Can someone give me a description of how the system acually works? Hows it know when you are slipping, etc?

Can different pads, rotors, etc somehow mess up the performance of the system?

 

ABS works by sensing different wheel speed from wheel to wheel under braking. If one wheel is spinning at 40 mph while the other is spinning at 10 it knows the wheel spinning faster is slipping. It then pulsates the brake fluid at varying rates until the same speed is reached. In some cars this is only measure between front and rear axles. In ours it is 4 wheel, meaning each wheel independantly is sampled. There is a sensor somewhere, it depends on design. If you have different equipment on either side of one axle it will throw it off. But aftermarket pads/wheels/rotors shouldn't throw it off.

 

The ABS system on all US FD's is a three channel system. This means the ABS can pulse the fronts independently, but the rears are pulsed together (this is why your ABS pump only has 3 outlets). The ABS system senses tire slippage by monitoring a sensor on each wheel hub. The hubs have toothed wheels that pass in front of this sensor and the sensor gets its speed measurement by counting how quickly these teeth are going by. If a tire were to lock and skid it also must stop turning, this is how the system knows a tire is locking. The system wil then temporarily release some pressure at that wheel (if it's a rear, both wheels) until the tire begins turning again.

Various systems are capable of reading wheel speed and varying the hydraulic brake pressure at different frequencies. I don't know what frequency the FD operates at but the higher the better as a higher frequency will allow shorter stopping distances. If a system's reaction were "slow" and you upgrade to race tires for instance, the tire may be able to decel the car so quickly that the ABS will feel a lock up is impending and begin modulating pressure because the ABS software is not capable of dealing with the increased grip and therefore more rapid decel rates. I have not heard of this being an issue in many years as the modern systems are very advanced. I know on my FD the ABS works just fine on race tires. Since the ABS system is actually monitoring the turning of the wheel hubs, different wheel and tire sizes will not effect its ability to function properly. Nor will different tires on different corners of the car (though that is not recommended).

 

DamonB, good responce. I thought we had 4 channel, regaudless. If you have a larger circumference wheel on the front right than on the front left (for argument) wouldn't it throw off the system as there would always be a speed differential? Kind of like with traction control.

 

The FD system is not smart enough to know. Strictly for ABS it just needs to know if the wheel is turning; relative speed differences between different wheels does not matter strictly for anti-lock. Now traction control or the various stability control systems (none of which the FD has) are very different. They will use the speed differentials you pointed out to figure not only the braking issues but also induce or correct yaw. Tire diameters different from stock in that case would cause some strange reactions in the system.


The FD ABS system is a four wheel system that operates on 3 channels as the rear wheels are treated by the ABS as a single unit; both rear calipers are always pulsed together regardless of which one is locking. 3 channel was common in the early 90's, but the last half of the decade brought hugely improved ABS and stability and control systems.

 

Having different tires (even different sized) on each side of the front shouldn't affect the ABS. It may affect the car (skew it) during braking, but the ABS can't see that. It skews the car because, as stated over and over, the brakes do not stop the car, the tires do - so one more grippy one on one side will pull the car in that direction. Just do as I do to test. I love to torture the ABS of the work Suburban (I hate that system - it has intermittantly failed on me twice!!! - ie - depress brake pedal and NO BRAKES! - quick repress and normal braking - but I digress). I put the two drivers side tires on the snow at the centre of the road (no traffic of course) and that left the two passenger side tires on dry pavement. Apply brakes hard enough to lock up drives side tires. ABS goes crazy trying to control brakes. This is similar to having much different grip tires on each side. To it's credit, usually the Suburban does not pull to the passenger side for more than a small fraction of a second. It certainly does not go out of control, and it would without a very good driver or ABS. Not sure which is resonsible in this case ;-).

 

ahh, ok i was confusing 4 wheel with 4 channel. Thanks again!

 

dont mean 2 change the subject....but has anyone experienced their ABS system going crazy after giving the car a jump?

cuz that happened 2 me a few months back... battery died cuz of my sub lol. gave the battery a jump, and from that point on, the ABS unit was always on... even when u took the key out, locked the doors, etc..u could still hear this hissing from under the hood. open the hood, come rite near the ABS unit, and low and behold, that's where the hissing came from (the ABS light was on in the dash as well...)

the hissing im talkin about u can actually hear every time u start ur FD...it comes on for a few seconds, then stops.... but w/ me, it kept on goin...

needless to say, havin this stay on drained my battery, so i needed to get a jump pretty much every day that week. until bout the 4th jump, when it stopped. just like that. never gave it much thought till waaay later on, when i was flippin thru the owner's manual, and it said the ABS unit may act funky (it said somethin like malfunction or the ABS light might come on in the dash) after the car is jumped, and its only temporary...

was this a unique experience? or has anyone else ever have this happen 2 them?