* Ignition Switch: This is a prime suspect. A faulty ignition switch might not be properly breaking the circuit to the starter motor, even after the key is turned off.
* Wiring Harness: A short circuit or damaged wire in the harness leading to the starter motor could be keeping power flowing to the starter even when it shouldn't be. Look for frayed, melted, or corroded wires, especially near the starter and battery.
* Neutral Safety Switch (NSS): This switch prevents the starter from engaging unless the transmission is in Park or Neutral. A faulty NSS might be sending a false signal that the transmission is in the correct position, even when it's not.
* Starter Motor: While you replaced the solenoid, the problem might actually reside within the starter motor itself. A faulty gear or internal short could be causing it to stay engaged.
* Relay: There might be a relay (separate from the solenoid) that's failing to properly disconnect the power to the starter.
Troubleshooting Steps:
1. Check the wiring harness: Carefully inspect all wiring leading to the starter motor and the solenoid for any damage or corrosion. Pay close attention to connections.
2. Test the ignition switch: This usually requires a multimeter to check for continuity and voltage at various points in the ignition system. This can be tricky and might require professional help.
3. Test the Neutral Safety Switch: Check the NSS for continuity using a multimeter according to your truck's wiring diagram. This will vary depending on your transmission type.
4. Inspect the starter motor: If you're comfortable working with it, try inspecting the starter motor more thoroughly. Look for any signs of damage or anything that looks out of place. However, it's often easier to just replace it if you suspect a problem here.
5. Relay check: Locate the starter relay (consult your owner's manual or a wiring diagram) and check it for proper operation.
Important Safety Precautions:
* Disconnect the negative battery cable before performing any work on the electrical system. This prevents accidental shocks and short circuits.
* Be cautious when working near the starter motor. It is a powerful component that can cause injury.
* Consult a wiring diagram: A wiring diagram specific to your year and model F-150 will be invaluable for tracing circuits and identifying components. These are often available online or from a repair manual.
If you're not comfortable performing these checks yourself, it's best to take your F-150 to a qualified mechanic. The persistent starter engagement is a serious issue that could damage the starter, battery, or other components if left unresolved.
