Austin Champ Fuel Gauge System

This page is the practical side of fixing a misbehaving fuel gauge, for an in-depth article by Andy Jackson, including fixing and history, Andy's article

The problems with the fuel gauge on the instrument panel can be:
1. it doesn't work at all.
2. it works part of the time, e.g. the Austin Champ is filled-up with petrol and the gauge works but when, say, the tank gets down to just below two thirds full the gauge goes to zero or it could be that when it gets down to one third full the gauge starts working again.

The fault of the gauge only working on any third, or two thirds, of the gauge is because the fuel gauge system is three separate systems in one.
The transmitter in the fuel tank has five wires: positive and negative and then three others. Each of the three others controls a separate circuit (the thirds of the fuel gauge) - if everything is working properly this fact is not noticed but if one circuit stops working then it will be.

(yes, I hear, 'Oh no!' but, if working on the tank sender unit be careful.)
1. If taking out the whole sender unit petrol will be exposed, and there will be fumes. To prevent static electricity sparks from yourself touch your bare hands on bare metal on the vehicle, also hold any metal tool by the metal part and touch bare metal with your hand.
2. Have fire extinguishers close by - I have both carbon dioxide and dry powder.
3. If working and testing electrics, and apart from when power is actually needed, it is better to isolate the batteries using your battery isolator switch (you have fitted one haven't you?).

The power starts from the Distribution Box (bottom right), through the INS (Instruments) wire to the Switchboard (top right), note INS LINK. From the Switchboard to the Instrument Panel (IP). From the IP as the + FUEL wire to the fuel gauge sender in the petrol tank - on the diagram it has FUEL GAUGE, whereas it should be, Tank Sender Unit. Note: on the wiring diag. the sender unit terminals are; + (plus) and - (minus), - but on the actual sender unit they are; +6 (plus) and -7 (minus).
Austin Champ Fuel Pump Wiring Diagram

(below) Looking into the rear of the Champ with the winchwell cover removed. The fuel sender unit wires go into the metal tube and up to the back rear of the Champ.
Austin Champ Fuel Sender Unit

(below) The electrical unit. The terminals from the top are, 3, 2, +6, -7, 1.
Austin Champ Fuel Sender Unit

(below) The underside of the electrical unit: the blue arrows show the two contacts that connect to the Torroidial Ring, these are often called, 'wings'; the red arrow shows the Bakelite tab that the pin on the tank unit goes into; the green arrow shows the lug that positions the electrical unit on the tank unit.
Austin Champ Fuel Sender Unit

(below) Contrast the New Old Stock unit above with the one below. Look at the tips of the wings and, below, they are worn through.
Austin Champ Fuel Worn-out Sender Unit

(below) The top of the tank unit that the electrical unit fits into. Note the pin that has to mate with the arm on the electrical unit.
Austin Champ Fuel Worn-out Sender Unit

(below) The tank sender unit. The dimension between the top and bottom casting is 13mm, on other military vehicles the dim. is different.
Austin Champ Fuel Worn-out Sender Unit

The fuel gauge does not work at all. The instrument panel is, or should be, sealed so it is less likely that the gauge itself is faulty. But, if looking at the instrument panel you can see rust and deterioration on the fuel gauge and speedo etc., then corrosion in the fuel gauge could be the problem.
The easiest place to start is at the back of the driver's side corner of the Champ where the rear wiring is. Find the + (positive) wire and pull out the connector (a Lucon type) - these connectors do corrode and can be the problem.
Switch the ignition on and use a voltmeter and check for 24v (24v is the nominal voltage but the reading will be the battery voltage, which should be just over 26v), if there is no voltage the problem is toward the front of the vehicle. If there is voltage then the problem is from the connector to the fuel sender unit.

No voltage at the rear corner:
The power to the rear (+) comes from the instrument panel (IP) and the IP is fed from the ignition switch.
Remove the instrument panel - I find it easier to remove the four bolts that go through the rubber mountings each side. Pull the IP gently forward, the speedo and\or the tacho drive cable may need to be undone.
Check the voltage on the 'out' wire, which is + FUEL, if no voltage check on the 'in' wire, INS (for instruments) from the ignition panel. If there is power coming into the IP but not coming out then the fault is in the IP.
If no voltage on the INS wire then remove the Switchboard and check the INS wire coming out, if nothing then the fault is in the Switchboard.

If there is voltage at the rear corner:
Remove: the winch well cover, the metal cover plate over the sender unit, and the black Bakelite cover - some penetrant like Plus Gas might be needed on the screws. The five wires in the sender unit can be seen. If there are poor\loose\corroded wires then rectify and try the fuel gauge again.

Put the voltmeter red wire on the +6 terminal and the black on the -7 terminal, there should be 24v. If not, try the voltmeter between the +6 terminal and a bare part of the Champ body. If there isn't power then it is a wiring fault between the back corner of the Champ and sender unit.

If the voltmeter shows power between +6 (battery main) and -7 (negative) try the mulitmeter on each of terminals 1, 2, & 3 and one of them should give a voltage reading depending how much fuel is in the tank. If any of one of the three terminals does give a reading, but the fuel gauge is still not reading, then that terminal wire is faulty from the sender unit to the instrument panel, check all connections with the voltmeter starting at the back right corner.

To fully test all three segments the electrical unit needs to be removed from the tank by removing the six screws, which could be seized. The electrical unit itself can only be fitted back in one position because of a lug on the unit but, try not to move the wiper arms as it comes out because its best to mark the position of the brown Bakelite slotted tab that locates on the pin in the actual tank unit so it can be put back in the right place.
Inspect the electrical sender unit underneath for corrosion\damage and especially the tips of the two wings, which can be worn through, and the integrity of the wired ring.
Reconnect the five wires, and move the wings around and watch the fuel gauge.

If the fuel gauge works OK then the problem is in the tank unit. Before removing the tank unit (sometimes called the, 'float unit', don't forget, earthing yourself and each tool as you are about to use it and the fire extinguishers, its preferable do this job outside - another alternative is to drain the petrol tank but there will still be fumes in it.
The tank\float unit can now be inspected to see if the cork is good, whether the mechanism is jammed, etc.

Spares notes:
The Austin Champ fuel sender came complete, with the float unit that goes in the tank and electrical unit. On a Champ the electrical unit, (Potentimeter) does not seem to have a part no. stamped on it - it is in the parts book, LV6/MT1/SM/FG2440 - others units have. One of the differences between a Champ one and one for another military vehicle is the length of the stem that holds the float arm but the electrical unit may well be the same so the whole unit is worth buying if the price is right.