How do i test my Fuel sending unit on my 1 ton dump has two tanks one is bad?

need to check and test the wiring and grounds --but possible the sending unit has failed

Bad sending unit, bad ground, bad gauge, I have seen sending unit go bad they will work part way.

The gauge is faulty or the sending unit in the fuel tank is bad ! There is away to test it, find a wiring diagram an locate the two wires at the fuel pump sending unit connector . Go to radio shack or some other electronics store an buy some resistors of differing resistance , I'm not sure of resistor sizes , I will look it up on my ford service info system. If you jump the right resisters into the fuel sending unit circuit you can make the gauge show 1/4 - 1/2 - 3/4 an full. If it move to those positions with the right size resistors it's not the gauge but the sending unit in the gas tank ! Diagnosis of Stuck Fuel Gauge Is it Gauge or Sending Unit

Sending unit wire is grounded or sending unit in tank is shot.

Most of the time its is the sending unit in the fuel tank. The test for it is to use a variable resistor that is hooked up to the wiring at the fuel tank and watch the fuel gauge if the gauge works ok thats makes 100% sure its the sending unit. At 245 ohms the gauge should read full, 180 ohms for 3/4 full, 160 ohms for 1/2 tank, 100 ohms for 1/4 tank and 42 ohms for empty. The color of wires at the fuel tank is purple wire and black wire. You can also use fixed value resisters. If the fuel gauge the wires from the sending unit to computer then to gauge for bad connections, shorts, broken wire and bad grounds. If the wires test ok that only leaves insterrment cluster or computer which even most dealers can't test eather one but the insterment cluster would be a more common problem then the computer. But about 95% of the time it will be the fuel level sensor in the gas tank that causes the problem. Good luck.

When you changed the sending unit, was this in the tank? or does it have a manual that bolts to the block. If it is on the block, drop the tank (or take the bed off) and replace the float.

carsandcomps,

Guru, Premium Expert @Fixya.com

Test the gauge by using a test light ,and grounding the test light clip to a good known ground,then touch the pink wire on the connector harness going to the tank,with the connector unplugged,and key on,the gauge will move slowly to the empty mark,and lower ,then remove the test light and it will go past full,if it does this,it is a good gauge,and the sending unit,was put in wrong,or the part was bad.

Yes the tank must be removed

Fuel Pump: Service and Repair
WARNING: The fuel system is under constant pressure, even when the engine is OFF. Always release fuel pressure before servicing any fuel
system component.
FUEL PUMP REMOVAL
1. Release fuel system pressure.
2. Fuel tank must be removed prior to fuel pump service.
3. Using a hammer and a brass drift, carefully tap the fuel pump lock-ring counter-clockwise to release the fuel pump.
4. Remove the fuel pump and its O-ring from the fuel tank, and discard the old O-ring.
5. Cover tank openings to prevent dirt from entering system.
FUEL GAUGE SENDING UNIT REMOVAL
1. Gently bend the sending unit locking tabs away from the sending unit.
2. Remove the electrical connector cover (red). Remove the sending unit wires from the electrical connector.
3. Use one screwdriver to push on the fuel pump body, and use another to release the sending unit locking tab from the sending unit.
4. Slide the sending unit assembly off of the fuel pump reservoir housing.
5. Pull the wires through the reservoir body and fuel pump cover plate.
FUEL GAUGE SENDING UNIT INSTALLATION
1. Bend the sending unit wires and lay them into the slot on the back of the sending unit.
2. Slide the sending unit into the slots on the fuel pump reservoir body. Slide the wires through the triangle shaped hole in the fuel pump reservoir
body. Check that the tab in the fuel sending unit locks into the detent in the fuel pump reservoir body.
3. Slip the wires onto the tab of the fuel pump reservoir body.
4. Slide the sending unit wire terminals into the electrical connector. Install the terminal locking cover.
5. Slide the fuel sending unit electrical connector into the bottom of the fuel pump electrical connector. Check that the connectors are securely
attached and fully pushed into each other.
FUEL PUMP INSTALLATION
1. Wipe seal area of tank clean, and install a new O-ring seal on pump.
2. Position fuel pump module in tank and secure with the lock-ring.
3. Using a hammer and a brass drift, carefully tap the fuel pump lock-ring clockwise to lock the module in place.
NOTE: Do not overtighten the lock-ring, because the fuel pump seal could split and leak.
4. Install fuel tank assembly.
NOTE: ASD and fuel pump relays remain energized for 7 minutes during the following test, unless the test is stopped or the ignition switch is
turned to the OFF position.
5. Using scan tool, place ignition switch in "RUN" position and use Actuate Output Test "Auto Shutdown (ASD) Relay", to activate fuel pump and
pressurize system.
6. Check for leaks.

disconnect the battery,drop the gas tank,disconnect the hoses and wiring harness,get tank out in front of you,take out sending unit take off pump,replace is opposite of removal

What was the original problem? Most fuel gauge problems are either a bad fuel tank sending unit or a bad ground wire going between the sending unit and the frame of the car. The gauge and wiring can be easily tested using only a 12 volt test lamp, once you have access to the wiring at the fuel sending unit. This often requires draining the fuel tank and lowering it from the vehicle.
To test the gauge in the dash and it's related wiring, connect the 12 volt test light between the wire between the gauge and the sending unit (the one with power in it with the ignition on). Make the connection near the fuel tank sending unit. Connect the alligator clip on the test lamp to a known GOOD ground. The bulb in the lamp should glow dimly. The dash gauge should read somewhere between 1/4 to 3/4 of a tank of gas. (Varies with make and model) This will verify that the gauge is either good or bad.