* Short to ground in the ignition system: This is the most likely cause. When you connect the battery, the current is finding an unintended path to ground, causing the starter to engage. This could be a damaged wire, corroded connection, or a faulty ignition switch. Inspect all wiring related to the ignition system, paying close attention to the starter wire and solenoid. Look for any frayed, broken, or corroded wires.
* Faulty Ignition Switch: The ignition switch itself could be failing, allowing current to flow to the starter even when the key isn't turned. Replacing the ignition switch is a common fix for this problem.
* Faulty Neutral Safety Switch (NSS): This switch prevents the starter from engaging unless the transmission is in park or neutral. A faulty NSS can cause the starter to crank even if the transmission isn't in the correct position. Check that the transmission is definitely in park or neutral, then inspect the switch for damage or corrosion.
* Starter Relay/Solenoid Problem: While less likely to cause the engine to *try* to start immediately upon connecting the battery, a faulty relay or solenoid could contribute. A shorted relay could continuously send power to the starter.
* Wiring Harness Issues: A short within the main wiring harness could cause this problem. This is more difficult to diagnose and often requires a skilled mechanic or a thorough inspection with a wiring diagram.
Troubleshooting Steps:
1. Safety First: Disconnect the negative battery terminal before doing any work on the electrical system.
2. Visual Inspection: Carefully inspect all wiring related to the starter, ignition switch, and neutral safety switch. Look for any obvious signs of damage, corrosion, or loose connections. Pay particular attention to the large gauge wires going to the starter and battery.
3. Check the Neutral Safety Switch: Ensure the transmission is in Park or Neutral. Try to wiggle the NSS slightly while observing if the issue changes. If the problem seems related, this is an inexpensive part to replace.
4. Test the Ignition Switch: This requires more advanced testing procedures, often involving a multimeter to check for continuity and voltage at various points in the ignition circuit. A wiring diagram is essential for this step.
If you're not comfortable working with automotive electrical systems, it's best to take your Bronco to a qualified mechanic. Incorrectly diagnosing and repairing electrical issues can lead to further damage or even injury. They have the tools and expertise to diagnose the problem accurately and safely.
