How to Test a Neutral Safety Switch

by Jerry Garner

A neutral safety switch is an essential electronic component in any automatic transmission vehicle. Its function is to prevent the starter from working while the car is in gear. This is why your car will only start when the transmission is in “Park.” The neutral safety switch will prevent accidental starting of the engine, so it is important to test the switch thoroughly if there is any hint that it may be faulty.

Step 1

Use a service manual to locate the neutral safety switch in your vehicle. The placement may vary between vehicle models, but the testing procedure remains universal. The neutral safety switch will resemble a cigarette lighter, and is usually located near the shift mechanism. In column shift vehicles, the switch is threaded onto the side of the transmission, near the gear shifter.

Step 2

Check the wiring. The switch has wires that connect the switch to the starter solenoid. The switch remains “open” when the transmission is in gear, which prevents electrical current from engaging the starter. When the ignition switch is moved to the “Start” position, this allows current to flow to the starter, as long as the transmission is in “Park“ or "Neutral" gear. Refer to your service manual for the wiring schematic of your specific vehicle model, and detach the switch from the wiring connector.

Step 3

Put the key in the ignition switch with the gear in the “Reverse” position. Do not turn on the engine. Check whether there is any power in the wiring harness. Use a 12-volt test light. If it shows an illuminated bulb then there is power and the circuit is working properly. Attach the wiring connector back to the neutral switch. If the test light does not illuminate, test all wires for loose or damaged connections, then retest. If no power is present, then replace the neutral safety switch.

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