Electrics often best left to experts

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IOL mot pic aug30 Diagnostic 1

The owner an elderly Mercedes C280 complained of three possibly linked faults. He told me that the ABS warning light would for no apparent reason come on and when this happened the traction control also stopped working, as did the cruise control.

Ideally, when such a multi-fault as this occurs it really is best to visit a main dealer for the vehicle who should easily be able to pinpoint the source of the trouble. The Mercedes owner looked a bit apprehensive when I suggested this and went on his way and I did not see him again.

Although many people prefer to entrust their cars to the local general workshop because they fear main dealer costs, the fact is that a specialist workshop with the correct electronic diagnostic equipment should be able to do the job far more quickly and save both time and money.

What are the likely causes of such a problem?

Well, looking first at the most expensive thing to put right; it could be an electronic control unit fault. Less expensive to rectify would be something amiss with the rear light clusters. Of course, you could be lucky and find it’s nothing more than a dodgy stoplight switch.

A DIY operation would be easy enough, if rather tedious. First arm yourself with a main wiring diagram and a continuity tester.

Check all the connections in the circuits and then go over the sections of the circuits with the tester, ticking off the cleared sections on your wiring diagram.

Carefully done, this should locate the trouble spot.

But if it doesn’t, the ECUs will have to be checked professionally.

Another entirely different problem involved a judder felt at the steering wheel when braking. I advised the car’s owner to have the wheels balanced and to check the condition of the tyres, although the most likely cause was run-out on the front discs.

Run-out is easily checked by clamping a pencil on the calliper with it its point against the disc. As the disc is turned, any run-out will be shown by a gap between the two due to a distorted disc or poor disc seating on the hub.

Front end disc run-out will normally be felt by vibration through the steering wheel, but if the cause is a fault at the rear brakes, it will be felt throughout the vehicle.

CHECK TYRES FOR DAMAGE

It might be worth swapping one or both wheels in turn from the front to the rear to see if this makes any difference. If there is, inspect the removed wheel or wheels for damage or distortion and the tyre for run-out. It is worth getting under the car occasionally to inspect tyres for damage.

Having checked the front discs for run-out, inspect the rear drums. Uneven wear may cause them to become oval, resulting in a pulsating pedal under braking.

Noise and judder could originate from wear and excessive play in the suspension and steering joints, so give them a look over too. - Star Motoring

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