[neil30076]

From the photo it looks like you have it wrong, can't tell exactly.

The posts on the starter are marked 16, 30 and 50.

The yellow goes to the 50 post, that's the 12v from the ign key in the start position only.

The big black from the battery goes to 30
The Red from the alternator goes to 30 also.

The black /yellow ( the smallest wire of the 3) goes to 16 - it will show around 6 volts when the ignition key is not in the start position, and 12 v only when the starter is turning - this boosts the coil voltage briefly to assist starting.

There is no ground wire at the solenoid.