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Strange brake behaviour

jimbothemagnificent

jimbothemagnificent

Messages
2,683
Hi guys,

I could use some advice and ideas on some very strange things my brakes are doing.

All pads and discs were replaced when I bought the car 1 year ago. Recently the ABS light has come on intermittently, however, fault code reader has drawn a blank. Several times while braking gently from very low speeds (around jogging pace), the pedal has twitched as if abs has cut in. The brakes don't feel 100%, though it could just be my imagination and paranoia. They still stop the car effectively, though I've lost all confidence in them now

I've had the wheels off and had a good snoop around, and I can't see anything wrong. Pads appear to have plenty of life left, and discs seem fine. Unfortunately I am flat broke this month, and I can't afford to take it to the garage to have it checked.

Does anybody have an idea of what is wrong? And do you think it's safe to drive?

All I can think of is either boiled brake fluid or an electrical fault.
 
Boiled fluid wouldn't cause that.

Check the ends of the wheel speed sensors for swarf/crap/grit

The ABS C/U could be getting an incorrect wheel speed signal and cutting ABS in early.
 
Excuse my ignorance, but what do the speed sensors look like?

That does sound like a very sensible interpretation. I though it could be a similar problem, but I don't know enough about abs to speculate.
 
ABS is one of my specialist subjects I teach :)

They will be located in the hub area, behind the disc.

They will have an electrical connection going to them so shouldn't be hard to find.
 
Amazing, seems I've found the right person! Mind if I pick your brains?

The way I imagine abs to work; is that the control unit compares wheel speed, and loss of speed, to a predetermined model of how quickly the car can stop. If the wheel speed decreases faster than the model says the car speed car decrease, then the cadence effect is applied via the brake servo.

I assume that the speed sensors detect rotational speed of the wheel electronically from the hub without contacting it directly, electromagnetism springs to mind. So brake pad and rotor debris could interfere with this signal causing incorrect readings, and therefore incorrect abs application.

This is all conjecture based on my basic understanding of physics and engineering....
 
Close.

Even simpler.

ABS control unit monitors the wheel speeds via inductive sensors at the hub using reluctor rings with teeth on.

Simply, when the ring stops turning, the abs unit opens a valve in that brake circuit to release pressure at that individual wheel, waits to see it rotating again and closes the valve and re-applies pressure by utilising a pump inside the unit. Up to 40 times per second. Cadence effect. Servo isn't involved.

Electromagnetism is correct.

Inductive wheel speed sensors, so a magnet with copper windings around it. When teeth of the reluctor ring pass if, they induce a voltage creating a sinusoidal ac wave form utilised to measure speed and movement. So RS debris could interfere with the signal.

Newer stuff will use magnetoresistive sensors to also measure direction of travel.
 
Ah, that makes a lot of sense. It does make me wonder why this isn't a more common problem, metallic debris must be abundant in that area.

Thanks for your help Stoo, I'll whip the wheels off and give it a good clean this afternoon, see if it helps.
 
I had similar symptoms in a Fiat Punto I once owned, cleaned up the sensors and all was good, sometimes some crap gets on them and causes the readings to be 'messed up', so try giving them a clean, it's the cheapest option.
 
Swarf on the sensor reduces the amplitude of the sensor signal and therefore fools the car in to thinking it's going slower than it actually is, so ABS will cut in sooner.
 
Update for solved problem!

The problem is caused by a hairline crack in the reluctor ring. The slight increase in distance between the teeth is causing a flatspot in the sine wave, this this is being interpreted as the wheel stopping, causing abs to be applied. The flatspot in the wave is so small that it is only detected at very low speeds.

I'll report back again when I have the money to fit a new reluctor ring.
 
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