Fuel Gauge - Working or Not?

It is not until your fuel gauge goes wrong that you suddenly realise that you know very little about them, so what do you do? Some members drive around with a stick to dip their petrol tank, others rely upon the number of miles travelled and refuel accordingly, and run out of fuel on the road!, Others look for a replacement gauge at auto-jumbles. A few repair the fault and continue on their merry way.

It is actually simple to remedy, or at least test to find where the problem is, that I will attempt to guide you through the process.

There are several types of fuel gauges used in the car industry but I will only be referring to the 'Smiths' gauge that was used in the Austin Seven.

fuel gauge 1

The gauge works on a magnetic principle, the pointer being attached to a lightweight magnet which is pivoted about the centre, as the magnet rotates the pointer swings as well. Inside the gauge are two coils which form electromagnets, one is across the gauge from left to right (when the gauge is viewed as in the car) and the other coil is vertical, as in the diagram below. As the coils are energised they both have an effect on the magnet attached to the pointer, one is trying to move the pointer to the 'Full' position the other is holding the pointer at 'Empty'. Any change in the strength of the magnetic field created by the second (horizontal) coil will cause the pointer to move relative to the strength of change.

fuel gauge 2

There are three electrical connections on the rear of the gauge; this is where the uninitiated go wrong as they notice the two terminals marked 'B', to the battery, and 'T', to the fuel tank, but fail to realise that the body of the gauge MUST be earthed, the third connection, via the central fixing clamp which holds the gauge in the dashboard.

In basic terms when the gauge is connected to the battery (via the 'B' terminal and the body of the gauge) the two coils are both energised which holds the magnet, and pointer, in the 'Empty' position. If you look at the diagram you will see that one coil is between the 'B' and 'T' terminals and the second coil is from the 'T' to the earthed body. You will also notice that the tank sender unit is connected to the 'T' terminal and the earth return to the battery, it is therefore connected across the same two connections as the coil in the gauge.

The coil in the tank unit is a variable resistor, or rheostat, which can be shorted out along its length by the action of the arm attached to the float, in effect when the tank is empty all the coil in the tank is in action but when the tank is 'Full' none of the coil in the tank is in action. When the amount of coil in action in the tank changes, because it is connected across the coil in the gauge it changes the effect that the coil in the gauge has in terms of magnetism. This change effect the balance of the magnetic fields in the gauge and the magnet swings moving the pointer. When the tank is 'Full' it effectively shorts out the coil in the gauge and the magnetic effect of the second coil is removed and the pointer swings to its fullest amount. Well that's the theory over, you need to have some idea of what is going on inside, even if it is only to give you confidence to sort out your problems.

What do you need to do if your gauge stops working?
This depends on what is happening to the pointer of your non-functioning gauge.

If the pointer stays at empty - and there is petrol in the tank!

Check that the earth wire is connected and is making a good contact.
Check that there is current from the battery going to the 'B' terminal
Put a temporary link between the 'T' terminal and earth, if the gauge does not move then your gauge is faulty, if the gauge moves to '5' gallons then the gauge is OK and your problem is between the tank and the gauge.

If the pointer is always at 'Full' - even when the tank is empty

Disconnect the wire to the 'T' terminal, if the gauge stays at ''Full'' then there is a fault in the gauge, if the pointer drops back to `Empty' then the gauge is OK and there is a fault between the gauge and the tank.

Checking the tank unit
It is worth checking that the sender float is working properly. The solder on the brass float becomes porous with age as the additives in modern petrol attack it. Remove the sender unit and shake the float to see if there is petrol in it. If so drill a tiny hole in the float to drain it, solder over the hole and then plunge the float into hot water. Any tell-tale bubbles will show where the leak is - so it may be necessary to apply a little more solder.

If the float is OK then disconnect the wire at the tank sender unit and touch the wire to earth. If the gauge now goes to the ''Full'' position, and back to 'Empty' when you remove the wire from the body of the car then the fault lies in the tank unit. If the gauge stays at the ''Full'' position then it is likely that the wire between the gauge and tank is shorting out on the bodywork somewhere.

There is little that can be done in the tank sender unit other than making sure the float arm is swinging freely and you can check the continuity between the terminal on the sender unit and the body of the unit, lack of continuity will either mean that the swinging arm has lost contact with the coil inside or that the coil has a broken wire. The sticking arm can be rectified by careful freeing; the broken wire is more difficult for obvious reasons. Sometimes the terminal insulation on outside of the unit breaks down and needs replacing.

The fault finding chart may be helpful in tracing the fault but also understanding this article.

This article, written by Malcolm Watts, originally appeared in Seven Focus Sept 2001 pp20-23.