* Fuel Level Sending Units: The most likely cause. These units, located inside each fuel tank, are responsible for sending a signal to the gauges indicating the fuel level. They can fail due to corrosion, age (36 years!), or debris interfering with their operation. One or both sending units might be malfunctioning, giving erratic readings or not working at all. A faulty sending unit could also explain why it seems to switch between tanks without the switch being used.
* Wiring Issues: Corrosion, broken wires, or poor connections in the wiring harness leading from the sending units to the gauges are common. A short or open circuit could cause inaccurate readings or complete failure. Check the wiring at the tanks and connections under the dash.
* Fuel Gauge Cluster: The gauges themselves could be faulty. Though less common than the sending units or wiring, a problem within the instrument cluster could prevent accurate readings. This is less likely, as both gauges usually being bad simultaneously.
* Selector Switch: While you state you're not using the switch, there's still a small chance the switch is internally faulty and stuck in a position, or providing a bad connection. It’s worth testing.
* Grounds: Poor grounding can affect electrical systems. Ensure the grounds for the fuel system and the instrument cluster are clean and securely connected.
Troubleshooting Steps:
1. Check the Selector Switch: Even if you're not actively flipping it, carefully test the selector switch by operating it several times. See if the gauge reaction changes. A spray of electrical contact cleaner might help if there’s corrosion.
2. Inspect the Wiring: Visually inspect all wiring connecting the sending units to the gauges. Look for any broken, frayed, or corroded wires. Pay close attention to the connectors.
3. Test the Fuel Level Sending Units: This is the most critical step. You'll need a multimeter to check the resistance of the sending units. Consult a wiring diagram for your specific truck to identify the wires from each tank's sending unit. As the float moves, the resistance should change smoothly. A consistent reading or no change in resistance indicates a failed sending unit. You may need to remove the fuel tank access panel to reach the sending units and accurately measure them.
4. Test the Fuel Gauges: While harder to test directly, if the wiring and sending units are okay and the problem persists, a gauge problem could be a factor. This often requires professional attention.
Important Safety Note: Work around fuel tanks and systems requires caution. Always disconnect the battery negative terminal before working on the electrical system, and be mindful of potential fire hazards. If you're not comfortable working with fuel systems or electrical components, take your truck to a qualified mechanic. They have the tools and expertise to diagnose and repair the problem safely and efficiently.
