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post #17 of (permalink) Old 31st October 2012
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Re: Laguna 2 NSF wheel bearing replaced now ABS fault ?

I did wonder if the ECU could be programmed for targets with different numbers of magnets or teeth on sensor wheels. In my case, since both front bearings were the same part number and one is not throwing up faults, I am assuming the system is happy. But I also checked the pulse count from a rear wheel and got the same number. Unfortunately, the first time around I did not spend enough time looking at the 'quality' of the pulse sequence and whether one or two were missing or intermittent.

So as not to cause confusion, but I think the principles are the same, my car is the Laguna 1.

On the back of the bearing there is a thin circular brown plastic ring set flush. If you did not know, you would just think it was part of the bearing construction. However, that is 'printed' with 44 magnets.

This is how I think it works: my sensors are what they call active which means there are chips inside, it is probably a Hall effect unit. You cannot do the usual ohmeter tests because the sensor needs power - even though it only has 2 wires.

Most ECU 's monitor the pulses coming off the sensor in real time and can activate the ABS within a wheel pulse or two. That is fast because there are about 44 pulses per wheel revolution which is only 8 degrees!

So, the sensor sits there getting pulses and the ABS computer knows from the first couple when to expect the next pulse to come along. As long as it does it in the time it expects, the ABS and SERV warnings are out and the system is happy.

The magnetic ring on my bearings is called a 'Target' and that is the error message you can get from diagnostics. If the error message was 'Sensor fault' then that would point to the actual sensor or its wiring. In my case there are 44 of these and the plastic material is thin and very fragile. If you were to catch the back of the ring with the drive shaft as you were refitting the hub, then you could 'puncture' the ring at that point and damage a magnetised section. If you think how the old cassette recording tape works, the circular printed magnetic plastic rings on the Lag 1 bearings are very similar.

These are the problems likely to cause sensor problems : Magnets missing or damaged breaking the sequential sequence of pulses the ECU must see, at higher wheel speeds sensors losing sensitivity (air gap too large) and then there is always the possibility of bad wiring.

From a garage diags check I knew that my right front wheel sensor fault was 'Target' error. Since the left front bearing was changed at the same time I assumed that if one bearing was ok then the right bearing magnet ring must be bad which I confirmed independently with my test method. The diagnostics were telling the truth.

The sequence of events as I think I understand it is the 'Target' fault code stays in the ECU and puts on both SERV and ABS. Incidentally, SERV is nothing but a simple linking inside the cockpit from its lamp to the ABS lamp so do not think there is anything more complicated. Both lamps will always light if the ABS lamp is on - same with the airbag light. The garage could cancel the ABS/SERV warning so the lamps were out before driving off. During the first 100 yards or so the ECU is checking the wheel pulses. If a fault code is already on it clears that code and the lamps go out After another 200 yards or more the fault lights will come back and stay on if the pulse sequence is bad. I am confident that when the wheel sensor pulses are ok, my fault lights will go out and stay out.

My second new bearing which this time is a FAG should be ready in the hub today. I am confident that when put back carefully on the driveshaft and given a preliminary check with my own method, both fault lights will go out and stay out.

As I said, I will post a follow up when I get to that stage if it helps.

If somebody who knows the ABS target ring arrangement on the Lag 1 and 2 could explain that, then my follow up will not mislead. I assumed it was integral with the bearing like my Lag 1, but alternative ways of doing it can use steel toothed wheels on driveshafts.
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