ABS problem
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From: Surrey, UK
ABS problem
Hi,
I seem to have a problem with my ABS. There is no indication from the warning light (it comes on with all the other lights when the ignition is switched on, and goes off as soon as the car is started) - but if I brake too hard and the ABS cuts in things get a bit scarey.
I can feel the pedal pumping, but a lot more happens. The whole car seems to be shuddering - even my passenger could feel it, which I could live with, except for the brake performance. For the first half second or so, everthing seems OK. Then the computer seems to lose control slightly - the car doesn't slide but it seems like the brakes are totally released for about half a second. Then they cut back in for half a second or so and the whole cycle repeats (although it's not quite such an even cycle).
What I would really expect is controlled braking, with pumping through the pedal being felt - but I wouldn't really expect the passenger to be able to tell, and I certainly don't expect the car to suddenly lurch forwards as the brakes are released. When this is happening the braking is dangerous - it takes far longer to stop than if I hadn't braked that hard, and when it happened at the weekend I nearly lost the front of the car.
I obviously plan to get it checked over - but does anyone have any ideas? I'm assuming it probably means the ABS brain has a problem. The traction control seems to work OK which I assume are the same sensors. The size of the wheels is standard.
Dave
90 C4
I seem to have a problem with my ABS. There is no indication from the warning light (it comes on with all the other lights when the ignition is switched on, and goes off as soon as the car is started) - but if I brake too hard and the ABS cuts in things get a bit scarey.
I can feel the pedal pumping, but a lot more happens. The whole car seems to be shuddering - even my passenger could feel it, which I could live with, except for the brake performance. For the first half second or so, everthing seems OK. Then the computer seems to lose control slightly - the car doesn't slide but it seems like the brakes are totally released for about half a second. Then they cut back in for half a second or so and the whole cycle repeats (although it's not quite such an even cycle).
What I would really expect is controlled braking, with pumping through the pedal being felt - but I wouldn't really expect the passenger to be able to tell, and I certainly don't expect the car to suddenly lurch forwards as the brakes are released. When this is happening the braking is dangerous - it takes far longer to stop than if I hadn't braked that hard, and when it happened at the weekend I nearly lost the front of the car.
I obviously plan to get it checked over - but does anyone have any ideas? I'm assuming it probably means the ABS brain has a problem. The traction control seems to work OK which I assume are the same sensors. The size of the wheels is standard.
Dave
90 C4
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From: Surrey, UK
Hi,
I don't think the rotors are warped - all new. It did this before the brake update, when I tried it once in an empty car park.
When I say the whole car is shuddering, I think this is because the brakes are cutting in / out with fairly big gaps between. If I tried to "simulate" ABS manually - by repeatedly slamming the brakes on / off as quickly as I could - then this is what it would be like. Almost seems like the ABS unit is far too slow. However, it may be intermittent since I tried it this morning and it felt a lot better.
Dave
90 C4
I don't think the rotors are warped - all new. It did this before the brake update, when I tried it once in an empty car park.
When I say the whole car is shuddering, I think this is because the brakes are cutting in / out with fairly big gaps between. If I tried to "simulate" ABS manually - by repeatedly slamming the brakes on / off as quickly as I could - then this is what it would be like. Almost seems like the ABS unit is far too slow. However, it may be intermittent since I tried it this morning and it felt a lot better.
Dave
90 C4
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From: Parafield Gardens
Guys,
first thing to know is that the purpose of the ABS is to release the brake pressure. This allows the wheel to speed back up instead of locking up. The interval between cycles is a function of how fast the wheel is slowing down and how fast it is speeding up. The wheels and tyres you have fitted play a critical role in this function. The other item in the equation is the road surface. A slippery road surface will result in longer pedal cycles because grip is lost and must be regained for the wheel to speed up.
I do not believe there is actually anything wrong with your ABS system but maybe as a precaution you should get the brake fluid replaced (all of it), a full and proper system brake bleed as per the manual and then see. Brake fluid has to be replaced in a C4 every 2 years maximum. The cycling of the pedal is exactly what is supposed to happen. It can be disconcerting at first. To be quite honest my ABS hardly ever kicks in even with really heavy braking. I would suspect your tyres as being the culprits. In a C4 the ABS and traction control unit are one in the same unit and "VERY" expensive to replace.
