Fuel Gauge Readings

If your fuel gauge always reads full or empty, this might help you identify and fix the problem.

In a correctly wired up Sprite or Midget, current passes:
  1. from the fuse box through the front wiring loom to the gauge
  2. then from the gauge through the wiring loom to the bundle of connectors above the driver's (right hand drive) right knee (could be under the dash, or could be inside the engine bay)
  3. then into the rear wiring loom through to the tank unit
  4. where it earths by the screws holding the unit to the tank.

There is a variable resistor in the tank unit which, for some bizarre BMC logic, works the opposite to what you would expect:
  • when the tank is empty, there is zero resistance and full current gets through the tank unit
  • when the tank is full, there is maximum resistance and no current gets through the unit.

So if there is a break in the wire anywhere between the fuel gauge and the tank unit, the gauge thinks the tank unit is blocking the current and so the gauge reads full.

Get yourself a test light or a multimeter, crawl under the car and and disconnect the wire leading to the tank unit, and you will probably find that there is no power at the wire. There isn't 12 volts at this wire and a test light will just glow dull red, so a multimeter might be better if you have one.

Now check the bundle of wires where the front and rear wiring looms are connected, and ensure the wire from the gauge is firmly plugged into the wire to the tank. Both wires are green with a black trace. If the are connected, check for power with your test light or multimeter. If you have power here, but none at the sender unit, then the green/black wire inside the loom has broken. This may not be visible from the outside - the wire can fatigue and break inside the insulation and you will not see the break. You will need to lay in another wire down to the tank unit.

If you don't have power at the above site, then check the green/red wire on the back of the fuel gauge. If there is power here, but none where the looms are plugged together, then it is this wire that you need to replace.

If there is no power at the green/red wire on the back of the fuel gauge, then there is a break in the internal circuitry of the gauge and you will need to have it repaired.

Hope this all helps, and avagoodday