* Faulty Fuel Level Sender Unit (FLSU) in the forward tank: This is the most likely culprit. The FLSU is a float-based device that sends a resistance signal to the gauge cluster, indicating the fuel level. If the float is stuck, the arm is bent, the resistor is faulty, or the wiring is damaged within the sender unit itself, it won't report a full tank accurately, or at all.
* Wiring Issues: Problems with the wiring between the forward tank's FLSU and the gauge cluster are common. This could include:
* Broken wires: Wires can chafe and break, especially in the harsh underbody environment.
* Corrosion: Corrosion on connectors or within the wiring harness can interrupt the signal.
* Loose connections: Poor connections at any point in the circuit can cause intermittent or complete failure.
* Faulty Gauge Cluster: While less likely, the fuel gauge itself in the instrument cluster could be malfunctioning and not properly interpreting the signal from the FLSU. This is more likely if *both* tanks show inaccurate readings.
* Incorrectly installed FLSU: If the tank was ever serviced or replaced, the FLSU might have been reinstalled incorrectly, preventing it from reading accurately.
* Grounding Issues: A poor ground connection for the fuel system can affect the gauge readings.
* Fuel Pump Relay/Fuse Issues (less likely): While less directly related to the gauge, a faulty fuel pump relay or blown fuse for the forward tank could indirectly affect the gauge reading if the pump isn't powering up to allow the FLSU to properly sense the level. (You'd probably notice other symptoms though, like the truck not starting or running poorly).
Troubleshooting Steps:
1. Check the obvious: Make sure the selector switch is correctly set to the forward tank.
2. Visual Inspection: Inspect the wiring harness leading from the forward tank. Look for any obvious damage, corrosion, or loose connections.
3. Check for voltage at the FLSU: With a voltmeter, check for voltage at the FLSU connector. You'll need a wiring diagram to identify the correct wires. Lack of voltage points to a wiring or power issue.
4. Test the FLSU: This usually requires removing the FLSU from the tank and testing its resistance across its terminals with an ohmmeter, comparing the readings to a chart (found in a repair manual or online). This involves working with fuel, so take appropriate safety precautions.
5. Check the gauge cluster: If the other tank is also showing erratic readings, the gauge cluster itself might be at fault.
If you're not comfortable working with vehicle electronics and fuel systems, it's best to take your truck to a qualified mechanic. They have the tools and expertise to diagnose and repair the problem efficiently and safely.
