I purchased a cheap 2000 with a history of cylinder head gasket problems it was a fully running car/engine but oil was getting into the water, (It seemed a good idea at the time), I suspected either a cracked head or block, as it is virtually impossible to test the block or head to determine which is cracked I purchased a used engine, I stripped down the replacement engine renewed anything that was required and also fitted time- serts in the block I also stripped and cleaned the starter motor(I bench tested it when I reassembled it), I refitted the engine and used all of the sensors that were fitted to the original engine.
When everything was fitted I tried to start the engine and all it would was click from the starter just like you have a flat battery or bad earth, it is also worth noting that while the clicking is happening the headlights come on.
I could not explain the headlights coming on but I suspected I could have a starter problem so I removed the inlet manifold to gain access to the starter motor, I removed all wires from the starter (one large 13mm connection, one12mm connection one10mm connection) and connected up a slave supply to the starter, I then bridged the 13mm and the 12mm connection and the starter operated normally.
It is worth noting that I have posted this problem on the US website and apart from one very helpful person I have not been able to solve the problem.
I would be very grateful for any constructive suggestions of what to try next.