Fuel gauge/sending unit 801

Can anyone tell me if/how you can check a fuel gauge. have two different ones, both have 3 terminals. one has S on one terminal (guessing s is connected to sending unit) another terminal has I or 1 (thinking I is for ignition) and a gnd. terminal. The other has a B terminal guessing Battery and T terminal guessing tank and gnd. terminal. can the gage be checked without the sending either with voltage or continuity and are my assumptions on the markings correct. Next what kind of continuity readings should you get on the sending unit. My reading are all over the place and intermittent on the sending unit. I have these parts out on the bench for testing. Thank in advance

The numbers that ring a bell with me are 35 and 235 ohms...min resistance of a sending unit and max, forget which is at which end of the scale but you could find out easy enough as you are there and can move the float.

Some folks short the "load" terminal of the meter to gnd to test a meter in troubleshooting, but what I have read about meters is that they are not designed for the full battery voltage across them...they will burn out an internal connection....that's why the minimum resistance of the sending unit is 35 ohms and not zero.

I don't know of all the connections of which you speak. On the float sending unit you have a wire wound variable resistor, usually a "Rheostat" which uses only 2 terminals and what would be the 3rd terminal in a variable resistor is just left dangling. The rheostat is a current sensing device and the variable resistor a voltage sensing device. Your tractor's electrical system may be setup as a variable resistor.

If that's the case you need 3 wires" (not necessarily in the following order) 1. Battery voltage. 2. Battery return. 3. Wiper connection to meter.

On the two terminal rheostat, the battery voltage would be on one terminal of the meter and the return for the battery would be on one terminal of the sending unit...usually the mounting plate. The isolated terminal would connect to the other terminal on the meter. As the fuel level changes, the wiper on the sending unit changes it's resistance which changes the current in the wire going to the meter which changes the deflection of the needle.

In a 3 terminal system it seems to me that the sender would power the meter. The wiper on the sender would be obtaining power from the resistor in the tank which is tied across the battery and sending that power to the meter terminal. The meter second terminal would be at ground...batt return.

Somewhere in all that the battery power would have to be switched on and off to prevent the system from draining the battery when not in use.

HTH for what I know about it.