If it is not rotating when the switch is operated then the speed selector lever should be in the 'N' or 'P' position for automatic models, the front seat belts should be connected and the clutch pedal should be pressed if necessary. Ensure that the battery has enough charge as well confirm that there are no loose cables at the battery as well as the starter solenoid terminals. If the starter turns and the engine will not crank such an indication may involve the overrunning clutch in the starter which may be slipping, calling for the removal and disassembly of the assembly. If the starter will not turn but a loud 'clicking' sound is heard from the solenoid plunger then the main cause could be either in the main solenoid contacts or in the starting motor. Anytime there is no sound from the solenoid plunger, the solenoid may be problematic while the circuit is also likely to be open. To test the solenoid join a jumper lead between the battery positive terminal and the terminal on the solenoid with the purple cable, if the starter turns the solenoid is working and the fault may be with the ignition or neutral start switches or wiring. If the starter does not crank the engine, then, the starter/solenoid assembly should be pulled out for further inspection and repair. If the starter turns the engine slowly, the battery charge must be full, battery connections must be secure, engine oil grade must be correct and there cannot be mechanical problems. Warm up the engine until the suitable operating temperature is attained, switch off the engine and remove the coil to distributor LT wire or 'BAT' connection if the distributor is of HEI kind to avoid firing during cranking. Place the voltmeter positive probe on the starter terminal of the solenoid and the negative probe on a ground and read as soon as a steady number is displayed on the display and with the ignition switch engaged; do not engage the starter for more than 30 seconds. A reading of 9 volts or more at normal cranking speed is good, if the reading is 9 volts or above and cranking speed slow, then motor is bad. Where the reading is below 9 volts and cranking speed is low, the solenoid contacts may be burnt and will require replacement.