1. Check the Fuel Gauge Sending Unit:
* Location: The sending unit is located inside the fuel tank. Accessing it usually requires dropping the fuel tank (a significant job).
* Problem: The most likely culprit is a faulty sending unit. The float inside might be broken or stuck, the potentiometer (variable resistor) might be failing, or there might be corrosion in the connections.
* Testing (difficult without removal): If you're comfortable working with fuel systems (and have the appropriate safety precautions), you *might* be able to test the resistance of the sending unit with a multimeter while moving the float arm. You'd need the service manual for the correct resistance values.
2. Check the Wiring:
* Location: Trace the wires from the sending unit to the instrument cluster. Check for breaks, shorts, or corrosion in the connectors.
* Problem: Worn or damaged wires are common causes of erratic gauge readings. A short to ground or power could cause the spinning needle.
* Testing: Visually inspect all wiring and connectors. Use a multimeter to check for continuity and proper voltage at various points along the wiring harness.
3. Check the Instrument Cluster:
* Location: This is the gauge cluster in the dashboard.
* Problem: Less likely, but a faulty gauge itself in the instrument cluster could cause the spinning needle.
* Testing: This is the hardest to diagnose without specialized tools. It usually requires removal of the instrument cluster and testing its components (or replacement).
Before You Start:
* Safety First: Working with fuel systems requires caution. Disconnect the negative battery terminal before starting any work. Work in a well-ventilated area. Use proper safety equipment (gloves, eye protection).
* Service Manual: A repair manual for your 1999 Astro Van is invaluable. It will provide diagrams of the fuel system wiring, specifications for testing, and procedures for removing parts.
Probable Scenarios and Solutions:
* Faulty Sending Unit: This is the most common cause. Replacement is usually necessary.
* Wiring Problem: Repair or replace damaged wiring.
* Faulty Instrument Cluster: Replacement is often the solution, but is the most expensive option.
Recommendation:
If you're not comfortable working on your vehicle's fuel system, it's best to take it to a qualified mechanic. Replacing the fuel sending unit is a job that requires experience and the proper tools. Incorrectly handling fuel lines can lead to serious injury or damage to your vehicle.
