Mercedes Benz 1520

Hello, good afternoon/morning/evening.

Does anyone happen to have the workshop manual for a 1970 Mercedes-Benz 1520 truck?

I'm having trouble starting it; the starter motor seems to be turning weakly. It's a 24V truck with two (new) 12V, 90Ah batteries. There are four wires going to the battery: one to the starter motor, one to the chassis, and the other two to a box inside the cab. I imagine this is the 24V to 12V converter, but when I tested this box, I got no continuity on the two wires that are supposed to be connected in series.

If anyone knows how to fix this problem, I would be very grateful.