Starting a diesel: batteries, starter and gearbox

Everything you have built so far is mechanical, and none of it does anything at all until this works.

The circuit that starts an engine is very simple and very large. Simple because it is a battery, a switch, a cable, a motor and a way back. Large because a starter motor turning a cold diesel against compression draws several hundred amps for a few seconds — which is why that positive cable is as thick as your finger, and why every connection in it matters.

It is a loop, and people only ever look at half of it. Follow it: battery, isolator switch, heavy positive cable, starter motor. That is the way out, and it is the half everybody checks.

Now find the way back. Every one of those hundreds of amps has to return to the battery, and it returns through the earth strap — the heavy black cable bonded from the battery to the engine block. The engine itself is part of the circuit. The current goes out along a red cable and comes home through the lump of iron you spent six phases building.