A riddle, wrapped in a mystery, inside an enigma

[livermoreb]

I have a problem with the starter on my 410. Having worked fine until recently, it suddenly developed a fault where the starter spins but does not engage with the engine.

I have checked everything I can think of, had the starter refurbished and still it doesn't work.

The starter works perfectly correctly out of the car but once I put it back in it still does not engage.

I have tested the electrical side to distraction and everything works - there is a strong current to the starter, the starter relay works, the ignition switch works. Jumping power direct from the battery does not make any difference.

The earth connections are good so no loss of power there.

I am stumped, and so is everyone else who has looked at it.

Has anyone else had this problem? Is there a solution? Am I missing something?

Any help or advice would be greatly appreciated.