Brake failure temporarily !!!

steviewonder7 · Unread post by **steviewonder7** » 07 Jun 2006, 21:42

Hi Folks,

Hope you can help me on this:-

Today shortly after pulling away from an arcade of shops I went to brake as i was coming to a junction,doing approx 20 miles an hour speed wise when I went to brake and the dam car wouldnt stop,the brake pedal was pulsating quite violently.I managed to stop the car on the handbrake without hitting anyone or any thing and severely giving myself heart palpatations and nearly brown pants.
After recovering from this brief escapade I looked under the bonnet of the car to see if I could see anything amiss.I couldnt see anything wrong.

My home is only half a mile from this little venture so I gingerly drove back there and the brakes acted like normal and I couldnt get it to do the same thing again.
The only thing I have noticed recently is that when driving the car and coming to a dead stop is the back end of the car rises very slightly.Its like the accumulator sphere is not doing its job and keeping up the system pressure.
Any of you kind folks got any ideas ?

Thanks in advance to any of your replies.
Steve

406 V6 · Unread post by **406 V6** » 07 Jun 2006, 22:09

Before having no brakes you should have the suspension lowering itself. At least i recall the emergency shutdown sequence was PAS-suspension-brakes.
How are the Acc. ticking intervals?
Does the car rise then fall again to it's height?

AndersDK · Unread post by **AndersDK** » 07 Jun 2006, 22:13

The violent pedal pulsating must have been the ABS cutting in. Dont worry about this - as weird ABS behaving most likely is caused by basic system problems.

Do the intial basic checks :
1) reservoir level - check at highest height setting ONLY

2) click interval from pump/regulator at least some 30 seconds
3) pump large dia feed rubber hose NO surface cracking - and tight secured on reservoir + pump studs.
4) Do the Citaerobics (up/downs) - and after this bleed the brakes.

This will ensure 99% of most likely problems are found & solved.

BTW : did you take off with the STOP lamp lit ? (sorry about this - but so many hydroCit owners does in fact do this)

jeremy · Unread post by **jeremy** » 07 Jun 2006, 23:00

Stevie - Try what Anders suggestes first but if you have to come back on here can you tell us if your car has anti-sink or not please. Anti-sink was introduced around 1994 and if your car has it it will have a dual outlet pump (majority for the steering, 2 cylinders for brakes and suspension) and additional 'anti-sink' spheres front and back.

Mandrake · Unread post by **Mandrake** » 08 Jun 2006, 00:59

Pulsing pedal with no braking sounds like a faulty ABS sensor or wiring to me.

I've had the same thing happen to me before when I had (intermitantly) faulty wiring to the front right ABS sensor. Normally ABS is supposed to disable itself below a certain threshold speed - approximately 20Km/hr.

Why ? Because the whole way ABS works is by measuring the speed of each wheel, taking the fastest rotating wheel as the "road speed", and any wheel that is doing a much slower speed than the fastest wheel, or not turning at all due to locking up has its brake pressure momentarily removed to allow the wheel to spin up again and regain traction.

(There's more to it than that but thats the basics)

If the ABS didn't disable itself then you could never fully stop - because stopping implies all wheels have stopped turning.

So the way around that is that below a certain threshold speed the ABS deactivates, to allow you to fully stop.

My theory of what happens is that a faulty sensor and/or wiring can cause extra spurious pulses to be sent to the ABS computer from that wheel, thus making the computer think you're going faster than the threshold speed.

For example say you're really doing 10Km/hr and trying to come to a stop, but extra pulses from one wheel make the computer think that wheel is doing 40Km/hr.

The computer will assume you're doing 40Km/hr with 3 of your wheels locked up!

