Austin Champ Fuel Gauge System, Andy Jackson


Andy Jackson,
A bit of background and theory.
The instrumentation system adopted for use in standardised British post-war vehicles and used on almost everything from Champs to Conquerors, was designed by Smiths Instruments of Wembley, Middlesex; a company with roots in the aircraft industry (in 1944 it was known as Smiths Aircraft Industries Ltd), and for the fuel gauge they adapted a system patented by them and originally designed for the position indication of aircraft mechanical components. This is known as the 'Desynn System' and consists essentially of a position transmitter unit electrically connected to a remote indicator.
The salient features of the system are the ability to indicate angular movements of greater than 360 (useful for applications like weather vanes but not really fuel gauges), its tolerance of very wide supply voltage variations, and the fact that the transmitter and indicator parts can be almost any distance apart.
The basic circuit is shown(below) Austin Champ Fuel Gauge, Fig. 1
The transmitter unit consists of a toroidal (circular) resistance tapped at three equidistant points and connected to the indicator unit by three wires designated Fuel 1, Fuel 2 and Fuel 3. Power to operate the system is applied at two wipers 180 apart that run on the toroidal resistor and are driven round the resistor by the position of the float in the fuel tank. These two connections are marked Fuel Pos (ignition-switched battery) and Earth. The wipers move over an arc of approximately 300 degrees between empty and full positions of the tank float.
The three wires, Fuel 1, 2 & 3 are connected to the indicator unit in the main instrument panel through the rear wiring loom. The indicator (below)
Austin Champ Fuel Gauge unit
is effectively a magnetic compass needle surrounded by three interconnected coils with the magnetic needle providing a "home" position for the indicator at 6 o'clock on the dial when in the idle state. With power applied, the currents flowing in the three coils cause the needle to exactly mirror the position of the wipers on the resistor in the transmitter unit. Unfortunately, while extremely accurate, there is no damping in the system which is why the fuel gauge swings wildly as fuel splashes about.
The components of the system are shown (below)
Austin Champ Fuel Gauge, components
What goes wrong?
With Champs now at or approaching 50 years old, the component parts may have suffered wear, corrosion, ingress of water, crossed wires or just plain damage. The fault finding suggestions that follow are designed to localise the problem before resorting to dismantling of the transmitter unit or the instrument panel.
A prime candidate for trouble is bad connections in the Lucon connectors in the Fuel 1, 2 and 3 lines. These carry very small currents and any corrosion or loose fitting connections will cause trouble. These are located in the connector blocks at the rear right corner of the body and at the rear of the instrument panel. Scrape the plug parts clean and scour the inside of the connector. After cleaning, check that the ID (Identification) sleeves on both wires match and a smear of silicone grease on the mating rubber parts will help maintain waterproofing.
After removing the tank unit cover plate, the connector block of the tank unit is easily accessible by removing the top cover. Check the terminal screws for tightness and neat termination of the wires.
Getting access to the indicator unit involves a lot of stripping down and is not really recommended unless you are familiar with electrical instrumentation. If you do, note that the Fuel 1, 2 and 3 lines become A, B & C on the indicator module itself. As the instrument panel is a sealed unit, the indicator unit is fairly reliable.
The other major source of trouble is the tank unit. Problems can be mechanical such as stiff float arm gearing, or electrical. If this is the cause of problems then there is little alternative but to remove it from the tank. This isn't too difficult but will probably destroy the cork gasket between the tank and unit. The flange of the tank unit has three 2BA tapped holes which can be used for jacking screws to assist removal. The correct position of the unit on the tank is indicated by a single screw larger than the others.
The resistance section can be removed leaving the tank unit in situ but it's awkward to get at and replacing it in the main housing and correctly engaging the wiper assembly with the driving pin is a fiddly job at the best of times and it is easy to cause irreparable damage. Take great care here!
The three resistance tappings are brought out to terminals 1, 2 and 3, and the two connections to the resistance wipers are terminals marked +6 (Fuel Pos) and -7 (Earth).
Fault finding Equipment required:
1. Multimeter capable of indicating 24 volts and measuring resistance reasonably accurately. Either a digital or analog meter is OK for this.
2. A 24v, 21 Watt vehicle lamp with wires attached.
3. 9 Volt PP3 battery.
Initial checks:
- With the ignition off, check that the gauge points to the white spot at 6 o'clock. Turn the ignition on and check that the needle moves to some other position (the needle will normally take the shortest route to the indicating position). Turn off the ignition and check that the gauge returns smartly to the home position. This checks that the indicator part is basically functional.
- Remove the three screws securing the resistance unit cover, remove, and connect the test lamp between terminals +6 and -7. Turn on the ignition and check that the lamp lights at full brilliance. This checks the battery and earth supplies to the tank unit and the current drawn by the lamp checks for high resistance faults in the connectors that a simple voltage test sometimes won't reveal.
