Relay between batteries.
I'm still working on my gone running lights and discover this:
My old Westfalia had two batteries, one to crank and the other one for the fridge and interior lights.
When the ignition was at the OFF position, there was no contact between the batteries.
When the ignition was at the ON position (on the road, driving) a relay was opened, allowing the alternator to recharge the auxiliairy battery at the same time it recharge the principal one.
The relay is used so the principal battery can never be drain out by the accessories (specially the fridge!).
Now, back to the Sprinter. The Mercedes tech suggest that I disconnect the battery to reset the SAM.
QUOTE: All of the lights are electronically fused by front SAM control unit. It is located under the steering on the left wall. All of the lights must be connected and on short circuits or interruptions present. Only then SAM is supplying the voltage to the lights. If the wiring got shorted out during assembly work, there might be a need to delete the fault codes and even disconnect the battery to reset the SAM. Disconnect all non-working lights and start connecting them one by one. Also cycle the ignition after connecting lights. Possibly there is a short circuit in one of the lights.
There are no fuses for these lights. If this test does not produce a clear result- fault codes in front SAM control unit must be checked with Star Diagnosis computer.
So, I disconnect the main battery (it's huge !!!) but nothing happened cause I still had power everywhere !!!! I could even start the engine.
The reason is that there is no cut-off between the main battery and the motorhome batteries hidden is the staircase. In my case, two more 12 volts batteries to disconnect. I tried the shut-off switch but it only cuts the current going for the camper section.
Not that I'm worried about discharging all the batteries but I think there should be a cut-off between the 2 systems. Or, is there one and it's not working properly ?