And of course it will release the brakes on 3 of the wheels. (Since the rear wheels are controlled together, if a rear sensor is faulty it will effectively release ALL the brakes)

After about a second the computer will usually realise there is a fault condition and deactivate itself (thus re-activating your brakes, and bring up the ABS light) or the fault condition itself may go away. (wires flexing with suspension movement etc)

Unfortunately FINDING which wheel it is can be very difficult if the fault is only transient... I wasn't able to find which wheel it was on my car for months until finally one day the ABS light came on permanently and by measuring the sensors from the ABS plug I worked out it was the front right sensor and fixed it.

I've had no more occurances of this "momentary missing brakes at slow speeds" problem since fixing it...so I'm fairly confident that spurious signals to the computer is what causes it...

Regards,
Simon

Mandrake · Unread post by **Mandrake** » 08 Jun 2006, 01:20

A few more thoughts...

If it is an ABS fault, it is possible to disable the ABS by removing its fuse.

This will bring the ABS light on permanently, and fail an MOT (naturally) but at least the brakes wouldn't be mysteriously cutting out while you're trying to find out what the problem is - the brakes will behave exactly like a car without ABS.

The other thing is it might be worth getting someone with a diagnostic computer to read the ABS fault codes - I'm not sure how much detail it goes into, but I believe it logs transient faults, and that may identify a faulty wheel...

Regards,
Simon

jeremy · Unread post by **jeremy** » 08 Jun 2006, 06:34

Simon

Does this mean that if the ABS is switched from monitoring to active mode the valve block should start chattering away happily on its own? As I understand it the modes are changed by operation of the brake light switch - so (on a private road slowly of course) if the brake light switch were closed (lights on) the computer will be in operating mode - but there will be no brake pressure - so brakes don't come on and the car could be driven for a while like that.

Incidentally you can monitor a sensor's performance by listening to it using a cassette recorder or portable amplifier.

Mandrake · Unread post by **Mandrake** » 08 Jun 2006, 07:30

Hi Jeremy,

jeremy wrote:Simon

Does this mean that if the ABS is switched from monitoring to active mode the valve block should start chattering away happily on its own? As I understand it the modes are changed by operation of the brake light switch - so (on a private road slowly of course) if the brake light switch were closed (lights on) the computer will be in operating mode - but there will be no brake pressure - so brakes don't come on and the car could be driven for a while like that.

Under no circumstances would an ABS system rely on something as potentially unreliable as a brake light switch to activate it...

The computer can tell everything it needs to know based on measuring the individual wheel velocities. The ABS computer is actively monitoring the wheel velocities whenever the ignition is turned on...

It could potentially have a pressure switch in the ABS block to measure when the brakes were being applied (much like the Hydractive 2 front brake pressure switch) but I doubt even that is necessary.

It would continually monitor the velocity of each wheel and it would have a lookup table of "realistic" decelerations. If all 4 wheels go from 50Km/hr to 0 Km/hr (total lockup) in a fraction of a second it knows the wheels just locked, as the car can't physically decelerate that quickly.

So it's course of action would be to release all the brakes for a fraction of a second, and see whether the wheels suddenly start spinning again...

Also if it measures a difference in relative wheel velocity significantly greater than what is possible on full steering lock, then it can use the fastest spinning wheel to get an idea of the actual car velocity, and assume that any wheels that are spinning a lot slower than that or stationary must be locked, and individually release them.

Because the ABS can only ever "release" the brake caliper, never apply it, naturally it has no effect when the brake isn't depresssed, so theres no need for it to know when the pedal is pressed...

Functionally a pretty simple algorithm, the challenge is in making the system ultra reliable in the face of a myrid of potential faults....and while 99% of the time ABS systems can detect and deal with faults in a safe way (by disabling themselves and warning the driver) there do appear to be very specific circumstances where they can be "fooled" into doing things like releasing the brakes when coming to rest...

Incidentally you can monitor a sensor's performance by listening to it using a cassette recorder or portable amplifier.

Yes, good advice, although trying to get it to play up on demand is challenging

I tried for months to get mine to reproduce the fault on demand and it never did... its not until it died completely that I tracked it down, which is why I suggested getting the fault codes read...

