abs coming in too early is this normal?

[bullyd]

every time i put any real pressure on my brake pedal my abs takes over, is this a characteristic of the e34 or is something wrong? there is no way my tyres were skidding, ive tryed it in various situations and the abs is cutting in way before the traction could possibly be lost! comments greatly appreiciated:)

[zygoteer]

ABS works by essentially comparing the relative rotational speeds of the wheels.
If some tyres are 'significantly' more worn than others then the effective rolling diameters of the tyres could make the rotational speed differences be beyond the control threshold making the system cut-in.

A more sensitive scenario could occur on a car using staggered wheels having slightly different tyre diameters front to rear.

[Jon K]

it would help too to know the situation this happened. Mine activate, but i was on 1/4" of ice on Kumhos

[bullyd]

ABS works by essentially comparing the relative rotational speeds of the wheels.
If some tyres are 'significantly' more worn than others then the effective rolling diameters of the tyres could make the rotational speed differences be beyond the control threshold making the system cut-in.

A more sensitive scenario could occur on a car using staggered wheels having slightly different tyre diameters front to rear.

im running 245 45 17 rear and 225 45 17 front,all bridgestone potenzas 'bout 3-4mm tread on rear 4-5mm on front

[bullyd]

it would help too to know the situation this happened. Mine activate, but i was on 1/4" of ice on Kumhos
tryed it in the dry under moderate braking , wet under less than moderate braking and abs cuts in, trust me there is no way im losing traction,

[Elekta]

I crashed at a 30 mph panic breaking situation...I sure wish the abs would not have kicked in b/c for sure my new rubber would have stopped me on their own.

abs is hard to work for us older folks because we just push harder when it engages when we should be releasing....would rather have skidding feedback frankly as my pumping the breaks might have prevented my bender.

in the paint shop today btw....it's been too long

[zygoteer]

... from your info ...
Front 225/45-17 has a rolling diameter of approximately 634 mm worn about 4 mm = 630 mm
Rear 245/45-17 has a rolling diameter of approximately 652 mm worn about 6 mm = 646 mm
(assumes approximately 9 mm 'new' tread depth - it varies between brands/models etc.)
Therefore difference = approx 2.5%
From what I know in the past, about Teves systems, this doesn't seem to be an excessive difference and would probably not cause your problem.

On this basis, I would look at the wheel sensors for a problem - if a sensor is missing pulses (symptomatic of a 'bad' sensor or dirt on the reluctors), or a sensor is picking electrical noise (usually caused by frayed wiring and/or bad connections) to give spurious pulses, then it would explain what you say.
Most sensors can be statically checked by measuring the electrical resistance with a DVM.

Uunfortunately it is my experience that getting sensors out to clean etc. can be a real problem - corrosion can make it such that extracting it will damage the sensor, so you end up fitting a new sensor anyway :(

[ScD]

I crashed at a 30 mph panic breaking situation...I sure wish the abs would not have kicked in b/c for sure my new rubber would have stopped me on their own.

abs is hard to work for us older folks because we just push harder when it engages when we should be releasing....would rather have skidding feedback frankly as my pumping the breaks might have prevented my bender.


If your ABS is working properly, it will stop you a lot quicker than any pumping of the breaks by a person. The most effective way to emergency stop in an ABS car is to stand on the stop pedal, nothing more.

[bullyd]

[QUOTE=zygoteer]... ]Most sensors can be statically checked by measuring the electrical resistance with a DVM. QUOTE]
whats the resistance meant to be? whats the threshold?

[632 Regal]

Wham! 245 45 17 rear and 225 45 17 front... I believe this is your problem.

[bullyd]

Wham! 245 45 17 rear and 225 45 17 front... I believe this is your problem.
why is this a problem i ran these tyres on my 520 with no problems? also this has only just came to my attention (im not saying the problem definitly wasnt there) but now it seems to be happening nearly everytime i push with any real pressure?

[632 Regal]

well, this is an obvious problem, why there was no issue on the other car I dont know. Try putting on a set of the same tires and see if that helps you out...hopefully someone lives close to you.

[romus]

Umm thats not the case in every situation

[bullyd]

i put on 225 60 15 basketweaves before work this morning and same issue!! (mabye a little worse) grr this is doing my head in! does anyone know where i can get info on how to test my abs sensors with a meter, i cant find it in bently but if its in there then ,hey just a page number will do :) cheers

[Dan in NZ]

If your ABS is working properly, it will stop you a lot quicker than any pumping of the breaks by a person. The most effective way to emergency stop in an ABS car is to stand on the stop pedal, nothing more.

