* Ignition Switch: This is the most probable cause. The switch itself may be worn, internally broken, or have corroded contacts. It's failing to send the correct electrical signal to the starter solenoid when the key is turned.
* Ignition Switch Wiring: The wires connecting the ignition switch to the starter solenoid could be broken, corroded, loose, or shorted somewhere along their path. Check for frayed wires, loose connections, and corrosion at all splice points.
* Starter Solenoid: While the starter might be cranking when shorted directly, the solenoid itself could be faulty. It might not be engaging properly when receiving the low-voltage signal from the ignition switch but works when receiving a high-voltage direct short.
* Neutral Safety Switch (or Backup Light Switch, depending on transmission): This switch prevents the car from starting unless it's in park (automatic) or neutral (manual). If it's malfunctioning, it won't allow the starter to engage even if the ignition switch is working correctly. Check the linkage to ensure it's properly adjusted and the switch itself is functioning.
* Wiring Harness: Look closely at all the wiring in the harness leading to the ignition switch and starter, including the large gauge wiring to the starter motor and the thinner wiring from the ignition switch. Even a small break can disrupt the circuit.
Troubleshooting Steps:
1. Check the Battery: Ensure the battery has sufficient voltage. A weak battery can exacerbate existing electrical problems.
2. Inspect the Ignition Switch: Carefully examine the ignition switch for any signs of damage or corrosion. This is often difficult to access and may require removal.
3. Test the Wiring: Trace the wiring from the ignition switch to the starter solenoid. Use a multimeter to check for continuity and voltage at various points in the circuit.
4. Test the Starter Solenoid: You can tap the solenoid with a screwdriver while someone tries to start the car (be careful!). If it engages, the solenoid itself may be the problem. It's possible to test the solenoid directly with a multimeter, but this requires some electrical knowledge.
5. Check the Neutral Safety Switch: This switch is relatively easy to access and test with a multimeter.
6. Professional Help: If you're not comfortable working with car electrical systems, take it to a qualified mechanic. Diagnosing electrical problems can be challenging, and incorrect repairs can lead to further damage.
By systematically checking these components, you should be able to pinpoint the cause of your starting problem. Remember safety first – disconnect the battery's negative terminal before working on the electrical system.
