will occasionally have the problem immediately if I just stamp on the brakes but it doesn't usually happen in that situation. The easiest way to make it happen is to brake hard from speed - but not hard enough for the ABS to cut in straight away. But as I slow, at the point I guess the ABS would begin to activate, (sometimes) the pedal goes hard and the car shoots forward. Almost like I used all my pressure up in the first part of braking (although I know that doesn't make much sense).

One of the reasons I'm taking it to a different garage is that I will have had two independents flush the system so I can be sure that it's been done properly. Hopefully, they will also check other things that have been missed by my regular garage.

Adrian

Dear Dave,
These problems have to be taken slowly and make sure that the people working on it are system experts. Gets a tad expensive otherwise. There is a techncial overview of the systems on the rennlist technical advisory page.
The noise you speak of tends to point to the master cylinder assembly which is mounted on the front of the brake booster.
The brake circuit and the ABS are in series with each other but isolated due to design.
Your last paragraph says to me you are losing brake pressure not having ABS problems. This suggests a leak of pressure in the master cylinder to the return line or you are losing boost pressure. With a hard pedal in a C4. You are not losing systempressure because the warn light would come on. I suggest that the master cylinder and booster be checked. Please understand that fluid from the booster system and brake system never mingle until the fluid reservoir. This is very important to understand. The boost pressure helps you move the pedal. The pedal movement determines the pressure in the brake system.
Summary,
Hard pedal "no brake pressure" you will shoot forward.
Ciao,
Adrian
911C4

DaveK

Hi,
Here is an update on my wondeful brake system...

The car went to JZM a week ago, and they called me the next day and told me it was fixed, and that it was the accelerometers - both were stuck. After Adrians comments above (which make logical sense to me), I really didn't believe that this was going to make any difference at all but they were very confident that they had fixed it, and I collected the car. It was fairly busy on the way home, so I only got a couple of opportunities for a heavy braking test. And the result was.......

Blimey!

I braked "hard" - same as I usually do when testing the previous attempted fixes - and the ABS didn't activate at all - I came to a very rapid stop. I have always felt that even when my ABS was "working" that it was a bit enthusiastic and cut in before it really needed to. It turns out it was VERY enthusiastic and was activating far more than it should. I finally understand some of the comments I've had in the past on this subject like "My ABS has only activated once in 3 years....", and the comments on how good 964 brakes really are.

I was away most of last week, so still have not tested extensively. But I have proved conclusively that under "heavy" braking which would usually have my ABS pulsing away (and sometimes releasing the brakes for a few seconds) the ABS no longer activates at all. I have to brake very very hard for the ABS to cut in now - and so far, when it does it works perfectly.

I am amazed at how much quicker the car stops now - I thought my problem was only occuring occasionally when the pedal went hard for several seconds, but it turns out it was happening much more than that, and in 99% of those cases the ABS should not even have been activated.

I'm fairly confident that this problem is now solved, and I feel a lot safer in my car (as do all the people in front of me).

I must admit I still don't understand why the accelerometers caused this problem - and why they never caused warning lights - but I can't argue with the results.

johnfm

I think these accelerometers are also in the C2 tiptronic - I may check these as, with Johnny G, my ABS activates approaching junctions & turning in at relatively low speeds - though I think it is irregular road surface that causes this - as the front tyres encounter rippled road surface, traction reduces enough to bring on a brake lock up and the ABS kicks in. Something for me to check now I suppose.