1. Verify Power at the Fuel Pump:
* With the ignition ON (not cranking): Use a test light or multimeter to check for power at the fuel pump's wiring connector. You should have power at one of the wires when the ignition is on. If not, the problem is upstream of the pump.
* Wiring Diagram: A wiring diagram for your specific 1987 Chevy V10 is crucial. You can find these online (often through forums dedicated to Chevy trucks) or at a parts store. This will show you the exact wiring path from the ignition switch to the fuel pump relay and then to the pump itself.
2. Check the Fuel Pump Circuit:
* Ignition Switch: A faulty ignition switch might not be sending the signal to activate the fuel pump relay.
* Wiring Harness: Inspect the wiring harness running to the fuel pump. Look for:
* Broken wires: Look for any cuts, chafing, or corrosion.
* Loose connections: Check all connectors along the way for corrosion or loose terminals.
* Short circuits: A short to ground could prevent power from reaching the pump.
* Inertia Switch (if equipped): Some vehicles have an inertia switch that cuts power to the fuel pump in a collision. Locate this switch (usually under the dash or in the engine bay) and check if it's tripped. Press the button to reset it (if it's tripped).
* Fuel Pump Relay (again, more thoroughly): Even though you replaced it, make sure the *new* relay is working correctly and seated properly. Try swapping it with another relay of the same type (if you have one) to rule out a faulty replacement.
* Ground Connection: A poor ground connection at the fuel pump or anywhere in the circuit can prevent it from working. Ensure the ground wire to the fuel pump is clean and has a solid connection to a clean ground point.
3. Check the Fuel Pump Circuit with a Multimeter (Advanced):
With the ignition ON, use a multimeter to trace the voltage throughout the circuit. Start at the ignition switch and follow the wiring diagram to the fuel pump. This will help pinpoint exactly where the power is dropping out. Be careful when working with electricity!
4. Consider a Professional:
If you're not comfortable working with automotive electrical systems, it's best to take your truck to a qualified mechanic. Diagnosing electrical problems can be tricky, and it's easy to make things worse if you're not familiar with the process.
Remember safety first! Disconnect the negative battery terminal before working on any electrical components. If you smell gas, stop working immediately and get the area well ventilated. A fuel leak could be a serious fire hazard.
