How do I test the fuel sending unit on a Ford 4500?

Armymutt

Member
I'm ordering up some new parts. I don't know if the fuel sending unit is bad, but the gauge doesn't move. I don't really want to pay more in shipping than I have to, so I'm looking for a way of testing the sending unit on the bench.
 
hook an ohm meter to the sender, them operate the arm. You'll know quickly if it's not working
 
Okay, the guys who really know this stuff will correct me if I'm wrong, but here's what I think is the case.

One side of meter will have voltage on it when the ignition switch is turned on. I assume it's full battery voltage, but it could be moderated to a lower voltage. The other side of the guage goes to the sending unit.

The sending unit is just a resistor that changes value as the float rises and falls with the level of the gas in the tank. One side of the sending unit is connected to the guage via the wire from the guage to the sending unit. The other side of the sending unit (not actually a wire, just the sending unit touching the fuel tank, is at ground level.

As the float goes up and down, the amount if resistance changes, hence more or less current flows through the guage and the pointer goes from empty to full.

So, having said that, I believe that you can attach the plus lead of your voltmeter to the wire that goes from the guage to the sending unit. If the volt meter reads zero volts, the coil inside the guage is probably bad. If there is some voltage there (the same as the voltage on the "hot" side of the guage), the sending unit is probably bad.

Then as someone else mentioned, with the sending unit out of fuel tank, you can measure the resistance of the sliding resistor by moving the float up and down. If the resistance changes, the sending unit is good.

At that point, if you have conflicting results, you just have to look closer - examine the wire, etc.

Good luck,

Tom in TN
 
Tom, you basically have it correct in how it works, but there is something called a voltage stabilizer on the back of the instrument cluster that provides the power to the two gauges (temp and fuel). The stabilizer receives 12 volts when the key is in the run position, and it basically switches on and of repeatedly when it has power so that the output is switching ion and off rapidly, and the average output over time is somewhere between 5 and 6 volts. The gauges ae magnetically dampened and as a result are slow reacting, and they operate off of the average voltage output from the stabilizer, so measuring the voltage at the gauge with any kind of meter, digital or analog, will result in bouncing reading, as the meters react too quickly to the changing voltage and will try to give you the instantaneous voltage instead of the average.

Best way to test everything is using an ohmmeter. While everything is still hooked up on the tractor, remove the positive cable from the battery and measure the following resistances:

1. Stabilizer to gauge should read zero ohms.

2. Other side of gauge to wire terminal on tank sender should also read zero ohms.

3. Tank sender mounting ring to negative battery cable should also read zero ohms (this tests ground from sender all the way back to the battery).

Then remove sender from tank and measure from the mounting ring to the wire terminal as you move the float arm and the resistance should vary as the arm moves. I'm not sure of the exact range of resistance in ohms that it should be, but it should change smoothly and gradually across the full range of motion of the arm.

To test the gauge itself, you c can just disconnect the wire from the sender and ground it with the key in the run position and the gauge should slowly move to "full".
 

To test the gauge itself, you can just disconnect the wire from the sender and ground it with the key in the run position and the gauge should slowly move to "full"."""""""

Bingo!!

Some sending units are low resistance.. meaning that they will read between zero ohms to about 230 ohms.. newer ones are higher resistance,,, meaning that they read between 30 ohms and 2400 ohms give or take a bit.

the wiper arm that rubs against the windings will usually corrode and also cut through the windings eventually.
 
Sorry to bring up an older post, but I installed all new gauges and a sending unit (purchased the kit for those era Fords) on my Ford 641. It has all new wiring and new everything. It was installed a couple years ago and just finished the tractor this week. Put a couple gallons of fuel in it and the gauge moved up a little as expected. Well since I finished re-doing the tractor, I figured I'd dump 5 gallons in it to make sure it had plenty of fuel and to check the operation of the gauge. For some reason the gauge only read about 1/4 of a tank....actually a shade less. I also noticed that the gauge doesn't sit still. Sometime it will "jump". Have I don't something wrong or should I remove the sending unit to check?

(quoted from post at 17:32:45 08/12/15) Tom, you basically have it correct in how it works, but there is something called a voltage stabilizer on the back of the instrument cluster that provides the power to the two gauges (temp and fuel). The stabilizer receives 12 volts when the key is in the run position, and it basically switches on and of repeatedly when it has power so that the output is switching ion and off rapidly, and the average output over time is somewhere between 5 and 6 volts. The gauges ae magnetically dampened and as a result are slow reacting, and they operate off of the average voltage output from the stabilizer, so measuring the voltage at the gauge with any kind of meter, digital or analog, will result in bouncing reading, as the meters react too quickly to the changing voltage and will try to give you the instantaneous voltage instead of the average.

Best way to test everything is using an ohmmeter. While everything is still hooked up on the tractor, remove the positive cable from the battery and measure the following resistances:

1. Stabilizer to gauge should read zero ohms.

2. Other side of gauge to wire terminal on tank sender should also read zero ohms.

3. Tank sender mounting ring to negative battery cable should also read zero ohms (this tests ground from sender all the way back to the battery).

Then remove sender from tank and measure from the mounting ring to the wire terminal as you move the float arm and the resistance should vary as the arm moves. I'm not sure of the exact range of resistance in ohms that it should be, but it should change smoothly and gradually across the full range of motion of the arm.

To test the gauge itself, you c can just disconnect the wire from the sender and ground it with the key in the run position and the gauge should slowly move to "full".
 
Here is a picture of my 641 just for fun. I'm waiting on sunny weather to take some better pictures.
28551.jpg
 

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