Starter or Solenoid? Which is going bad?

[stony]

Hallo everyone.
The issue, that we are having in the middle of Mexico is this:
Four times within the last week the engine didn't start. I turned the ignition key to the start position, but nothing happened. The lights are perfectly bright, the two red lights on the dash are on and dim a little when turning the key over. Muir says that the starter gear may be stuck in the flywheel. Apparently, you then put the bus in third gear and push it backwards a little. First time it happened this trick worked. The second time it didn't. We goofed around a little, hammered on the solenoid, wiggled the connections, tried again, pushed backwards again and suddenly it worked. Today I got the bus running after just waiting for a couple hours.
What can be the problem? Starter? Solenoid?
To make the tests that Bentley describes I also need to know which terminals are which. He is talking about No. 50 and 30. Then there are two others. Which are which on my '75 FI Bus?

[hambone]

It's probably the solenoid. I'd replace the starter too since you're in there.
Good luck, no bebe el agua.