Q For some time, my neighbour has been finding he has a flat battery when he comes to try to start his Mercedes E220 in the morning. He has had a switch fitted so that he can isolate his battery from draining overnight, and has been advised that his problem lies with the ECU – or, at least, the part of it that deals with the central locking, immobiliser, alarm, etc. What would you suggest he do next? SP Skelton
A If the Mercedes in question is fitted with electrically-operated seats then this is the first place I would check for a battery drain. The seat control module under the seat is a well-known point of current drain, as are the switches, which are prone to fail. If this is not the cause, and the problem is either the comfort module or the ECU, then the only course of action would be to replace the faulty unit. But before doing this, check that the alternator is not allowing a drain back through the voltage regulator. Another common reason for battery drain is the boot light. Very often a faulty switch in the boot catch can allow the light to remain illuminated unseen in the boot area.
Have A Problem Vehicle? Get In Touch…
If you have a problem vehicle, Car Mechanics has the answer in the shape of technical editor Steve Rothwell. Email your questions firstname.lastname@example.org.