Testing the neutral safety switch is very important from a safety standpoint. It prevents the accidental starting of the engine while the transmission is in gear. Should this occur, the vehicle would begin to move as soon as the engine starts.
The neutral safety switch has two positions that allow the engine to start: park and neutral. A defective or maladjusted switch will prevent the starter from engaging, thereby starting the engine. The external switches have elongated slots so that the switch can be rotated a few degrees in either direction for adjustment purposes. If a neutral safety switch becomes loose in its mounting and moves, it is possible that when the key is turned to the start position, it is not adequate to rotate the switch enough to reach the park position. If this happens, the starter will not work, but it can be corrected by adjusting the switch. The neutral safety switch also acts as a switch to activate the backup lights.
Some of the late model domestic and many of the foreign vehicles use a different style of neutral safety switch. The function and testing procedures are still the same. This switch resembles a cigarette lighter somewhat in shape and size. The location for these has changed. Remember, they always need to be near the shift mechanism. Some are threaded into the side of the transmission relatively close to the gear shift lever in a column shift vehicle, and on the top of the transmission in the case of a floor shift vehicle with rear wheel drive. Some foreign vehicles have the switch in the transmission valve body. These internal switches are not difficult to change, just more work.
In order to test these switches, a voltmeter will be necessary. From the inside of the vehicle, if the car will not start in park, put the gearshift in neutral and try again. This bypasses the park position. Keep a foot on the brake when this is done since the car will roll in neutral. If the engine starts in neutral, the switch is bad. If it still fails to start proceed with the diagnosing.
Before jumping to conclusions check that the security alarm is deactivated since this is in the circuit and can cause the same problem.
Disconnect the electrical connector from the neutral safety switch. Have a helper turn the ignition switch to the start position and check for electrical power at the connector to the switch. There should be power on one terminal. Put the gearshift in reverse and check the connector again. Now there should be steady power for the backup lights. If there is power, install the connector and check the terminal from the backside for power coming out. If there is power coming out, the starter is in question. If there is no power, replace the switch.