ABS Problems at Low Speed

[Jakg]

Originally Posted by HarryM1BYT

I'll bet this turns out to be nothing to do with the sensors, ABS or the ring.

If you have any other suggestions I'm all ears...

[HarryM1BYT]

Originally Posted by Jakg

If you have any other suggestions I'm all ears...

Wear and play in suspension parts, allowing a wheel to turn slightly out of line when braking - something along those lines..

[Mike Noc]

Originally Posted by Jakg

When braking very very gently at low speeds (<15 MPH), I sometimes get ABS kick in very lightly.

Stopping from faster speeds is fine, and ABS kicks in normally if you go for an emergency stop.

Had the same problem and found it was caused by a pattern sensor not quite being near enough to the magnetic reluctor ring on the bearing. Worked fine at speed but when the wheel was rotating slowly it started missing the odd segment.

Cleaned the backplate and made sure the sensor was seated correctly but still had to file around a mm off the back of the sensor bracket to sort it.

[HarryM1BYT]

Originally Posted by Mike Noc

Had the same problem and found it was caused by a pattern sensor not quite being near enough to the magnetic reluctor ring on the bearing. Worked fine at speed but when the wheel was rotating slowly it started missing the odd segment.

Cleaned the backplate and made sure the sensor was seated correctly but still had to file around a mm off the back of the sensor bracket to sort it.

That would be something a scope would easily identify.

Going back to basics...

The ABS is triggered, because it sees enough of a difference in rotation speed between one or more of the wheels. The usual cause of a difference being that wheels being braked are slipping on the road surface, so to maintain grip and balance, it reduces the braking effect on the wheels sufficient to get the rotational speeds to match.

Under inflated tyres, excess movement in the suspension and similar can all cause the ABS to trigger when it should not. As also can the sensor simply not responding at low speed as pointed out by Mike above, or metallic dirt on the ring producing distorted or extra pulses.

[Jumper75]

Originally Posted by HarryM1BYT

That would be something a scope would easily identify.

I agree, any cheap handheld scope will do it. A DSO (Digital Storage Oscilloscope) will do even better as you can freeze the trace.

Handheld scopes for low speed work like this are remarkably cheap on ebay. Probably not accurate, but for automotive work you just need to look at simple pulses.

[HarryM1BYT]

Originally Posted by Jumper75

I agree, any cheap handheld scope will do it. A DSO (Digital Storage Oscilloscope) will do even better as you can freeze the trace.

Handheld scopes for low speed work like this are remarkably cheap on ebay. Probably not accurate, but for automotive work you just need to look at simple pulses.

Absolutely! I bought an open source one - An MP3 player, which can be converted to a full colour storage scope, reusing the screen + case and other parts, with a specially made PCB and software. It is very accurate and I flagged it up on this forum as a good buy for diagnostics on cars, at the price. It has doubled in cost since then.

[Jakg]

Originally Posted by Phil-T4

Depends on how irregular it is, if the switching state is at the same rate as the other sensors it will be happy.

So is a T4 session to identify this problem likely to be a waste of money?

Originally Posted by HarryM1BYT

Under inflated tyres, excess movement in the suspension and similar can all cause the ABS to trigger when it should not. As also can the sensor simply not responding at low speed as pointed out by Mike above, or metallic dirt on the ring producing distorted or extra pulses.

My tyres are all fine (same brand, pressures and virtually same tread).

My suspension is all in good order (generally new bushes all around, wishbones are good, springs are all good if not slightly corroded and the dampers are all a bit tired).

I could buy some sort of problem with the sensor being the issue (eg a faulty sensor or dirt etc) but I need some way of working out which corner is the problem.

[HarryM1BYT]

Originally Posted by Jakg

So is a T4 session to identify this problem likely to be a waste of money?
My tyres are all fine (same brand, pressures and virtually same tread).

My suspension is all in good order (generally new bushes all around, wishbones are good, springs are all good if not slightly corroded and the dampers are all a bit tired).

I could buy some sort of problem with the sensor being the issue (eg a faulty sensor or dirt etc) but I need some way of working out which corner is the problem.

It keeps coming back to using a scope - Do you have, or know anyone with a scope you could borrow?

[Jakg]

Originally Posted by HarryM1BYT

It keeps coming back to using a scope - Do you have, or know anyone with a scope you could borrow?

I've managed to borrow a scope, no idea how good it is (not seen it yet) but I'll take a look at each sensor tomorrow with it (while driving, of course).

No idea what I'm looking for so hoping whatever it is jumps out at me.

[HarryM1BYT]

Originally Posted by Jakg

I've managed to borrow a scope, no idea how good it is (not seen it yet) but I'll take a look at each sensor tomorrow with it (while driving, of course).

No idea what I'm looking for so hoping whatever it is jumps out at me.

Absolutely any scope will cope with showing the pulses. No need to drive it and watch the scope, just jack a wheel up and spin it slowly and regularly whilst watching the scope.

What you are looking for is some irregularity in the waveform, irregular pulses, double pulses, or missing pulses as the wheel is rotated. Mark the wheel with a bit of chalk if you see one and confirm it several times.

It should be a neat square wave on the screen, with DC coupling (look for a switch marked AC/DC coupling).