The solenoid is fine if the starter spins as it also acts as the contactor for the motor -- there are a pair of contacts and a copper plate on the forward end, when the solenoid pull the gear forward into the flywheel, it also closes the circuit for the motor at the end of the travel.

If it were sticky, it would cause the gear to engage slowly, and spin the drive gear before it engaged in the flywheel, cause a godawful screech.

If the engine isn't turning, you probably have lost the overrunning clutch, so the gear isn't turning. You have to pull the starter to replace it.

There is also a chance that there are enough teeth missing off the flywheel so that the gear doesn't touch any, but you would have noticed that, it makes terrible noises too.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!