ABS when reversing

YLS · May 30, 2018

Twice today when reversing my brake pedal has juddered/vibrated when pushing it to stop, whilst going backwards. The feeling like when the ABS pump is doing it's thing. No lights on the dash at the time or after. Not done it forwards at any time.

Does ABS work in reverse?
Does it only work above a certain speed? Both times I was doing roughly 10mph.
Anything else it might be other than ABS?

Had the corroded reluctor ring problem recently on the NSR only, now fixed. OSR looked fine when I checked it...

Nottsknots · May 30, 2018

Yep, it should.

Dont forget, most of the braking comes from the front wheels under normal driving, so the discs and pads are bigger; you're probably putting all the load on the tiny rears which dont slow you very well, the fronts are close to locking up with no load, and forcing the ABS to kick in.

YLS · May 30, 2018

Yeh, I understand that, but this was reversing in a petrol station the first time, hardly a speedy J turn! Maybe oily though. Second time was reversing around a corner, so a little bit speedy... Was wondering if there was a common complaint about it, seems not. I'll keep an eye on it. Just odd it did it twice today and never noticed it before. Brakes behaved fine in all other ways, and I did test them as best I could.

YLS · May 31, 2018

Did it again today. Fairly swift reversing bit nothing silly, and then braking. No chance at all a wheel lost traction though. It's a definite rapid juddering through the pedal (like ABS) and a noise in time with it, possibly from the back end. Could a brake pad be rattling around, and it seats well going forwards but rattles/judders in the caliper going backwards? Would that give feedback like that to the pedal? 'Warped' discs do, so I guess it could?

YLS · May 31, 2018

Tried it out again. Reversed in a straight line and kept some gas on, and left foot braking fairly firmly. The juddering continued, I could hear it and feel it though the pedal. After about 10 metres all the ABS/brake related lights came up on the dash. Engine/ignition off, back on, drive forwards and the lights clear and all back to normal.

Went over all the brakes and everything is tight (7mm allen bolts and caliper bracket/hanger bolts). Only thing I noticed was the OSR caliper could be noticably moved on the slider pins, as in give is a good wiggle and you could feel play/rattle in it associated with the sliders. Not along the axis of the pins, but as if the pins were too small a diameter in the bushes (they're not, it's just an example!).I'm guessing this is the cause...

Brakes and ABS still work 100% ok going forwards.

1. Could sloppy slider/guide pin bushes this cause an ABS light to come up? Perhaps the ABS module is sensing pulsing fluid (like I can feel through the pedal) and then flagging it up?
2. Are slider pin bushes replacable? There are relatively new pins on all calipers, but never replaced the bushes. They're remanufactured calipers so would assume they're all good to go, but perhaps they've worn already.

marco_polo · Jun 1, 2018

Sounds like the slider bushings have worn with pressure from the drag of the disc, and the calliper is effectively now sitting on the **** slightly. The pads could have worn to this wonky angle, so everything feels fine going forwards.

I'd replace the calliper and rear pads with new ones and retest.

YLS · Jun 1, 2018

Thanks, that sounds plausible. I was struggling to work out how a wobbly caliper would cause a juddering sensation, as it would surely just align with the disc once the brakes were applied. But as you said, if the pads have worn unevenly as a result of worn guide pin bushes then that might explain it.

The caliper itself was replaced about 6 months ago. Pistons moved freely, no drag on the disc, and never hotter than the other discs. I'm not concerned about the caliper itself tbh. If the pads weren't sitting right though, that could load the pins/bushes differently and wear them out faster.

I've ordered some replacement guide pin bushes (didn't realise they were the rubber bits, I thought the rubber bits were seals over the actual metallic bushes...) so I'll swap them out and see what happens. Perhaps the remanufactured caliper has cheap bushes made of rubber cheese... If it changes the noise then at least it points me in the right direction.

It's got the MOT next week, so hopefully any problems will be identified on that. Might be a case of new discs/pads and pin bushes on the back.

Thanks for the suggestions.

YLS · Jun 5, 2018

Fixed now. I replaced the slider pins and bushes but it made zero difference. Did a bit more experimenting, like reversing at idle and gently applying the brake on and off over 100m distance. If I held the pedal down it made the juddering noise (could feel it from the back end) but interestingly could hear it from the front as well. I said at the start it sounded/felt like the ABS pump, and it was. I could hear the pump working, and feel the pedal dropping in stages in time with the pump.

A few months ago I had the reluctor ring problem. I spaced out the sensors on both sides as a temporary fix. Only one side was a problem though, so I replaced the ring and sensor on that side and everything was fine. I left the spacer in on the other side as everything was working fine. Turns out, it was working fine driving forwards, but for whatever reason it was triggering ABS problems when reversing. Took the spacer out again and it's all back to normal now, and in for the MOT now. Fingers crossed no other problems!

So for anyone putting a spacer in as a temporary fix for the reluctor ring problem, the thickness is critical. Too thick and it'll throw errors going forwards. Slightly too thick and it will appear fine going forwards, but trigger ABS problems in reverse.

Nottsknots · Jun 5, 2018

Cool. Thanks for posting that - I'd never have guessed :woohoo:

ABS when reversing

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