Ciao,
Adrian
911C4
first thing to know is that the purpose of the ABS is to release the brake pressure. This allows the wheel to speed back up instead of locking up. The interval between cycles is a function of how fast the wheel is slowing down and how fast it is speeding up. The wheels and tyres you have fitted play a critical role in this function. The other item in the equation is the road surface. A slippery road surface will result in longer pedal cycles because grip is lost and must be regained for the wheel to speed up.
I do not believe there is actually anything wrong with your ABS system but maybe as a precaution you should get the brake fluid replaced (all of it), a full and proper system brake bleed as per the manual and then see. Brake fluid has to be replaced in a C4 every 2 years maximum. The cycling of the pedal is exactly what is supposed to happen. It can be disconcerting at first. To be quite honest my ABS hardly ever kicks in even with really heavy braking. I would suspect your tyres as being the culprits. In a C4 the ABS and traction control unit are one in the same unit and "VERY" expensive to replace.
Ciao,
Adrian
911C4
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From: Surrey, UK
Hi,
Thanks for the answer. Brake fluid is only 4 months old - it was changed when I had all new discs and pads fitted. Tyres are the same age, and are good tyres - Bridgestone SO2s. I have 993 16" wheels, but have seen the same problem on original D90s when the ABS cut in once before.
When the problem occured at the weekend it was on a level road in dry warmish weather. I did admittedly brake way too hard from about 40mph and under these circumstances I expected to feel the pedal pumping. What concerns me is that I can feel the car surge forward 2 or 3 times before it comes to a halt, and it takes a lot longer to stop than if I hadn't caused the ABS to cut in.
I'll take it to the garage and get a second opinion. I certainly hope it is behaving normally since I expected big bills if it's not, but I'll be surprised. I've had ABS on cars before and I could feel the pedal pump, but not the whole car surging. The brakes release for long enough on my car that it actually feels like it doesn't stop any quicker than if I was skidding.
Dave
90 C4
Thanks for the answer. Brake fluid is only 4 months old - it was changed when I had all new discs and pads fitted. Tyres are the same age, and are good tyres - Bridgestone SO2s. I have 993 16" wheels, but have seen the same problem on original D90s when the ABS cut in once before.
When the problem occured at the weekend it was on a level road in dry warmish weather. I did admittedly brake way too hard from about 40mph and under these circumstances I expected to feel the pedal pumping. What concerns me is that I can feel the car surge forward 2 or 3 times before it comes to a halt, and it takes a lot longer to stop than if I hadn't caused the ABS to cut in.
I'll take it to the garage and get a second opinion. I certainly hope it is behaving normally since I expected big bills if it's not, but I'll be surprised. I've had ABS on cars before and I could feel the pedal pump, but not the whole car surging. The brakes release for long enough on my car that it actually feels like it doesn't stop any quicker than if I was skidding.
Dave
90 C4
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From: Parafield Gardens
Dear Dave,
I would advise that you have the system bled. I wonder how this facility you had change the fluid did the job. Ask them. You may have air in your system. This will cause the ABS solenoid valves to chatter. The C4 ABS and Traction control systems must be bled in accordance with the manual and a Bosch Hammer must be used.
Ciao,
Adrian
911C4
I would advise that you have the system bled. I wonder how this facility you had change the fluid did the job. Ask them. You may have air in your system. This will cause the ABS solenoid valves to chatter. The C4 ABS and Traction control systems must be bled in accordance with the manual and a Bosch Hammer must be used.
Ciao,
Adrian
911C4
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From: Everywhere
I have had a similar experience with my race car. In my 90 964 coming down into a braking zone @ 130mph I have had this "skipping" with the result of me overshooting my turn in point. I could not get the car slowed down enough and as a result needed to maintain my straight line. It has only happened at this track at this corner. I had a friend driving my car and a he had a similar experience. We attributed this to the uneven pavement in the braking zone. I felt that the wheels where skipping on the uneven pavement giving me a feeling of having no brakes, very disconcerting at these speeds. When I altered my line entering the corner the skipping subsided. I have not had this happen at any other tracks.
