Fuel sending units

Fuel sending units

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The sender is grounded to the tank by the mounting screws but the tank needs to be grounded to the fuselage by a wire that goes from a tab on the tank to a screw on the fuselage root rib. Check for this ground between the tank and any good ground on the fuselage with an Ohm-meter. If the sensor is bad (eg. 2-4 ohms empty, 24-30 ohms full) you will need to drain the tank. It often takes a bit of worrying to get the sender, gasket, and float in place through a misplaced inspection hole. Been there, done that!

The Stinson fuel gauge is fairly easy to test: an open circuit to the sender shows full ++, a short to ground shows empty, a 15 ohm resistance to ground is 1/2 full.

The fuel level measurement system is basically identical to what General Motors used in automobiles from the ’30s until digital and computerized autos came along.  It consists of a gage which has a bimetal strip with some wires wrapped around it connected through a direct current source to a variable resistor that is in the fuel tank with an arm and a cork float to change resistance in proportion to the amount of fuel in the tank and the change in resistance results in a change in current through the circuit and through the wires wrapped around the bi-metal strip and the bi-metal strip is heated in proportion to the current through the wires and the bimetal strip which bends as a function of temperature and is attached to a lever or gear system that moves the visible pointer in proportion to the amount of current through the gage which is in proportion to the resistance of the variable resistor in the tank which changes according to the level of the float.
Unless someone has connected the two terminals on the gage directly across a battery and melted the wires inside, the gage itself rarely fails.  Like someone else mentioned the wiring from the gage to the resistor in the tank goes through a connector at the wing root and then is under a terminal on the sending unit in the bottom of the fuel tank.  The circuit is completed through grounding of the wing to the fuselage.  There was originally a grounding strap in the upper wing root accessible under the metal strip between the top of the fuselage and the wing.  
But, the most common failure is the sending unit itself.  Stinson uses a single gage switched by a switch above the fuel selector valve handle to determing which sending unit in which wing it is connected to.  Page 107 of the General Service Manual shows the wiring diagram for the fuel measurement system.
It is all pretty simple and a cheap volt-ohmmeter from Harbor freight is all that is needed for trouble shooting.   
Larry Wheelock  `


I am not Larry, but I am sitting here with an original fuel gauge in my hand. It is so original that it still has a small cardboard note on the left terminal (as viewed from the back of the gauge). The note reads: “Connect sender wire to this terminal. Warning- If hot wire touches, sender will be damaged.” The right hand terminal has a 1/2inch diameter device connected to it with a second connection at the end of the device. I believe this device is a resistor to allow a 6 volt gauge to be used with a 12 volt system. The 12 volt supply connection would be made to the end of this pellet.