Car clinic: my BMW's speedometer and mileometer have broken. Help!

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Car clinic: BMW speedometer

Q. The antilock braking system (ABS) warning light is illuminated on my 1999 BMW 523i. The speedometer and mileometer stopped at the same time that it came on. My garage has replaced two wheel sensors and one wheel bearing but this has not helped. Any better ideas?
MM, London

A. It might seem peculiar for three things to fail at once but in fact these systems are all interlinked. In the past, the speedometer was operated by a rotating cable driven by the gearbox. But in a car fitted with ABS, because the braking system requires information about the speed of each individual wheel in order to operate, it makes sense for that same data to be sent to the speedometer and mileometer as well.

Your BMW’s ABS works by way of a speed sensor attached to each wheel hub. (An associated part of the speed-sensing equipment is housed in the wheel bearing, which explains why your garage tried changing one of those.) The sensors are quite exposed and, after some years, they can corrode. If just one sensor goes wrong, as a safety default the ABS computer packs up. The brakes will still work, but without the antilock function; the speedometer and mileometer will stop working altogether.

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The ABS system will record the problem as a fault code, but this might not be very informative to the sort of basic code reader used by many independent garages. Only one of the more advanced code readers found in BMW specialist garages or main dealers will be able to determine the specific sensor that is not sending a signal. The mechanic also needs to discover whether this lack of signal is because the sensor is faulty or is because of a break in the wire running from the sensor to the computer.

So the skill lies in reading the fault code and then investigating the exact nature of the problem. Simply changing components before carrying out a proper diagnosis is the mark of an inexperienced technician. We suggest you go elsewhere and, once the actual problem has been identified, seek a refund for the components that were changed unnecessarily.

Sunday Times Driving car clinic expert: Tim Shallcross

Tim Shallcross used to train AA patrols to fix cars. Now he advises the Institute of Advanced Motoring – read more from Tim here.

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