Weird abs fault
One of our VW LT28 hire vans was returned recently with a totally wrecked n/s/r wheel bearing and half shaft. (Been making a funny noise for weeks said the hirer).
The ABS sensor and ring were damaged and the wiring melted. Everything was replaced using genuine VW parts. The half shaft / bearing / sensor ring / sensor assembly was taken from an engine damaged van that we use for spares, and is known to be good. Two new rear discs and new pads were fitted all round.
The problem is, when braking with a firm steady pressure there is a single thud from the ABS unit which is felt through the pedal. This happens between 5 - 8mph and It doesn't always happen if the pedal pressure is varied or released during braking. The ABS works as it should under heavy braking, and the warning light works normally. No fault codes are recorded after a road test. If the ABS is disabled, the brakes work normally, but without the thud.
I've replaced the ABS unit, (with a known good one), bled the brakes, and adjusted the n/s/r sensor to ring clearance. The problem persists.
The local VW dealer has been helpful, but - like me - have run out of suggestions and opinions.
Can anyone help ?
{vehicle details added to subject header - DD}
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Is there any changeover of parts on this model/year?
Is there the possibility of a different number of teeth on the rings?
Similar looking, but different sensor types (passive / active?) - I suppose if the ABS ECU's between the vans have the same part number all my suggestions count for nowt!
good luck none!
Number_Cruncher
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The ideal test for this problem would be to compare the output of each sensor at the time the fault occurs using a four channel oscilloscope,however you may not have access to one of these.You might like to inspect each wheel sensor and verify that each has the same resistance reading and that the sensor to ring clearance is equal on all axles front and back.You should find something which is causing the output of one sensor to be different from the other three.As the fault only occurs at very slow speeds I'd suspect your sensor clearances are not exact .
David Davies
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Many thanks to NC and DD.
The number of spokes on the sensor ring did occur to me as being relevant, but the VW parts dept.say that all of the new and used parts are compatible. It looks like a count is on the cards ! As a matter of interest, they say that whenever they replace a sensor ring the mechanic doing the job always gets another mechanic to confirm that the ring has minimal runout - it's that critical. The transplanted half shaft does have a bit of runout, (couple of thou) but it wasn't a problem on the donor vehicle.
I only have access to a basic code reader which displays no fault code after a road test.
It did occur to me that the thud happens when the ABS switches off (about 5mph according to VW) but the local mechanics insist that this would show up as a fault code. (That's why I changed the ABS unit).
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