MERC B-CLASS BLOWING FUSE

This 2011 B180 was brought to our workshop this week from another garage which couldn’t get to the root of the trouble. The problem was it was blowing a fuse intermittently which was then bringing on the battery light and management light. The previous garage had plugged the diagnostic into it and was getting a fault code relating to the stop/start alternator. The strange thing about this fault was that the fuse that was blowing was not listed as controlling the alternator circuit. The first thing we would normally do is to disconnect the alternator and see if that stops the fuse from blowing, but in this case the wiring is inaccessible unless the alternator is completely removed from vehicle.
The alternator was removed and checked and found to be ok. So we started to look more closely at the circuits that were controlled by this fuse even though they weren’t coming up on the diagnostic!
The circuits all related to the lambda sensors, purge canister solenoid and swirl flaps.
We could easily disconnect the lambda sensors and see if the fuse blew, which we did and found that the fuse still blew. The swirl flaps on this vehicle were vacuum operated so we couldn’t see how that would pop a fuse. But we got access to the control for the flaps and found we could move them very easily as a common fault with these is they can seize up due to oil and soot from the engine.
The only option for us now was to examine the loom from the main ECU in the engine bay, as wiring to this could affect unrelated circuits which might use a common positive connection. After a few hours of careful examination of the main engine loom a tiny nick on a cable which was not directly connected to any of the devices that was on this fuse or alternator was found across the top of the engine. The wirer was reinsulated and the loom refitted correctly so as to not cause problems in the future. The car was road tested and given back to the happy customer.