How to check a fuel gauge - currently not working
Discussion
OK chaps, not necessarily a TVR specific question but they are all pretty much the same, well the older ones are at least 
I have a 1950 car with a non functioning fuel gauge, so it could be a fault in the sender unit or a faulty gauge, I suppose it could be wiring but lets assume that's OK. If I use a multi-meter is it easy to check the sender unit but putting it across the two connections? Presumably if it's working it will move the gauge? Likewise if the fuel gauge is OK but not receiving a current from the sender then how do I test this? Can I put a 12v current through it and see if it moves? A bit of help would be appreciated before I start ripping the dashboard apart! 
Thanks Rich...
I have a 1950 car with a non functioning fuel gauge, so it could be a fault in the sender unit or a faulty gauge, I suppose it could be wiring but lets assume that's OK. If I use a multi-meter is it easy to check the sender unit but putting it across the two connections? Presumably if it's working it will move the gauge? Likewise if the fuel gauge is OK but not receiving a current from the sender then how do I test this? Can I put a 12v current through it and see if it moves? A bit of help would be appreciated before I start ripping the dashboard apart! Thanks Rich...In principle yes.
But
1, the sender unit on older cars is normally a wire wound resistor, the float altering the the position of the contact on the resistor track giving different values of resistance depending on the float position. The sender unit uses the metal of the tank to provide its earth path, corrosion and often rubber mounts stop this earth path. No earth path - no current can pass and so no reading! Therefore you should check the continuity of sender body to earth - simply use the resistance scale of your multimeter from the sender unit body to a good earth point - preferably back directly back to the battery, not just to the metal of the tank.
If that is ok you should then get a higher resistance reading when you test from the sender unit feed wire through the sender unit to earth- if you can move the float to vary the reading so much the better. Some sender units give no resistance when the tank is full and some give no resistance at empty, hopefully you will be somewhere in between if you can not move the float.
2, The gauge will be fed with current from a voltage stabiliser unit, usually mounted behind the dashboard. The current then passes through the gauge and down the wire to the sender unit and back along the earth path. The position of the gauge indicating the resistance from the sender unit being caused by the position of the float. You can test the gauge by passing 12 volts through it - it should go past full scale but please,please,please use a low amp inline fuse if you are testing it in situ in the car, as passing the starting current your battery is capable of producing through a short circuit in the dashboard wiring is going to result in a few melted wires at best or a fire.
But
1, the sender unit on older cars is normally a wire wound resistor, the float altering the the position of the contact on the resistor track giving different values of resistance depending on the float position. The sender unit uses the metal of the tank to provide its earth path, corrosion and often rubber mounts stop this earth path. No earth path - no current can pass and so no reading! Therefore you should check the continuity of sender body to earth - simply use the resistance scale of your multimeter from the sender unit body to a good earth point - preferably back directly back to the battery, not just to the metal of the tank.
If that is ok you should then get a higher resistance reading when you test from the sender unit feed wire through the sender unit to earth- if you can move the float to vary the reading so much the better. Some sender units give no resistance when the tank is full and some give no resistance at empty, hopefully you will be somewhere in between if you can not move the float.
2, The gauge will be fed with current from a voltage stabiliser unit, usually mounted behind the dashboard. The current then passes through the gauge and down the wire to the sender unit and back along the earth path. The position of the gauge indicating the resistance from the sender unit being caused by the position of the float. You can test the gauge by passing 12 volts through it - it should go past full scale but please,please,please use a low amp inline fuse if you are testing it in situ in the car, as passing the starting current your battery is capable of producing through a short circuit in the dashboard wiring is going to result in a few melted wires at best or a fire.
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