fuel gauge or?

how can i tell if my gauge or fuel sending unit is bad on a 265 diesel? i changed the float and still doesnt work

Hi '48Jr,

As a qualifier, I have not worked on a 265 Massey, yet. However, I have a 65 and have some electrical background.

Here is the theory: the sending unit is a variable resistor from "ground" (the tractor metal/Neg side of the battery) to the gauge. On the 65 diesel, the positive side of the gauge comes from the battery, through a fuse for all the 12v things like lights, then through a pressure switch mounted behind the oil pressure gauge. When the oil pressure comes up, the fuel gauge gets the positive applied, and reads the resistance of the sender to give an indication of fuel level - the lower the resistance, the higher the fuel level. Your 265 may or may not have the pressure switch, and instead come from a terminal on the key switch (if it has 3 positions: 'off' 'on' and 'Crank', or 4: 'preheat', 'off', 'on' and 'Crank')

To test: if you have a volt meter, you can use that, or if not, a 12v bulb (like an 1156, 1157 or 194) in a salvaged lamp socket or with wires soldered on and taped over will work as a test light. If the lights work, the fuse is ok. Next, remove the wire from the tank sending unit. With the key switch 'on', touch the wire to a solid ground - if the gauge moves, the sender isn't working properly, perhaps as a result of not having a good ground connection to the tank. That can be fixed with a jumper wire from a casting point to a tank mounting bolt. If the gauge doesn't move, look for a pressure switch beneath the oil pressure gauge. Start the tractor, check for the oil pressure gauge reading ok and see if you have +12v on both sides of the switch, then try touching the sender wire to a solid ground. If the gauge doesn't move, the gauge is bad. If you have no +12V on the PS, there is a problem between the battery and the pressure switch (perhaps a broken wire, or a bad key switch). If you have +12 on only one side of the pressure switch with at least 15PSI oil pressure, the pressure switch is bad. If you don't have oil pressure, you have bigger problems than just a non-functioning fuel gauge.

Hope that helps,
Well Worn