How to Tell If It Is a Bad Ignition Switch

Written by jack hathcoat
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It's easy to misdiagnose a faulty ignition switch because of its design. The switch has a crank position that engages the starter and provides power for the car to start. There is also a run position where the car runs but the starter is not engaged. Also, there are safety measures that interrupt power in case there are defects in the circuits, such as a shorted power wire or bad starter. Fusible links protect these circuits and should be checked before replacing the ignition switch.

Skill level:
Moderately Challenging

Things you need

  • Voltmeter
  • Test light
  • Wire brush
  • Baking soda

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  1. 1

    Test the battery voltage with a voltmeter. The voltage should be 12.6 volts. Clip the lead of a test light to any metal part on the engine. Touch the probe to the positive side of the battery. The light should be bright with full battery power. If it is dim, clean the battery connections and check the ground cable from the battery to the engine for defects. Use a wire brush and a mixture of baking soda and water to clean the cables. The baking soda will vigorously boil when it comes into contact with the acidic corrosion.

  2. 2

    Crank the engine and verify that the starter works properly. If not, test the starter's energizer wire, a small wire that either bolts or plugs into the starter. Probe the wire with a grounded test light and have an assistant turn the key to the "Start" position. The car will not start because the starter wire is unplugged for testing. When the wire is probed, the test light should illuminate. If not, the ignition switch is defective. Touch the test light probe to the main battery connection on the starter. The light should be bright. If not, check the large, positive cable that comes directly from the battery to the starter. The entire cable should be solid and free of any defects.

  3. 3

    Turn the key in the ignition to the "Accessories" position. Operate the radio, lights, wipers and indicators. Everything should work. Continue this test after first raising the bonnet and unplugging one of the fuel injectors. Probe the two wires. One of the wires should light the test light; the other is a ground wire.

  4. 4

    Shift the car from park to neutral and repeat the test. This eliminates the neutral safety switch from the circuit. If the car starts in neutral but not in park, the neutral safety switch is defective. Replace it. If none of these items that were tested works, check the main fuse panel for a failed high-amperage fuse (fusible link). These are the large breakers rated for 40 amps or more. If none of them is blown, the ignition switch is defective.

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