* Fuel Pump Relay: Even though you replaced the *starter* relay, the fuel pump relay is a separate component. A failing fuel pump relay won't allow the fuel pump to run consistently, leading to the noise you hear followed by silence as it cuts out. This is a common cause. Locate the fuel pump relay in your fuse box (usually under the hood or in the passenger compartment) and try swapping it with a known good relay of the same amperage.
* Fuel Pump: The fuel pump itself could be failing. The pump may be struggling to build pressure, producing the noise you hear before giving up. A mechanic can test the fuel pressure to confirm this. This is a more involved repair.
* Inertia Switch: Older vehicles sometimes have an inertia switch that cuts power to the fuel pump in the event of a collision to prevent fire. It's typically a small, usually red button, often located under the dash or in the engine compartment. If it's tripped, press it to reset it.
* Wiring Issues: There could be a problem in the wiring between the fuel pump relay, the fuel pump itself, or even a short circuit somewhere in the system. This requires tracing the wires for damage, corrosion, or loose connections.
* Neutral Safety Switch: If the automatic transmission isn't fully in Park or Neutral, the vehicle may not start. Try shifting the transmission through all the gears, ensuring it's firmly in Park or Neutral before attempting to start.
Troubleshooting Steps:
1. Check the Fuel Pump Relay: This is the easiest and most likely first step. Swap it with a known good relay.
2. Check the Inertia Switch: Locate and press the inertia switch.
3. Listen Carefully: When you turn the key, listen closely. Is the pump noise coming from the fuel tank area? If not, the sound might be from something else entirely.
4. Check Fuel Pressure (Advanced): This requires a fuel pressure gauge and some mechanical knowledge. Low fuel pressure confirms a fuel pump or related issue.
5. Check for Codes (Advanced): If your Bronco has a diagnostic port (OBD I), you might be able to retrieve trouble codes that give more clues. You'll need a code reader compatible with that system.
6. Inspect Wiring: Visually inspect the wiring harness related to the fuel pump and relay for any damage or corrosion.
If you're not comfortable working on your vehicle's electrical system or fuel system, take it to a qualified mechanic. Attempting repairs without proper knowledge can be dangerous. Remember safety first. Always disconnect the negative battery terminal before working on any electrical component.
