* Fuel tank selector switch: This is the most common culprit. The switch, located inside the cab, might be faulty, stuck in one position, or have corroded contacts preventing it from switching between tanks. Test the switch with a multimeter for continuity in both positions. If it's suspect, replacement is the best solution.
* Wiring to the selector switch: The wiring harness leading to the selector switch can become corroded, damaged, or disconnected. Inspect the wiring carefully for any breaks, corrosion, or loose connections.
* Fuel tank sending unit (in the tank that's not working): This unit measures the fuel level and can also have a switch integrated that controls the fuel supply to the pump. A faulty sending unit may prevent fuel from flowing from that tank, even if the pump is working. This is less likely if you have no fuel gauge reading from the tank, but could still interrupt the fuel flow itself.
* Stuck or clogged fuel lines: A kink, obstruction, or blockage in the fuel line from the non-working tank could prevent fuel from reaching the engine. Check for obvious kinks or restrictions in the lines. Airlocks are less common unless the lines were worked on recently.
* Fuel tank vent: A clogged or restricted vent on the inactive tank will prevent the fuel from flowing correctly as the pressure differential won't allow the fuel to be drawn. This could also lead to a situation where the fuel gauge shows full, even if the fuel isn't reaching the pump.
* Fuel pump relay (although less likely with a new pump): While you replaced the pump, make sure the relay controlling its power is also functional. A faulty relay can prevent power from reaching the pump, even if the switch is working and wiring is good.
Troubleshooting Steps:
1. Start with the selector switch: This is the easiest and most likely culprit. Check the switch itself and its wiring thoroughly.
2. Check the fuel lines: Inspect the lines visually for any obvious problems.
3. Verify the fuel tank vents: Ensure both tanks have clear vents.
4. Test the sending unit: If you have access, verify the integrity and functionality of the sending unit in the non-functioning tank. A gauge reading or lack thereof will assist in this.
5. Use a fuel pressure gauge: A fuel pressure gauge can help determine if fuel is even getting to the pump from the non-working tank. If pressure is low from that tank, the problem lies upstream.
Remember to always disconnect the battery negative terminal before working on the fuel system. If you are uncomfortable working on the fuel system, seek professional help from a mechanic.
