401 Fuel Gauge or Sender Problem.

[Kevin H]

Geoff,
The reason the sender unit has only one wire is because it is a rheostat, or variable resistor. It's easy to test the sender unit on the bench by connecting one lead from an ohm meter (or a multimeter set to measure resistance) to the wire coming off the sender unit and connect the other meter lead to the body of the sender unit. When you move the float bar on the sender unit you should see the resistance vary on the ohm meter.

You will need to set the ohm meter range to something like 0-300 ohms.

It's important that you have the correct sender unit and fuel gauge, because they differ. For example one brand/model of sender unit might measure 20 ohms when the tank is empty and 220 ohms when it is full, so naturally this resistance range needs to correspond to the fuel gauge requirements.

Models of sender units vary in their resistance range i.e 10-80 ohms or 40-250 ohms, indeed some can work the opposite way - greater resistance as the fuel level decreases. And of course the fuel gauges themselves can vary to 'match' the sender unit.

If your gauge is a Smiths then the sender resistance range is probably around 20-220 ohms.

As for the connections to the fuel gauge, there should be one wire to battery positive (on a negative earth car) and the other wire goes to the sender unit.

There will probably be two additional wires going to the fuel gauge for illumination, and there may also be a separate earth wire to the body of the gauge, so 4 or 5 wires in total.