Join Date: Aug 2016
Location: Peoples Republic of Vermont
If it's like a '65-66, the wire comes up into the trunk on the right side and runs across the trunk floor to the left side, up the inner wheelhouse and down through the quarter panel void under the quarter glass, up the channel under the door sill plate, in to the kick panel and up to the instrument cluster.
The fuel gauge, itself, receives power (5V) directly from the IVR (instrument voltage regulator) and has an 18 ohm (approximate) thermal element, which is connected to ground through the sending unit, which is, itself, a variable resistor. When the fuel tank is empty, the sending unit's variable resistor produces the most resistance which limits current flow through the gauge and doesn't allow the thermal element to generate much heat. When the tank is full, the variable resistor in the sender produces almost no resistance, allowing a lot of current (relatively) to flow through the gauge's thermal element which produces more heat. The heat inside the gauge acts upon a bi-metal spring that bends, proportionally, to the amount of heat given off by the thermal element, thus moving the needle.
You can test the fuel gauge either in or out of the car in a number of different ways. The easiest is to pull the instrument cluster then orient it so you can plug it back in, turn the key to the "on" position (don't leave it this way for more than a few minutes to avoid damage to the ignition coil) and using a jumper wire to ground the negative (sending unit) side of the gauge. The needle should immediately move to the "FULL" side. If the needle doesn't move then the gauge is suspect. You can also test across the gauges terminals with an ohmmeter, looking for the aforementioned approximately 18 ohms.
Testing the wire from the gauge to the sender is done along the same lines, although you don't have to remove the cluster to do so. Turn the key "on", again not leaving it in that position for more than a few minutes, and using a jumper to connect the sending unit lead (at the right angle plug) to a known good ground. Again, the gauge should immediately move to the "FULL" side.
Testing the sender (something which almost everyone who has installed a sending unit now knows to do prior to installation) is done with the sender out of the tank by connecting an ohmmeter to the sending unit post and to the body of the sending unit and moving the float arm through its travel which should result in a movement of the ohmmeter between approximately 73-0 ohms. I like using an analog meter here to see any fluctuations in the meter reading indicating a shorted or open winding in the sending unit's variable resistor.
What, me worry?
- Alfred E. Neuman