Very easy......
Change the thick red cable with cotton insulation, which is called T30 from the alternator connector to the same T30 port in the starter. Original wire size is 4mm2. There are two cables of these size coming out from B+ terminal on the alternator. One is going to the starter, and another of the same size goes to the car cabin to feed all the circuits trough the main light switch as a hub.
Increase the size of the cable going to the starter, which at the end is responsible of charging the battery. Change it from 4 mm2 to 10 mm2 which is the size that came on w111-3.5 later. 10mm2 is equivalent to a wire gauge of AWG-8. Use only full copper marine grade cable; that one is resistant to oil, water and heat. Of course avoid any Chinese product, buy American, made in USA, OR German as well.
With this arrangement an alternator of maximum 53A+28A may be installed. That is an 80Amp alternator.
It is advisable to change as well the cable T30 that goes from alternator to cabin which is 4mm2 in size as mentioned before. Not because you are increasing any of the original loads, but because of cable insulation aging. In all the many many wiring harnesses from Mercedes I have refurbished, I have not seen yet ANY case on which the insulation of this particular cable was intact. The cotton/resin insulation used on it cracks with the pass of time, making it brittle and loosing its insulation properties. You have to take in mind that this is a NON FUSED circuit line where is imperative to have a perfect insulation for safety reasons.
A common symptom of bad T30 lines insulation is a higher battery discharge rate. When situation becomes worse even a fire can happen.
If you do not want to take the time to remove the original T30 line going to cabin, leave it there, but disconnect both ends, and pass a new marine grade cable. You will remember this advise for life and thank me for letting you know of the danger involved.
Best regards
Eng. Leonardo Peterssen
