Mercedes Benz 1520

Hello, good/you afternoon/days/nights

someone for those coincidences will have the manual workshop of the Mercedes Benz 1520 truck of 1970.

I tell you: I have problems with the start I feel that the starting engine rotates without force, it is a 24V truck of two batteries (new) both of 12v and 90amp, this truck brings 4 cables to the battery one goes to the starting engine, another goes to the body and the other 2 To a box I imagine that it is the converter from 24V to 12V, but when I test this box I do not give me continuity the 2 cables that are supposed to go in series

if anyone knows how to repair this problem would be really grateful