Agreed. I read an interesting article in a motoring magazine, where they tested 2 different cars with ABS, and with the abs disabled. None of the drivers could match the ABS on the roads for stopping distance, plus the abs lets you keep control of the vehicle.

On dirt/metal roads it was different though, abs took longer to stop than locking the wheels up, as locked wheels push a ramp of dirt in front of the wheel. They concluded that the abs was still better than without, even on dirt roads, as you can still steer. I do a lot orf driving on metal roads, and in a car without ABS, if you have to stop suddenly, it's way too easy to lose control if you hit the breaks a bit hard.

REMEMBER: STOMP, STAY, STEER!

[bullyd]

Agreed. I read an interesting article in a motoring magazine, where they tested 2 different cars with ABS, and with the abs disabled. None of the drivers could match the ABS on the roads for stopping distance, plus the abs lets you keep control of the vehicle.

On dirt/metal roads it was different though, abs took longer to stop than locking the wheels up, as locked wheels push a ramp of dirt in front of the wheel. They concluded that the abs was still better than without, even on dirt roads, as you can still steer. I do a lot orf driving on metal roads, and in a car without ABS, if you have to stop suddenly, it's way too easy to lose control if you hit the breaks a bit hard.

REMEMBER: STOMP, STAY, STEER!
they obviously wernt using my abs:p

[Paul in NZ]

i have only felt my abs kick in once,,on a wet night brakes on cossing over painted signs on the road....

[632 Regal]

did you pressure bleed it? there could be some air trapped in the ABS unit...
i put on 225 60 15 basketweaves before work this morning and same issue!! (mabye a little worse) grr this is doing my head in! does anyone know where i can get info on how to test my abs sensors with a meter, i cant find it in bently but if its in there then ,hey just a page number will do :) cheers

[zygoteer]

whats the resistance meant to be? whats the threshold?

The Thresholds are set by your ABS control unit.



There are generally two types of ABS wheel sensors fitted to European designed cars.

Prior to about 1998, its mostly the ‘Variable Reluctance’ type, after that the ‘Magneto Resistive’ type is predominant - you need to know which type your car has - I don't know.

Test Method for ‘Variable Reluctance’ (2-wire) Sensors
This type gives a voltage from its pickup coil only when the wheel is rotated.
With ignition off, measure the coil resistance with an electrical test meter.
A short-circuit or open circuit is obviously a bad sensor, most typically come out at about 700 to 1500 ohms. – significantly more or less than this range is definitely suspect.

If this is OK, jack up the wheel and slowly rotate the wheel with the sensor connected to an electrical test meter, and measure the AC voltage across the two wires.
Typically expect from 50mV to 800mV – it will be rotation speed dependant !

Test Method for ‘Magneto Resistive’ (2-wire or 3-wire) Sensors ... sometimes called 'Magneto-strictive', and has characteristics similar to 'Hall-Effect' devices
This type requires a power supply from the ABS control unit.
With ignition key on (engine off) measure the voltage on the wires with respect to vehicle ground with an electrical test meter, .
If you have a 3-wire sensor, then one of them will be 0V = ground.
Sometimes the sensor cable is a screened 2-wire, the screen (outer braid) being ground.
If you have a non-screened 2-wire sensor then the ground is achieved through its mechanical fit to the hub – this kind tends to give trouble with older cars !
The remaining two wires are the voltage supply (most are nominally 12V DC – some odd Italian systems use 5V) and the rotation signal output voltage of around 1V.

Once you have determined it has a supply voltage (if it hasn’t then you have a wiring or control unit fault), jack up the wheel and slowly rotate the wheel, with the sensor signal connected to an electrical test meter, and measure the AC voltage.
The signal output voltage will vary from about 0.75V to 1.75V AC as the wheel is rotated.
When the wheel is static, the output voltage should be either the lower value or the higher value, anything else in between, or significantly above/below, is almost certainly a bad sensor.

For all types :
If you have access to a ‘scope, the waveform will be a distorted square wave, when rotating the wheel.
You need to see all the edges, of the pseudo ‘square-wave’, being similar in slope and over/under shoot (do a complete revolution of the wheel to check all poles in the hub).
If you get cyclic differences, then maybe the hub poles are damaged or just dirty with magnetically sensitive dust/mud.
For a reliable ABS action, the control unit needs to ‘see’ all sensors having very similar voltage AND waveform characteristic.
Notes :
1. If the sensor is not correctly positioned in the hub, then it will give odd results.
2. It is not uncommon for worn wheel bearings to affect sensor signals.

[bullyd]

The Thresholds are set by your ABS control unit.



