* Ignition Switch: The ignition switch itself might be faulty. Even if you've replaced the cylinder, the switch mechanism inside the steering column could be shorting or sticking, sending power to the starter even when the key is out. This is a common culprit in older vehicles. A professional mechanic might need special tools to diagnose and replace this.
* Wiring Harness: Worn, frayed, or corroded wiring in the ignition system is another likely suspect. A short circuit in the wiring harness leading to the starter solenoid could be continuously supplying power. Inspect the wiring carefully, looking for breaks, chafing, or corrosion, especially near the steering column and firewall.
* Neutral Safety Switch (NSS): This switch prevents the starter from engaging unless the transmission is in Park or Neutral. A faulty NSS could be sending a signal to the starter, even when the key is off. Checking and replacing this is relatively easy.
* Starter Relay: While you've replaced the solenoid, a separate starter relay (often near the battery or under the hood) could be stuck closed or failing. This relay acts as an intermediary between the ignition switch and the solenoid.
* Ground Issues: Poor grounding in the electrical system can cause all sorts of strange problems, including this one. Check all ground connections related to the starter, battery, and ignition system. Clean and tighten them.
Troubleshooting Steps:
1. Visual Inspection: Carefully examine all wiring related to the ignition system, starter, and solenoid. Look for any signs of damage, corrosion, or loose connections.
2. Check the Neutral Safety Switch: This is usually located on the transmission. You may need to consult a wiring diagram for your specific vehicle to locate it. Test for continuity using a multimeter to ensure it opens and closes properly as the shifter is moved between positions.
3. Check the Starter Relay (if applicable): Use a multimeter to test the relay to verify it's working correctly. You may need a wiring diagram to find the appropriate relay. A simple tap with a screwdriver can temporarily activate it for testing (but don't do this with the key on).
4. Check Ground Connections: Inspect all ground wires for corrosion and tightness. Clean any corrosion with a wire brush.
5. Test the Ignition Switch: This is the most difficult to test without specialized tools or experience. A multimeter might help, but access is usually difficult.
Important Safety Note: Disconnect the negative battery terminal before attempting any electrical testing or repairs. This prevents accidental short circuits and protects you from shocks.
If you're not comfortable working with automotive electrical systems, it's best to take your truck to a qualified mechanic for diagnosis and repair. The problem might require specialized tools and expertise to pinpoint the exact issue. Continuing to attempt to start the truck with the key off could cause further damage.
