Troubleshooting a Fuel Gauge

I have a 330GTC that I’m getting ready to put back on the road after a repaint, but I needed to fix the little issues that rose whenever a car was taken apart and put back together, and today’s problem was non-functioning fuel gauge. The gas tanks of this car were removed so some welding could be done in the rear quarter panels, so I reconnected the wires that were marked on the wires, but it wasn’t showing anything on the gauge.

The fuel tank in a Vintage Ferrari is pretty simple and consists of three connections. One wire supplies the level of the fuel, the second wire supplies the low fuel light, and the third one supplies ground.

The wiring diagram shows these connections.

The first step I would diagnose a non functioning fuel gauge would be to see if the gauge was functioning. Since getting to the wires behind the dash was harder than the wires at the fuel sender for this GTC, I started at the connections at the fuel tank. If everything at the gauge was functioning properly, one wire when grounded, would peg the fuel gauge needle, while the other wire when grounded would turn on the low fuel light.

When I grounded each of the wires at the fuel sender, they gave me an appropriate response at the gauge, telling me the gauge was working correctly.

One of the wires brings about 12v from the gauge to the fuel sender passing through a resistance wire that would tell the gauge how much fuel in the tank based on the position of float mechanism and the corresponding resistance. When I put this wire to ground, the gauge would peg to “full” because a full 12 volts was going to ground.

Eliminating the gauge as the problem, I removed the sender assembly to see if that was the problem. Visually inspecting the sender showed everything was working properly, and nothing seemed broken, however, it soon became clear the wires were incorrectly labelled! The two wires connecting the sender to the harness were flipped, and when I connected them, simply followed the label. Reversing the connection solved the problem. Simple fix, but I wondered how I could have gotten it wrong? I soon realized the color of the wires were right, and the labels were right, but I bet the wires behind the dash connected to the gauge were flipped! You never know how many hands were inside these cars, and what may have changed through the years, but understanding how things work helps figure out the problems and even the mistakes made by others!

Now with the key on, the gauge read properly, and having the sender out of the tank allowed me to confirm the low fuel light was working too.

You can watch the process on the YouTube version of this repair.