There are generally two types of ABS wheel sensors fitted to European designed cars.

Prior to about 1998, its mostly the ‘Variable Reluctance’ type, after that the ‘Magneto Resistive’ type is predominant - you need to know which type your car has - I don't know.

Test Method for ‘Variable Reluctance’ (2-wire) Sensors
This type gives a voltage from its pickup coil only when the wheel is rotated.
With ignition off, measure the coil resistance with an electrical test meter.
A short-circuit or open circuit is obviously a bad sensor, most typically come out at about 700 to 1500 ohms. – significantly more or less than this range is definitely suspect.

If this is OK, jack up the wheel and slowly rotate the wheel with the sensor connected to an electrical test meter, and measure the AC voltage across the two wires.
Typically expect from 50mV to 800mV – it will be rotation speed dependant !

Test Method for ‘Magneto Resistive’ (2-wire or 3-wire) Sensors ... sometimes called 'Magneto-strictive', and has characteristics similar to 'Hall-Effect' devices
This type requires a power supply from the ABS control unit.
With ignition key on (engine off) measure the voltage on the wires with respect to vehicle ground with an electrical test meter, .
If you have a 3-wire sensor, then one of them will be 0V = ground.
Sometimes the sensor cable is a screened 2-wire, the screen (outer braid) being ground.
If you have a non-screened 2-wire sensor then the ground is achieved through its mechanical fit to the hub – this kind tends to give trouble with older cars !
The remaining two wires are the voltage supply (most are nominally 12V DC – some odd Italian systems use 5V) and the rotation signal output voltage of around 1V.

Once you have determined it has a supply voltage (if it hasn’t then you have a wiring or control unit fault), jack up the wheel and slowly rotate the wheel, with the sensor signal connected to an electrical test meter, and measure the AC voltage.
The signal output voltage will vary from about 0.75V to 1.75V AC as the wheel is rotated.
When the wheel is static, the output voltage should be either the lower value or the higher value, anything else in between, or significantly above/below, is almost certainly a bad sensor.

For all types :
If you have access to a ‘scope, the waveform will be a distorted square wave, when rotating the wheel.
You need to see all the edges, of the pseudo ‘square-wave’, being similar in slope and over/under shoot (do a complete revolution of the wheel to check all poles in the hub).
If you get cyclic differences, then maybe the hub poles are damaged or just dirty with magnetically sensitive dust/mud.
For a reliable ABS action, the control unit needs to ‘see’ all sensors having very similar voltage AND waveform characteristic.
Notes :
1. If the sensor is not correctly positioned in the hub, then it will give odd results.
2. It is not uncommon for worn wheel bearings to affect sensor signals.


thanx zygoteer, most informative, ill check the sensors at lunch today

[tim]

I know you guys know this but...
There is a way to disable the ABS, if you don't mind the chistmas tree of warning lights on your dash. At the track it doesn't matter- and thats where I learned it. The abs controller can be physically removed and the brakes will operate like a standard non abs system. It's designed to piggy back on the basic system, so if there is a module failure, the braking sytem will remain operational. Just remove the torx screws and pull the module off. Cover the connector with a baggie, and tape tin foil over the ABS controller sensor thingees where the module attached. Now you have conventional brakes, and you're not really hurting anything.

I recently had an abs module issue and I sent it to this guy, and he rebuilt it. Perhaps you could call him and see if there is a known issue with your unit. He seems to know everything about these things as his name suggests.
http://www.modulemaster.com/

[bullyd]

my rear right brake disk (rotor)was warped!
i had the wheels off today and my rear right brake pads were worn a shitload more than the left side, so, whilst at work i had a parts order come in for a friendly brake specialist we supply and opted to take my bimmer on the delivery, whist i was there i asked if he would take a look and he put a guage on it and said it was warped by a fair way, so i get home tonight armed with new disks and pads, fit them, now she brakes a lot harder before my foot feels like its being shaken from my ankle:D :D
the abs still feels slightly early but it was pissing down out so i may have lost traction on a wheel but shes 100% better than she was:D
thanks for all your posts,

[632 Regal]

We all learned something today, thanks for sharing!:)

[R1daveg]

Good info.
BTW, hows your front end bushes? I just had my front-end done, and the early ABS issue has gone. I'd heard of this issue before, as bush wear allows oscillation, which gives rapid wheel speed change.

[bullyd]

Good info.
BTW, hows your front end bushes? I just had my front-end done, and the early ABS issue has gone. I'd heard of this issue before, as bush wear allows oscillation, which gives rapid wheel speed change.
all front end bushes were changed very recently but handy info for people doing a search later,cheers for the info