Regards,
Simon

steviewonder7 · Unread post by **steviewonder7** » 08 Jun 2006, 09:02

Just to answer Jeremy's question,the car does not have antisink features and therefore the extra sphere etc etc.

I will be looking at the car in more detail today,I have the day off,and trying some of your sugeestions people.Thanks thus far for help/input on the matter.

Thanks Steve

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steviewonder7 · Unread post by **steviewonder7** » 08 Jun 2006, 21:23

hi folks,

Its been a long day and these are the things I've been able to do on the car today to try an establish this brake problem:-

I've checked the abs sensors reading wise with an avo at the abs ecu multiplug.All readings well within specified limits

Checked all fuses in fuse box next to abs ecu....All o.k

Checked reservoir level when car set at max height....O.K again

Clicking interval from pump/regulator before any work carried on the car,time was approx 54 seconds between clicks.The thing is after working on various things the clicking interval reduced to 44 seconds.!!!!

I bled the brakes front and rear.No air what so ever was evident,and I used at least a bottle of lhm fluid to replace that that was bled out of the braking system.(all nice and green to)

Did citrobics after at least a half a dozen times.

Remade both ends of the hose that feeds the hydraulic pump.(I wasn't convinced that it was totally sealed at the pump end(very slight weep evident at the pump end.(why on eartth do citroen make both ends of this hose different sizes ?? Was going to renew hose until I found this out)

To answer Anders question,my dashboard Stop light is functioning just fine.

After re pressurising the hydraulic system a few times and doing citrobics
I took the car out for a run around the block,seemed o.k ,brakes have a slightly firmer feel to them.I just hope I dont get a repeat account of the brake temporarily failing again,not to say that its been cured though.
Dont know what to do next !!

Any more suggestion folks?

AndersDK · Unread post by **AndersDK** » 08 Jun 2006, 21:41

Its very important to understand that the ABS system is fitted - as an extra - its not the main part of the brakes system. The ABS is fitted to give you that extra bit of safety in case the car starts braking un-evenly on emergency braking.

Meaning : you can rip the plug to the ABS block if you have a feeling its starting to get its own mind - which is defo a lot more dangerous than not having an ABS system at all.

There is no more to the ABS system really.

Having said that - then there is large chance you have found the problem : "brakes have a slightly firmer feel to them" which could have been any air bled out the brakes.
Air in the brakes of a hydroCit does not give that spongy pedal feeling wellknown from "normal" cars.
Instead the pedal feels hard as there is a delay from pedal push to brakes acting. This is because the hydraulic pressure needs a finite time to compress any air present.

Now say that this was the problem for one wheel only -
Then this wheel would run on while the opposite wheel would slow down - and your ABS system immediately detects this as a problem and kicks in.
We are talking front brakes here - as this is where large forces are involved.
NOTE : exactly the same thing would happen if one wheels brakes was wet, dirty or disturbed by a chipstone.

Drive careful -

Gregg1100 · Unread post by **Gregg1100** » 08 Jun 2006, 21:47

Hi,
I get a momentary brake failure when I reverse out of parking space first thing in morning. The first dab on pedal gives no results, the second dab is ok. Thereafter brakes are fine all the time I drive the car that day.
Xant 1, VSX 16v, 2 ltr.
Greg

jeremy · Unread post by **jeremy** » 08 Jun 2006, 22:29

Gregg - has your car got anti-sink?

steviewonder7 · Unread post by **steviewonder7** » 09 Jun 2006, 08:17

Thanks to your reply Anders.If I read you correctly,if my abs is causing this braking problem,it would be safe to immobilise the abs from the braking circuit and use the brakes like a conventional car if need be?

Thanks steve

BonceChops · Unread post by **BonceChops** » 09 Jun 2006, 08:58

This might sound like a daft question -

Are you sure there was no oil or something like on the road that would have casued the ABS to chatter as it tried to stop the wheels locking up?