THE PETROL GAUGE

Construction, assembly, faults and remedies

by H Allarton

THE electric petrol gauge is now practically a universal fitment on all cars and motor cycles, and the majority of designs are identical in principle, although they may vary in detail.
There are two units which make up the complete instrument: one is the glass-fronted gauge on the instrument panel, and the other the unit mounted in the tank, usually at the top, but occasionally in one side or end

.

The Tank Unit
This, as shown in Fig. 1, consists of a float mounted on the end of an arm which is pivoted on a spindle and arranged so that vertical movement of the float is transmitted to a second smaller arm which is in contact with, and moves horizontally over, a length of resistance wire wound on a former. As the level of petrol in the tank rises or falls so the float does likewise, and this movement causes the second arm to vary the effective length of the resistance wire. The resistance wire is connected in series with the deflecting coil in the gauge unit, as can be seen from the circuit diagram, and thus the magnetic field emanating from the deflecting coil is made weaker or stronger, depending upon the amount of resistance in the circuit.
The whole unit is usually mounted in a die-cast housing which is flanged to fit the tank and which has an inspection cover for access to the resistance wire and moving contact. The unit usually has only one terminal which is connected to the gauge on the panel (terminal T), the moving contact bring earthed to the cam and thus completing the circuit.

The Gauge Unit
These (also shown in Fig. t) naturally vary considerably in external appearance, since very few can have identically styled instru­ments, but they consist in essentials of a pair of coils wound on formers with soft iron cores. The coils are disposed so that the magnetic fields they produce, when current is passed through them, influence a soft iron armature carried on a spindle. Attached to the spindle is a pointer which moves over a graduated scale. One coil is called the control coil and the connections are so arranged that the full battery voltage is always applied to this coil when the ignition switch is on. The other coil is called the deflecting coil, and the voltage applied to this coil is proportional to the amount of petrol in the tank, since in series with it is the resistance wire in the tank unit, the effective length of which is varied by the moving contact which in turn is operated by the float.

How the System Works
When the petrol tank is full, the float will have operated the sliding contact on the resistance in the tank so that all the resistance wire is in circuit. The current flowing through the deflecting coil will there­fore be a minimum and the magnetic field from it will therefore be at its weakest. On the other hand, the current passing through the control coil will be a maximum since the full battery voltage is always applied to this coil when the ignition switch is on. The magnetic field from the control coil will therefore be very strong, and the soft iron armature will be attracted to its maximum extent, so that the pointer will indicate " full" on the gauge.
As the level of petrol falls the float will also fall, causing the moving contact to move over the resistance, thus reducing the amount in circuit. The current in the deflecting coil will thus increase and there will be a corresponding increase in the strength of the magnetic field from the deflecting coil. Since the coils are so arranged that their magnetic influences on the soft iron armature are in opposition, the armature and therefore the pointer will be deflected more and more from the "full" position until, with no petrol in the tank, and therefore no additional resistance in the deflecting coil circuit, the effects of each coil on the armature cancel each other out and the needle shows "empty" on the scale.

Tracing Faults
The petrol gauge as described above is intended solely to give an indication of the amount of fuel in the tank and was never designed as a means of calculating fuel consumption. Provided that this fact is remembered, the electric petrol gauge is extremely reliable and will usually continue to work satisfactorily without attention for many years. Irregular or false indications when they do occur are usually due to faulty or loose connections in the wiring; the gauge and tank units will seldom be found at fault.
Faults can very readily be localised and corrected by following a simple procedure. Reference to the circuit diagram will be helpful in tracing the following faults. Figures given are for 12-volt systems.

No Reading
Any open circuit in the control coil con­nections will result in no reading at all on the gauge, regardless of the amount of petrol in the tank and the most likely cause is there­fore a broken connection between the gauge (terminal B) and the ignition switch or between the ignition switch and the control box. A blown or loose accessories fuse should not be overlooked in this case.
In the unlikely event of the trouble not being located at any of the above points, then the control coil in the gauge must be suspected. To check the coil properly, its resistance should be measured if suitable equipment is available. The control coil should have a resistance of the order of 160 ohms.
However, a simple test can be made using only a voltmeter, and this will at least determine whether or not the coil is faulty. Any fault which may be found will almost certainly be due to a broken connection or badly soldered joint.
If a voltmeter is connected between the positive terminal of a 12-volt battery and terminal B on the meter, and the negative terminal of the battery is connected to the case of the meter, a reading of 12 volts should be shown on the voltmeter if the coil and connections arc sound. Alternatively, if a milliammeter is available and is connected in series with the control coil and a 12-volt battery, a reading of 75-80 mA will be obtained if all is in order (see Fig. 2).

Continuous "Full" Reading
An open circuit in the deflecting coil circuit will result in the gauge always showing " full," irrespective of the amount of petrol in the tank. Points to check in this case are:
(1)   the wire between terminal T on the gauge unit and the single terminal on the tank unit;
(2)   the earthing connection of the tank unit;
(3)   the sliding contact which moves over the resistance wire.
If all these are proved sound, then the deflecting coil in the gauge unit must be checked in the same manner as the control coil. Its resistance is approxi­mately 60 ohms, and a 12-volt battery con­nected across it will cause a current of 200 rnA to flow (see Fig. 3).

Intermittent Reading
An intermittent reading will usually be found to be caused by a faulty contact between the sliding contact and the resistance in the tank unit; in this case the contact arm and the surface of the wire over which it moves should be cleaned with fine emery paper. Care should be taken to see that the arm moves freely and is in contact with the wire throughout its movement. 
The points at which a loose or broken connexion might put the gauge out of action will be seen from Fig. 41. It is important that both the gauge and the tank unit should earth properly. The cable from the tank unit may be earthing at some point if the gauge shows a full tank without cause. If it is the tank unit terminal which is earthed, the unit will require repairing.