If the lamp only glows faintly, look for dodgy connectors in the Fuel Pos and Earth lines back to the instrument panel block. Connect the test lamp between the +6 connection and a good chassis earth and If the lamp glows brightly, the Fuel Pos line is OK. Check the earth line back to the instrument panel. Note that the Earth wire connects to a four-way Lucon connector marked R1 NEG here. The Fuel Pos line is fed from the 10 Amp circuit breaker via the ignition switch and INS LINK connector. Check the tank unit
Label each of the five wires so you know where they go back and remove them from the tank unit. Temporarily insulate the Fuel Pos wire to prevent accidental short circuits.
With the multimeter, check between chassis and all five tank unit terminals for infinity resistance. Any readings less than infinity indicate insulation breakdown within the resistance unit. Water penetration is a common cause of this failure.
Check the resistance between terminals 1 & 2, 1 & 3 and 2 & 3. ALL readings should be between 400 450 Ohms. Any significant variation between readings indicates a problem in the resistance unit. Check between terminals +6 and -7 for a resistance of approximately 500 Ohms. This value is independent of the amount of fuel in the tank but a very high resistance or no continuity indicates a problem in the resistance unit. Corrosion on the wipers or the resistance track, excessive wear or slack tension of the wiper arms on the resistance are the most likely causes. Resistance readings between terminals +6 or -7 and any of terminals 1, 2 or 3 will depend on how much fuel is in the tank etc. and so are no use for fault finding.
Check the gauge and Fuel 1, 2 and 3 lines.
With the five wires to the tank unit still disconnected, identify the Fuel 1, 2 and 3 lines. With the multimeter set to measure resistance, check for infinity resistance between each line and chassis earth. Any reading indicates an earth contact fault on the wire with the lowest resistance reading. Follow the faulty wire forward to the instrument panel to localise the fault. Connect the Fuel 3 line to the negative terminal of the 9 Volt battery and touch the Fuel 1 line on the positive terminal. The gauge needle should move to around the 10 o'clock position. Now touch the Fuel 2 line to the battery positive terminal; the needle should move to the 2 o'clock position. If either of these tests fails, repeat the tests directly on the wires coming from the instrument panel. If the tests are successful here, look for a disconnected or crossed wire in the loom between the front and the back of the vehicle.
Dismantling the tank unit If the testing points to a problem in the tank unit, there is little choice but to remove it to gain access. At this point the army would just bin the whole unit and replace it with a new one; however we don't usually have that luxury, so an attempt at fixing it is the only option. Once removed from the tank, check that the unit operates without binding and that the float arm will fall freely under its own weight. To remove the resistance module, pull the float arm up to the fullest extent and tie the float arm to the main body to prevent movement; remove the six screws and gently pry the unit upwards and remove without twisting. There should be a thin rubber or composition gasket under the rim. Note the locating pip cast in the Bakelite body of the resistance unit. To help reassembly, mark the position of the slider unit as removed before moving it and note how the driving pin engages with the resistance wipers (See Fig. 3). (left)
Check that the wipers and the top of the resistance winding are bright and clean and that the wipers exert a pressure onto the winding wire. If the wiper is so worn that it has a hole in it, it is sometimes possible to twist the arm so that an unworn part runs on the resistance track.
Look for corrosion on the assembly and clean carefully with nothing more abrasive than a clean cloth or the very finest grade emery paper. Do not lubricate the resistor track or wiper assembly. Contrary to popular belief, WD40 is sticky death to delicate electrical equipment!
If the resistance winding itself is broken, repair is difficult if not impossible; however the resistance module is common to all the Smiths type installations so one from a Ferret, Saladin, Pig or Scammell etc. tank unit can be cannibalised.
Mechanical stiffness or binding is straightforward to fix but before undoing anything, carefully mark the relative positions of all the components and the position of the driving pin relative to the casing with the float in the fully up position, otherwise the gauge will not indicate accurately when reassembled.
When reassembling the unit, be careful to accurately locate the driving pin into the slot on the slider assembly. This is made of thin paxolin and is very easily broken. Once you have the two parts mated together, operate the float arm and check that you can hear the wipers moving on the resistance before proceeding further.

Spare parts identification
LV6/MT1/SM/1658FG Tank unit, fuel gauge FV15745 Complete tank unit (Champ only)
LV6/MT1/SM/FG1992/M Cover, terminal board 6680-99-801-9235 Resistance unit cover
LV6/MT1/SM/FG1993 Washer, sealing 6680-99-801-9236 Cable entry gland & cover seal
LV6/MT1/SM/FG2440 Potentiometer 6680-99-800-0311 Resistance unit
LV6/MT1/SM/FG642 Washer, rubber 5330-99-433-2947 Resistance unit to tank unit
LV6/MT1/SM/FG558 Gasket, fuel gauge transmitter 6680-99-801-9162 Cork gasket to tank

FAULT FINDING CHART Austin Champ Fuel Gauge, fault finding