Charlie
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From: Surrey, UK
Hi,
Quick update on this. The car is now booked in to have this looked at - it scared me to death last night when I failed to stop for a red light. It feels like the ABS cuts in too soon and once it does, braking is awful.
After this happened last night, I tried a "test" on an empty industrial estate to prove I really did have a problem:
1) Got up to 40mph, braked hard. First half of braking (about 1 second) was fine. Then the ABS cut in - even though I didn't change pedal pressure - and braking reduced a lot. Enough that I was actually thrown back in the seat as the car shot forward!
2) Over the same stretch of road, got up to 40mph then switched the ignition off and braked hard. No wheel lock up and I stopped in less than half the distance.
Something is definitely wrong - I'll give an update when I find out what. A full bleed will be the first try at fixing it.
In the meantime I don't think I'll use the car too much - if you see a white 964 coming up behind you, get out of the way!
Dave
90 C4
Quick update on this. The car is now booked in to have this looked at - it scared me to death last night when I failed to stop for a red light. It feels like the ABS cuts in too soon and once it does, braking is awful.
After this happened last night, I tried a "test" on an empty industrial estate to prove I really did have a problem:
1) Got up to 40mph, braked hard. First half of braking (about 1 second) was fine. Then the ABS cut in - even though I didn't change pedal pressure - and braking reduced a lot. Enough that I was actually thrown back in the seat as the car shot forward!
2) Over the same stretch of road, got up to 40mph then switched the ignition off and braked hard. No wheel lock up and I stopped in less than half the distance.
Something is definitely wrong - I'll give an update when I find out what. A full bleed will be the first try at fixing it.
In the meantime I don't think I'll use the car too much - if you see a white 964 coming up behind you, get out of the way!
Dave
90 C4
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From: Parafield Gardens
Dear Dave,
Seems to me that the ABS circuit which is always full of fluid but unpressurised is actually dumping the pressure on you. This does suggest insufficient fluid or air in the ABS circuit lines. I hope it is nothing too expensive but I also know that a failed solenoid in the ABS hydraulic unit or sequence control (relay mounted on top of the hydraulic unit)can also cause similar problems. The solenoid valves are supposed to cycle on and off, maybe they are not doing this. I am tending to think unmonitored fault here simply because what should happen is a logged fault will result in the ABS and Traction control circuits being switched off and normal braking resumed. It seems like your ABS is not switching off even though something is obviously wrong. ABS only faults on C4s are very rare.
What I recommend is the Bosch hammer hooked up and a readout of the fault codes logged in the control unit. You will need the Hammer anyway to bleed the whole system.
From your description of failing to brake, it certainly looks like a major loss of pressure in your system. The hydraulic pump should be screaming under these circumstances. Can you hear it running. The fact that you tested the brakes with ignition off (not recommended by the way especially if you have a pressure problem) suggests that your brake circuits are okay. ABS and Traction control circuits are electrically and mechanically isolated when power is removed.
Ciao,
Adrian
911C4
Seems to me that the ABS circuit which is always full of fluid but unpressurised is actually dumping the pressure on you. This does suggest insufficient fluid or air in the ABS circuit lines. I hope it is nothing too expensive but I also know that a failed solenoid in the ABS hydraulic unit or sequence control (relay mounted on top of the hydraulic unit)can also cause similar problems. The solenoid valves are supposed to cycle on and off, maybe they are not doing this. I am tending to think unmonitored fault here simply because what should happen is a logged fault will result in the ABS and Traction control circuits being switched off and normal braking resumed. It seems like your ABS is not switching off even though something is obviously wrong. ABS only faults on C4s are very rare.
What I recommend is the Bosch hammer hooked up and a readout of the fault codes logged in the control unit. You will need the Hammer anyway to bleed the whole system.
From your description of failing to brake, it certainly looks like a major loss of pressure in your system. The hydraulic pump should be screaming under these circumstances. Can you hear it running. The fact that you tested the brakes with ignition off (not recommended by the way especially if you have a pressure problem) suggests that your brake circuits are okay. ABS and Traction control circuits are electrically and mechanically isolated when power is removed.
