10-03-2018, 08:39 PM
As others have said, the sender is a simple variable resistor. A sliding contact moves in an arc over a thin rectangular insulating card wound with resistance wire. For the early sender type used in 6 volt Sevens (Smiths G33 system), the winding resistance is about 25 to 30 ohms, rather lower than many later types and modern replacements. Careful dismantling and inspection should reveal any electrical problems, and a cheap multimeter will come in useful. The mechanical parts should be checked for a pinholed float etc.
It's not immediately obvious that the resistance wire is earthed to the casing of the sender by trapping it between the casting and an insulating bush which carryies the "hot" connection through its centre. This earth connection can corrode over time, giving a poor or nil connection.
It's also not immediately obvious that the tank is (or was originally) insulated from the car by rubber dogbone washers, and the earth return from sender to battery is via the copper petrol pipe to the pump and thence to the engine block. Clearly, any plastic or rubber pipe sections in place of copper will break this connection.
Let us know how you get on - more advice available on request !
It's not immediately obvious that the resistance wire is earthed to the casing of the sender by trapping it between the casting and an insulating bush which carryies the "hot" connection through its centre. This earth connection can corrode over time, giving a poor or nil connection.
It's also not immediately obvious that the tank is (or was originally) insulated from the car by rubber dogbone washers, and the earth return from sender to battery is via the copper petrol pipe to the pump and thence to the engine block. Clearly, any plastic or rubber pipe sections in place of copper will break this connection.
Let us know how you get on - more advice available on request !