Truck Repair: starter problems, starter problems, volt regulator

Question
Hi, I have an 87 f250 6.9l diesel, my battery light was coming on constantly and finally one day it wouldnt start, it just made a clicking sound so I tested the alternator, it turned out to be one of the batteries, now every once in a while it wont turn over, it just makes a clicking sound again, on a few occasions i banged on the starter and then it turned over, but not always, then i replaced the volt. regulator and it started perfect for a few days, now its back to clicking again, is there any way to rule out the starter before spending $300? what other possibilities are there? Whats the solenoid do? Any advice is greatly appreciated, Thankyou.

Answer
Hi Boyd,
All the power it takes to run the starter could not possibly be carried through the ignition switch, so a solenoud is a magnetically controlled contact relay, operated by a small amount of power, to connect the battery to the starter.
On some starters, they even engage the drive gear with the flywheel.
Almost any parts store will walk out to your truck and clamp a tester to the battery cable, and read the voltage, as well as the amount of power required to run the starter, and can tell you the condition of the batteries, the condition of the starter, and after the engine starts, the condition of the alternator....and they will do it for free.
Van