Ciao,
Adrian
911C4
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From: Surrey, UK
Hi,
I haven't noticed the pump going - but I think you're definitely correct that it will be. When I booked the car in this morning, one thing the mechanic suggested was to start the car, wait a few minutes to make sure the system was fully pressurised and then pump the pedal hard 6-10 times to see if the pump activated.
I tried this - and in fact, the pump activates for about 5s after the second pump of the pedal. This is apparently much sooner than expected so it does support the low pressure idea.
From this, I guess I don't have an ABS problem - I have a lack of pressure in the brake system and this is playing havoc with the ABS.
I know that I didn't get a "full" bleed when the brakes were replaced. It's the first thing they'll try - and no doubt the cheapest fix - so let's hope it's that.
Dave
90 C4
I haven't noticed the pump going - but I think you're definitely correct that it will be. When I booked the car in this morning, one thing the mechanic suggested was to start the car, wait a few minutes to make sure the system was fully pressurised and then pump the pedal hard 6-10 times to see if the pump activated.
I tried this - and in fact, the pump activates for about 5s after the second pump of the pedal. This is apparently much sooner than expected so it does support the low pressure idea.
From this, I guess I don't have an ABS problem - I have a lack of pressure in the brake system and this is playing havoc with the ABS.
I know that I didn't get a "full" bleed when the brakes were replaced. It's the first thing they'll try - and no doubt the cheapest fix - so let's hope it's that.
Dave
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From: Parafield Gardens
Dear Dave,
The pump should crank up after 3 or 4 heavy pushes. Anyway, yes I would certainly start with a full system bleed done 100% correctly. Possibly replace the fluid as well and see how you go,
Ciao,
Adrian
911C4
The pump should crank up after 3 or 4 heavy pushes. Anyway, yes I would certainly start with a full system bleed done 100% correctly. Possibly replace the fluid as well and see how you go,
Ciao,
Adrian
911C4
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From: Surrey, UK
Hi,
Well this now appears to be fixed - got the car back last night and it now feels like I have steady braking when the ABS cuts in. A bleed solved the brakes completely releasing problem.
If I brake slightly too hard, then the ABS cuts in and feels OK. If I completely stamp on the brakes then it still feels like it's not braking as hard as it could - doesn't feel like it's really on the limit of traction, but the mechanic says it feels normal to him now and I have nothing to compare it to. At least it does slow down rather than feeling like the brakes are only on 20% of the time which it sometimes did before.
Dave
90 C4
Well this now appears to be fixed - got the car back last night and it now feels like I have steady braking when the ABS cuts in. A bleed solved the brakes completely releasing problem.
If I brake slightly too hard, then the ABS cuts in and feels OK. If I completely stamp on the brakes then it still feels like it's not braking as hard as it could - doesn't feel like it's really on the limit of traction, but the mechanic says it feels normal to him now and I have nothing to compare it to. At least it does slow down rather than feeling like the brakes are only on 20% of the time which it sometimes did before.
Dave
90 C4
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From: Surrey, UK
Hi,
Standard size SO2s. I should probably clarify that it doesn't cut in that often - it's just that when it did a couple of months ago, and scared me to death by releasing the brakes - I then got a bit paranoid about it. So before I took it in, I was standing hard on the brakes to see if I really did have a problem. Now that it's fixed, I've stood on the brakes a few times to see that it definitely is OK.
If I drive "normally" then it virtually never cuts in, even id I brake hard. I just want to be sure it is working OK since I don't want to over-do the brakes one day and find the problem has come back.
Dave
90 C4
Standard size SO2s. I should probably clarify that it doesn't cut in that often - it's just that when it did a couple of months ago, and scared me to death by releasing the brakes - I then got a bit paranoid about it. So before I took it in, I was standing hard on the brakes to see if I really did have a problem. Now that it's fixed, I've stood on the brakes a few times to see that it definitely is OK.
If I drive "normally" then it virtually never cuts in, even id I brake hard. I just want to be sure it is working OK since I don't want to over-do the brakes one day and find the problem has come back.
Dave
90 C4