ABS Problems at Low Speed

[Jakg]

Originally Posted by okenora

here you go easy in situ ABS sensor test

Perfect - will try this over the weekend.

Removed the ABS fuse in the meantime and everythings fine so it's definitely the ABS.

[Jakg]

Had a look at this today.

Replaced all the brake fluid and found a leaking union at the drivers side rear - problem still present

Checked all the ABS sensors with a multimeter (as per FrenchMike's instructions) - all seem to oscillate between 0.7 and 1.7 volts. One was reading slightly higher (+0.05v) than the other 3 across the board (drivers side rear).

All the sensors' cables look ok as well after a visual inspection.

Is the next stop a T4 (I don't want to plug an O-Scope in)?

[HarryM1BYT]

Originally Posted by Jakg

Had a look at this today.

All the sensors' cables look ok as well after a visual inspection.

Is the next stop a T4 (I don't want to plug an O-Scope in)?

Maybe one of the T4 guys could comment, but I'm not sure a T4 would identify it, with no obvious fault to identify. You need something to check the regularity of the pulses from each wheel, with each wheel actually turning at a regular rate.

If you have a fluid leak, I would be inclined to concentrate on that as a priority - maybe the leg with the leak is causing a localised pressure drop, reducing the the effectiveness of that brake and triggering the ABS? - clutching at straws there, because I have never studied the hydraulic side of ABS.

[paulozy]

had same fault last year (only happened under 10 mph) scoped all sensor and they all give a nice sinewave. A month or so later my abs light come on, mine was the offside rear abs sensor and since replacing had no trouble since.

[Jakg]

Originally Posted by HarryM1BYT

If you have a fluid leak, I would be inclined to concentrate on that as a priority

Even without ABS problems I wasn't exactly comfortable with the idea of the possibility of brake fluid leaking out!

It was just a loose union - I'm blaming the garage who replaced my rear brake lines last year. Wasn't leaking, just wet (no real fluid loss in the master cylinder) - slightly surprised this wasn't pointed out at the MOT though. Tightened it up and it seems ok now.

Just a shame that wasn't causing my problem

[Phil-T4]

Originally Posted by paulozy

had same fault last year (only happened under 10 mph) scoped all sensor and they all give a nice sinewave. A month or so later my abs light come on, mine was the offside rear abs sensor and since replacing had no trouble since.

The ABS sensors actually output a square wave signal, where 0.5V is considered to be Off and anything higher than 3V is considered to be On.

[paulozy]

Originally Posted by Phil-T4

The ABS sensors actually output a square wave signal, where 0.5V is considered to be Off and anything higher than 3V is considered to be On.

yes you are absolutely correct, it was squarewave

[HarryM1BYT]

Originally Posted by Phil-T4

The ABS sensors actually output a square wave signal, where 0.5V is considered to be Off and anything higher than 3V is considered to be On.

So, is the T4 able to detect what appears to be a slightly irregular waveform from one of the sensors Phil?

[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.

[HarryM1BYT]

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.

Mmm, so if the ABS is mostly happy, a T4 would be equally happy.

He needs a scope then to actually watch the pulses for irregularities on each channel.

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