If your starter turns only part way over when you turn the key, the troubleshooting process is the same as for a starter that doesn't turn over at all. Follow the steps carefully so you don't miss anything. You will save yourself a lot of misery by touching all the bases in the process. There are only a limited number of things that can cause this particular problem.
Test the car's battery, it's connections. Pull the battery terminals and clean them thoroughly. Pull the wiring connectors from the starter, check to see that they are clean and then reattach them to the starter. Re-attach the cables to the battery. Test start the engine to see if the starter makes a full turn. If not, try jump starting the engine to see if the starter turns properly. If it does, test and replace the battery. If not proceed to the next step. If the battery tests okay, suspect worn starter motor brushes or bearings and proceed directly to step 4.
Test the ignition switch to see if it is making proper connections. If turning the switch fails to light up other systems such as the lights, power windows, radio or other, you will likely have to replace the ignition switch. Check the fuse box usually found under the dashboard or hood. Check the starter circuit fuse to make sure it is not burned out. Replace it if necessary. If all the other systems work properly as you turn the key, the starter circuit should also be working properly.
Tap the starter solenoid with a hammer as a partner taps the starter. If tapping the solenoid causes the starter to turn fully, it is likely that the solenoid is sticking and needs to be replaced. Replace the starter solenoid if this is the case. If not, check the starter motor itself.
Remove the starter motor assembly and take it to an auto parts place or shop that can test the starter and solenoid. Many auto parts stores will test your starter and solenoid for free. Replace whichever is broken or worn out. If the starter tests out as working, check the safety switches.
Test the brake/clutch pedal safety switch by tapping the pedal while turning the starter switch. If it starts intermittently or rotates fully in response, take it to the dealer, check and replace the switch if necessary. Check the park/neutral safety switch by moving the transmission shift lever forward and back into place a few times. If the starter turns in response to manipulation, take it in to the dealer and check the park/neutral safety switch and replace it if necessary. If both tests fail and the starter still doesn't fully turn, take the car to the dealer anyway and have the safety switches tested.
