The "I" terminal as you named it needed to be grounded.

I am by no means an expert on battery starter cable sizing but I did contact a friend who knows much more than me and I'll attempt to describe our discussion. You are correct using charts and cable vendor information but this does not address safety in the event of unforeseen circumstances like a crash.

There are at least three design points to be considered when spec'ing out batery starter cable sizes.

(1) The first is the maximum current rating. A cable can be de-rated when in hotter environments, like engine compartments.

(2) The second design point is voltage drop. Cable size current draws and length of wire all play a roll.

(3) The third is failure mode scenarios such as vibration or an accident.

Between the first and second items, voltage drop is the overriding item with 12 volt systems. This is because 12 volt systems use more current than 110 volt system. Higher currents mean higher voltage drops (Ohm’s Law). There are tables available on the internet which show how much voltage drop a given circuit will have given the wire gauge, the current load, and the length of the wire.

How much voltage drop the system tolerate starters will spin fin at 10 or 11 volts provided adequate current is available.

Now, on to point (3). We still have to deal with the possibility of short circuits. Usually, the high current of a short will quickly fry the contact point of the short and open the circuit. This means that the wire will not have time to heat up, but one never knows what can happen.

For an unrelenting hard short, the energy will be expended inside the battery, in the wire, and in the item causing the short. Fuses can help this situation but there are only so many things you can do and you can’t protect everything. There is a lot of energy in a battery and there is always some situation where it can be unloaded in the wrong direction.

A #4 wire will handle about 1000 amps or so for a long enough time to drain a battery. It won’t be pretty. The wire will get very hot. As it heats, its resistance increases, which means the current will drop. If you use too big a wire, then all the energy gets expended inside the battery – probably not a good idea for a device containing acid. You probably would prefer to have the wire melt (*which is basically what a fuse does).

I'm wondering why no one here has tried a solid state version.

Maybe too high tech, maybe cost once you add the required heat sink, maybe overall size due to the heat sink requirements or maybe availability. It would need to handle around 80 amps. It would be good for a million plus starts but most are only rated to work down to -20 degrees C. Just when I need it most! I suppose you could add a heater to preheat the solid state relay before you glow. So I'll throw in complexity.

I put an ad in the for sale section. Maybe the pic and video will explain it better?

http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/359811-sale-manual-mercedes-glow-plug-controller-w123-300d-240d.html#post3385882

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked