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For the last number of months, I'd had severe steering wheel shakes under heavy braking, but only when the car had been moving for a longish time (- probably more than 40 minutes). The ABS ECU had logged faults something like 'front left speed sensor intermittent fault'. I'd removed the sensor a couple of times (wheels still on ths car) and there was quite often, a bit of debris stuck to the bottom of the sensor. I cleaned this off, put it back and went on my way, but the problem kept coming back.
A few days ago, I removed the front wheels so I could remove the two ABS sensors again. This time I was able to also look through the now empty sensor hole, at the reluctor ring below (on the hub). When I rotated the hub, I could see debris clogging up some areas of the reluctor ring - see below:
[attachment=2:fa28nwxu]paul.adshead.ABS.ring.clogged.jpg[/attachment:fa28nwxu]
After 20 minutes with a screwdriver, some turps, an old toothbrush and the compressor/air-gun, this is what they looked like (around the full rotation):
[attachment=1:fa28nwxu]paul.adshead.ABS.ring.clear.jpg[/attachment:fa28nwxu]
What I guess had been happening here, is that at certain portions through the axle rotation, the ABS sensor could not distinguish between the clogged 'gaps' and the 'peaks' on the ring and thus the ABS computer would not have been seeing any 'state changes'. As the ABS computer is measuring the time between impulses from the peaks on the ring, to work out axle speed, then at cetain parts of the rotation, the ABS computer would see one long pulse, instead of the regular stream of short pulses. If you happen to be braking when the ABS computer sees this one long pulse, then it interprets this as a low-speed/locked wheel and thus drops hydraulic pressure to that wheel. As the wheel is NOT actually locked/skidding, but has good traction and as the other brake on the axle is hard applied, then one wheel pulls rearwards a LOT harder than the other. Thus causing the steering to pull to one side. As the ABS system pulses the faulty side back on/off again, then you see this in the steering wheel as VIOLENT shakes, side to side. The only way to remove the steering shakes is to totally come off the brakes and gently apply them again - not good when you barrelling into a corner.....
The crud that I scraped out of the teeth on the reluctor ring, was a gloopy, gummy rubbish. It was apparrent that whoever had been in there beforehand, thought it would be a great way to hold the sensor firmly into the hub-carrier, by covering the sensor in this rubbish. :-(
RAVE says the following on the subject:
11. Lubricate new sensor bush with silicone grease.
See LUBRICANTS, FLUIDS AND CAPACITIES, Information.
12. Fit sensor bush.
13. Lubricate sensor with silicone grease.
14. Push sensor fully into bush until it contacts
reluctor ring. Correct sensor position will be
gained when vehicle is driven.
I only hope that the 'official' "silicone grease" isn't this crap I scraped out off the ring. If so, there will probably be A LOT of vehicles around with this same issue, where too much grease has been applied. I didn't use silicone grease, but instead used some of my trusty 'coppercote'. As coppercote is a very 'soft' mixture, any that should get onto the teeth on the ring would be thrown off again - not be stuck there forever and cause on-going problems.
Whilst I was in there, I thought I'd take a pic of the "sensor bush" too. Quite why RAVE insists a new bush needs to be used every time the sensor is removed, makes no sense at all to me - there is plenty of adjustment in the bendable teeth on the bush.
A few days ago, I removed the front wheels so I could remove the two ABS sensors again. This time I was able to also look through the now empty sensor hole, at the reluctor ring below (on the hub). When I rotated the hub, I could see debris clogging up some areas of the reluctor ring - see below:
[attachment=2:fa28nwxu]paul.adshead.ABS.ring.clogged.jpg[/attachment:fa28nwxu]
After 20 minutes with a screwdriver, some turps, an old toothbrush and the compressor/air-gun, this is what they looked like (around the full rotation):
[attachment=1:fa28nwxu]paul.adshead.ABS.ring.clear.jpg[/attachment:fa28nwxu]
What I guess had been happening here, is that at certain portions through the axle rotation, the ABS sensor could not distinguish between the clogged 'gaps' and the 'peaks' on the ring and thus the ABS computer would not have been seeing any 'state changes'. As the ABS computer is measuring the time between impulses from the peaks on the ring, to work out axle speed, then at cetain parts of the rotation, the ABS computer would see one long pulse, instead of the regular stream of short pulses. If you happen to be braking when the ABS computer sees this one long pulse, then it interprets this as a low-speed/locked wheel and thus drops hydraulic pressure to that wheel. As the wheel is NOT actually locked/skidding, but has good traction and as the other brake on the axle is hard applied, then one wheel pulls rearwards a LOT harder than the other. Thus causing the steering to pull to one side. As the ABS system pulses the faulty side back on/off again, then you see this in the steering wheel as VIOLENT shakes, side to side. The only way to remove the steering shakes is to totally come off the brakes and gently apply them again - not good when you barrelling into a corner.....
The crud that I scraped out of the teeth on the reluctor ring, was a gloopy, gummy rubbish. It was apparrent that whoever had been in there beforehand, thought it would be a great way to hold the sensor firmly into the hub-carrier, by covering the sensor in this rubbish. :-(
RAVE says the following on the subject:
11. Lubricate new sensor bush with silicone grease.
See LUBRICANTS, FLUIDS AND CAPACITIES, Information.
12. Fit sensor bush.
13. Lubricate sensor with silicone grease.
14. Push sensor fully into bush until it contacts
reluctor ring. Correct sensor position will be
gained when vehicle is driven.
I only hope that the 'official' "silicone grease" isn't this crap I scraped out off the ring. If so, there will probably be A LOT of vehicles around with this same issue, where too much grease has been applied. I didn't use silicone grease, but instead used some of my trusty 'coppercote'. As coppercote is a very 'soft' mixture, any that should get onto the teeth on the ring would be thrown off again - not be stuck there forever and cause on-going problems.
Whilst I was in there, I thought I'd take a pic of the "sensor bush" too. Quite why RAVE insists a new bush needs to be used every time the sensor is removed, makes no sense at all to me - there is plenty of adjustment in the bendable teeth on the bush.
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