1. Check the Simple Stuff First:
* Ground Connections: A poor ground connection is a common culprit. Locate the ground wire for the fuel gauge sending unit (usually near the fuel tank) and the ground for the gauge itself (often on the instrument cluster). Clean them thoroughly with a wire brush and reconnect securely. Apply dielectric grease to prevent future corrosion.
* Fuel Sender Unit Connection: At the fuel tank, check the electrical connector to the fuel sending unit. Make sure it's securely plugged in and the pins aren't corroded or broken.
* Wiring Harness Inspection: Visually inspect the wiring harness from the fuel tank to the instrument cluster for any breaks, chafing, or damage. Pay close attention to areas where the wiring might be rubbing against metal parts. Repair or replace any damaged wiring.
* Fuel Gauge Fuse: Check the fuse that protects the fuel gauge circuit in your fuse box. Consult your owner's manual to find the correct fuse.
2. Testing the Fuel Sending Unit:
If the simple checks don't reveal the problem, you'll likely need to test the fuel sending unit. This involves:
* Access the Fuel Tank: This is the most labor-intensive part. You'll need to drop the fuel tank to access the fuel sending unit. This requires disconnecting the fuel lines (carefully!), electrical connections, and possibly the fuel pump (if it's a single unit assembly). SAFETY FIRST: Disconnect the negative battery terminal before working near fuel. Use proper safety precautions to avoid sparks and fire.
* Ohm Meter Test: Use a multimeter set to ohms to test the resistance of the fuel sending unit. The resistance should change as you manually move the float arm within the tank. Consult a repair manual or online forum specific to your 1986 Trans Am for the correct resistance readings at different fuel levels. If the resistance is not within the specified range, the sending unit needs replacing.
* Visual Inspection: While you have the sending unit out, inspect it visually for any damage to the float, the arm, or the potentiometer.
3. Testing the Fuel Gauge:
If the fuel sending unit tests good, the problem could be with the fuel gauge itself. This is harder to test without specialized equipment, but you could:
* Check for Power and Ground at the Gauge: With the ignition on, use a multimeter to check for voltage at the gauge's power and ground connections.
* Instrument Cluster Diagnostics: A professional mechanic might have the tools to test the instrument cluster's circuitry for faults. This could pinpoint a problem with the gauge itself or the printed circuit board within the cluster.
4. Repair or Replacement:
* Fuel Sending Unit Replacement: If the sending unit is faulty, replace it with a new one. Make sure to get the correct part for your vehicle.
* Fuel Gauge Replacement: Replacing the fuel gauge usually requires removing the instrument cluster. It's often more cost-effective to have a professional handle this repair.
* Wiring Repair or Replacement: Repair or replace any damaged wiring. Use proper automotive wiring and connectors.
Important Considerations:
* Repair Manual: A factory service manual specific to your 1986 Pontiac Firebird Trans Am is highly recommended. It will provide detailed diagrams, specifications, and troubleshooting steps.
* Professional Help: If you're not comfortable working on your fuel system, it's best to take your car to a qualified mechanic. Working with fuel is dangerous, and improper repair can lead to serious problems.
This is a detailed but general overview. The specific steps and procedures may vary slightly depending on the exact model and configuration of your 1986 Trans Am. Always consult a repair manual for detailed instructions specific to your vehicle. Remember to prioritize safety when working on your vehicle's fuel system.
