Battery warning light keeps coming on

[Skydiver17]

2009 Mercedes C200 Sport CDI Auto

We have had the vehicle 2 years. 5 months ago the battery warning light came on. The garage has replaced the battery, alternator, glow/relay lamps and checked the electrical output and belt. They even took it to the Mercedes garage next door, to which they are linked, and hooked it up to a diagnostic reader – no faults found. They had the car for over a week. Of course during this time because they had fitted a new battery, the warning light did not appear. We took the car back and after a few weeks the warning light came on again. Eventually the car would not start as the battery was drained. We have resorted to buying a battery charger as the garage is at a loss as to what the problem is. They have suggested that it might be the SAM unit in the boot of the car, but they cannot be sure. Is there not a test to find out whether the SAM unit is talking to the alternator?

 

[Martin Theron]

To check if something is draining the battery you need to connect an ammeter in line between the battery pole and the battery cable. If there is a drain on the system, pull out the fuses one by one. The fuse that causes the drain to disappear is the system draining the battery. Now you know where to start looking for the problem. It often happens that a stuck relay causes this problem.