Again.....its good you already know how a realy works.
Starter motors have an internal relay.
The big wire which should be the red one is infact a direct power from the battery, and always has power - for this reason its best to disconnect the battery befor working on a starter motor.
The other smaller black/white wire is the "start" position trigger.
There is no earth wire as the starter motor earths itself through the gearbox/engine block, but it doesn't hurt the have another earth strap somehwere in this area to the chassis.
When you turn the key to "start", it triggers the SM's internal relay spinning the main motor, while also powering the solinoid to through out the gear.
If the small black/white wire is broken disconnected then nothing at all will happen.
If the big red wire is broken/disconnected you should still hear the click of the internal relay and solinoid, but the motor will not spin.
:wink: