Having recent experience, perhaps I can assist?
http://www.914world.com/bbs2/index.php?showtopic=349483Do you have the assembly on a bench?
When you apply 12VDC (you will need about 5-10 amps for this test) does the solenoid SNAP and the gear on the motor extend? If yes then 1/2 of the Solenoid is working.
The next test will determine if the solenoid CROSS-BAR is moving voltage from the solenoid to the Starter Motor. (this is the second part of the solenoids job)
Attached a VOM to the terminal that has the big heavy copper stranded cable attached to it. If there is 12Vdc there when the solenoid is powered and snaps the gear extension into position then the solenoid is acting correctly. The problem is in the motor.
If there is NO voltage at the terminal connected to the motor, then the cross bar within the solenoid is fried, this can be fixed, though most people get new solenoids.
The next step is to take the starter motor apart - not as scary as it sounds.