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QThe ABS light on my 1997 Golf operates randomly. My local garage, a dealer and a specialist have tried to fix it. The ECU diagnosed a faulty rear offside sensor. It was changed but the problem persisted. A new wire was run from the sensor to the ECU, with no joy. The sensors were swapped over right to left. The ECU still said that the rear offside was faulty. Finally, the ECU was changed: still no difference. The light comes on when I get the car out of the garage. I drive off and turn left or right and it’ll go out and stay out for a day-or-two.
If I go over a bump the light may come on or may not. What can be done about such ECU faults?
Jonathan Bridger, via email
AWe have a threesome of similar Ecu-based issues. The one thing they all have in common is that we think the ECU is blameless. In your case, seven separate components are in agreement that the fault is at the rear offside wheel. It’s hard to argue with this. It’s a shame they’ve all been placed on the naughty step for their efforts.
We’d turn our attention to the magnetic reluctor ring from which the sensor gets its signal. If it’s badly corroded, it’ll either need replacing or its squaretoothed profile will need gently restoring with a file. It may be damaged or have missing teeth. Look out for contamination by grease or brake dust, too, and make sure it has the same number of teeth as its counterpart on the nearside. It could also be improperlymounted or be flapping about because of a loose wheel bearing, which might explain its sensitivity to bumps. Jack the car and give the wheel a good shake to check the bearing for play.
Our other two examples relate to errors that the respective ECUS attributed to their camshaft position sensors.
The first example ran for a while, then stalled and wouldn’t start. After substituting the camshaft and crankshaft sensors and also the ECU, the problem remained. Then, we noticed that it had virtually no oil pressure when hot and that the timing chain was hydraulically tensioned. Result: the timing chain lost its tension when hot and the twin camshafts ran out-of-time with the crankshaft. The ECU was unable to correctly calculate the ignition and injector timing – and it had been correctly reporting a camshaft position fault all along.
The second example is a reader’s query that’s
still unsolved. The ECU reported a camshaft position issue, so the sensor was replaced.
The fault persisted and the ECU fell under suspicion. The reader is unable to verify, however, whether the belt driving the four camshafts has been changed – or even retensioned – in the car’s long life and 72,000 miles. We suspect that the timing issue is being caused by free play in the long belt run between the pulleys – or perhaps even a pulley that’s skipped a tooth.
ECUS do sometimes develop faults, of course. Recent and common models can usually be sent off for repair, but most older units are not catered for. Some live in damp areas under the bonnet or in footwells.
Their metal casings corrode, which allows water under their hermetic seals. If you can take the casing apart and dry the unit on a very low heat in an oven, it may resume correct function.