C Class Coupe multiplexed wiring problems

[Frank]

Can anyone help with 13 pin socket wiring for a new 'C' Class coupe.I had a detachable towbar and electrics installed at the factory.The 13 pin socket was only fitted with 10 pins.Pins 10,11 and 12 were ommitted.This is evidently common in German manufactured cars.I have had pins 10 and 11 fitted by the UK dealer but I am not confident that they have wired them correctly.As this car has a multiplexed electrical system, checking out the socket with a multimeter does not make much sense.For example pin 6 for trailer brake lights shows 12v with ignition off,but operates a lighting board correctly.The pins 10 and 11 should be ignition switched live and earth according to the UK standard but the dealer has wired them to an MB data sheet as ignition switched live and permanent live.The car knows when the lighting board is attached even though there is no micro switch on the socket.

 

[dieselman]

Fristly, are you sure the lighting circuit is multiplexed. Probably not, but there may be a controller unit which is controlled by the multiplex CAN system.
Are you sure you are metering the pins with respect to the appropriate ground. If so you may well find the brake lights are brought on by a transistor shorting the return side to ground. This arrangement is quite common in machinery where there is a single +ve supply and the loads are operated by clamping the rerturn to ground ( active low).

The hook up detection is probbaly performed by one of the lines changing state once the connection is made.

If the installer has fitted the exttra pins in accordance with the MB data sheet, and it all works, then I would be happy to leave alone.

If you have a complete wiring diagram post it, as I will then be able to give more detail.

 

[Frank]

13 Pin Socket

Thanks for your reply.The trailer lighting circuit as installed by MB definitely works as it should with bulb failure indications also correct.I asked my dealer to supply me with a copy of the MB wiring diagram that they were working to but they gave me a copy of the british circuit for caravan power circuits.The car is going in for another attempt tomorrow, so Iwill post the result.I see what you mean about the system changing state once the trailer is detected as this appears to be the case with the brake lights, but I was loath to plug in the van power circuit which was expecting a live and earth for the refrigerator and battery charging. :?

 

[Frank]

The car went in to outpatients today and was fitted with a good old conventional split charge relay.I have not checked it out,but am assured that everything is OK.It has taken 3 visits to get to this stage,so I hope that they have it sorted.