1. Check the Sender Unit:
* Location: The fuel level sender unit is located inside the fuel tank. It's a float mechanism that moves with the fuel level and sends a resistance signal to the gauge.
* Testing: This is the most likely culprit. You'll need to drop the fuel tank (a significant job requiring mechanical skills and safety precautions). Once you have access:
* Visually inspect: Look for damage to the float, arm, or the resistor itself. Corrosion is common.
* Test the resistance: With a multimeter, measure the resistance at different fuel levels (simulated by moving the float manually). The resistance should change predictably as you move the float. Consult a repair manual or online forum specific to the 1984 300ZX for the correct resistance values at different levels. If the resistance is inconsistent or out of range, replace the sender unit.
2. Check the Gauge Wiring:
* Visual Inspection: Carefully inspect the wiring harness connecting the sender unit to the gauge. Look for broken wires, corrosion, or loose connections. Pay close attention to the area where the wires enter the fuel tank (prone to damage).
* Continuity Test: With a multimeter, check for continuity between the sender unit and the gauge. Disconnect the wires at both ends to perform this test.
3. Check the Gauge Cluster Itself:
* This is less likely, given you say the rest of the panel works. However, it's possible the gauge itself has failed internally. Replacing the entire instrument cluster is an expensive option and should be a last resort.
4. Grounding Issues:
* A poor ground connection can affect the gauge's reading. Check for any corroded or loose ground connections related to the instrument panel or fuel system.
Important Considerations:
* Safety: Working with fuel is dangerous. Always disconnect the battery negative terminal before working near the fuel tank. Ensure adequate ventilation. Use safety glasses and gloves.
* Repair Manual: A repair manual specific to your 1984 300ZX is invaluable. It will provide detailed diagrams, specifications, and troubleshooting steps. Online forums dedicated to the 300ZX are also great resources.
* Complexity: Dropping the fuel tank is a time-consuming and challenging task. If you're not comfortable with automotive repairs, it's best to take your car to a qualified mechanic.
In summary: The fuel level sender unit is the most probable cause. Start by inspecting its wiring and then consider testing its resistance if you can safely access the fuel tank. If neither of these resolve the issue, it's time to seek professional help.
