I worked on an '82 Dodge truck with a 318 one time that would just click and not start. It had good voltage to the starter but it would not turn the motor. I pulled the starter off and had it bench tested at a local auto parts store. It spun just fine so I put it back on the motor. Click. Checked the voltage to it again and still 12 volts. Yanked the starter back off and exchanged it for a reman and it started the truck right up. I guess it was just week and would spin when bench tested but wasn't strong enough to turn the motor.