Fuel Gauge Problems



A fuel gauge which tells lies is not an uncommon problem. In the worst case it reads full or empty all the time and it is usually quite easy to find the reason for this. More difficult is when it under or over reads and sometimes the easiest course of action is to remember by how much it over or under reads and make allowance for this when reading the gauge.





Getting a true reading is just about impossible even if the gauge is in perfect condition due to the shape of the tank. The sender unit is on the right hand side of the tank and measures the depth of the petrol at this point. The tank is quite shallow and if the car is on a camber or going round a bend the depth of petrol on the left hand side may be very different hence a misleading reading will be obtained.















The gauge has two components, the gauge head on the dash and the sender unit in the tank. The circuit is as shown. There are two terminals on the back of the gauge head which are marked B and T. The B terminal connects to the battery through the ignition switch and the T terminal connects to the sender unit in the tank. The gauge head body must be connected to a good earth point. There is a single terminal on the sender unit which is connected to the T terminal on the gauge head. The body of the sender must also be connected to a good earth point.














The sender unit is a variable resistor (rheostat). The float in the tank moves up and down with the petrol level and moves the wiper arm across the resistor changing the resistance of the sender as it does so. When the tank is full the resistance is at its maximum and when the tank is empty the resistance is at its minimum.










The gauge head measures the resistance of the sender showing this as the gauge reading. The gauge head contains two coils both of which connect to the T terminal. One coil connects to the B terminal and the other connects to earth. There are two nuts with sliders on the back of the gauge head which are used in manufacture. They should never be touched.

Before doing any checking make sure that there is a good earth connection to the gauge head body. This is not normally a problem area.

If the gauge reads permanently full, clip a jumper wire to the T terminal on the gauge head and connect the other end of the jumper wire to a good earth point. If the gauge now reads empty the gauge head is working and there is a fault in the sender unit or the wire between the gauge head and the sender unit or a bad connection between the sender unit and earth. If the gauge head is working, remove the jumper wire and connect the jumper wire between the terminal on the sender unit and a good earth point. If the gauge now reads empty the wire between the gauge head and the sender unit is in good condition. Remove the jumper wire from the sender unit terminal and connect it to the body of the sender unit and leave the other end connected to earth. You may need to scrape the sender unit to get a good connection to it. If the gauge still reads full there is a problem within the sender unit and it will have to be replaced. If the gauge now reads empty the problem is a poor earth connection to the sender unit. The sender unit body is earthed through the petrol tank but since the tank is mounted on rubber bushes it does not always earth well.

If the gauge reads empty all the time check that the B terminal on the gauge head is live when the ignition is on. If the terminal is live and the gauge still reads empty then the gauge head has a fault, the wire between the gauge head and the sender is shorted to earth, the sending unit is shorted to earth internally, the float has a hole and has sunk to the bottom of the tank or there is no petrol in the tank.

To find where the fault lies, remove the wire from the terminal on the sender. If the gauge now reads full, the sender is faulty. If it still reads empty, disconnect the wire from the T terminal at the gauge head. If the gauge now reads full, the wire between the sender and gauge is shorted to earth. If it still reads empty there is a fault in the gauge head or the electrical supply to it.

A gauge which works but reads either high or low is probably caused by a wrong sender unit having been fitted. The float arm may be too long or too short or may be bent. Removing the sender is often difficult as the screws which hold it in place have threads which protrude into the tank and may be rusted in and very difficult to remove. If the error in reading is not too great it may be better to leave it and just make a mental adjustment when reading the gauge.

Important Note To gain access to the sender unit it is necessary to remove the right side rear wheel (off side in UK). Before removing the wheel, jack the car up and place an axle stand under the axle. Do not work on the car with the wheel removed and the car supported only on the jack supplied with the car. This jack is unstable and can easily slip or fold under.



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