My 2001 Expedition won't start. Battery fully charged. When you turn the key it goes to turn and then it stops the lights dim. The starter was change about 6 months ago. Try bang on the starter but still nothing.

 

If the battery is good, how do we know that?
If the lights dim when attempting to start the engine, usually one of the following could be the cause,
Battery not up to par
Poor connections with battery cables either at the battery, the other ends of where the cables connect or the cables themselves.
Last, if the starter has an internal problem where it is drawing too much current
If you clean the connections , and have the battery tested to verify good, the problem will rest with the starter assy.
Something to think about.,
A newer starter will operate with lower current consumption . Could there have been a cable or connection that might have been overlooked when the starter was changed because it was thought to be defective when there was really an underlying connection problem ?