Fuel Gauge Issues

System Basics

Knowing how to fix something is always easier if you know how it works. 

There are four main parts to the fuel gauge system:

1. Fuel Gauge - Takes the signal wire (variable ground) from the sending unit and displays fuel level based on ohms of resistance.
2. Constant Voltage Regulator (CVR) - Regulates 12v input into water temp/oil pressure gauge to a 5v power that is used for both fuel/ammeter and water temp/oil pressure.
3. Wiring Harness - The signal wire from the gauge into the sending unit and ground wires from the sending unit into the chassis.
4. Sending Unit - An in tank wiper style unit connected to a float. As the float moves up and down based on the fuel level the sending unit changes resistance to ground from 73 ohms when empty to 10 ohms when full. 

**ALERT!**

Full battery power goes through the fuel/ammeter gauge. It is wise to disconnect the battery when working with this gauge otherwise be very careful not to ground it out while performing tests. 

Trouble Shooting Steps

Order of operations depends on the exact symptoms your experiencing. Below is a general guideline for progressive trouble shooting. These are written for a Scout II but the same concepts apply to Scout 80/800. Remember, keep it simple. 

1. Check Symptoms - Do your other gauges work? If not, you may have different issues. 
2. Fuses - First rule, Keep it simple. Check all the fuses thoroughly to make sure you have the correct size, amps, and that they are all in good condition. Click here for correct fuses. 
3. Check Connections - Often the problem is a loose nut or loose circuit board peg on the back of the gauge. The fuel gauge gets power from the water temp gauge, so pull both the fuel and water temp gauges out far enough that you can check and tighten any loose nuts. Carefully pull the wiring plugs straight off the back of the gauge and check for any loose/wobbly pegs on the circuit board. Also, examine the circuit boards and wiring on the back for any burns or damage. Fix any issues found and retest gauges. 
4. Test the Constant Voltage Regulator (CVR) -  There are three connections on the CVR. They consist of voltage in, voltage out and ground. The CVR grounds through the gauge housing into the dash. Since you have pulled the gauge off the dash you will have to use an alligator clip to make a new ground connection for the tests to work. Now turn the key to the ON/ACC position, use an ohmmeter to test the input voltage to confirm it is 12v–13v. Next test the output voltage to confirm it is ~5v. This will be an oscillating voltage up and down but should average around 5v. If your using a test light this is show as a pulsing light. 
5. Test the Gauge -  With the key in the ON/ACC position and power going to the fuel gauge you can ground out the sending unit signal wire. This should send the gauge needle to "full" meaning the internals are functional. This can either be accessed behind the gauge (wire/plug #36), at the sending unit, or on the passenger side rear frame rail. 
6. Test the Signal Wire - Using an ohmmeter, test resistance from the signal wire (coming off of the center stud) at the sending unit to the gauge plug port #36 shown in the above diagram. It should measure between 0–1 ohms. Next, check to make sure the ground wire at the edge of the sending unit also has 0–1 ohms of resistance. 
7. Test the Sending Unit - The sending unit has two wires, a signal wire (brown) and ground wire (red). These go from the top of the sending unit and junction at the passenger side frame rail by the rear crossmember. Using an ohmmeter test resistance between the two wire connections. Without dropping the tank you will only be able to get a reading based on how much fuel is in the tank. Otherwise to get an accurate reading drop the tank and pull out the sending unit. Then with the float dropped down make sure it reads at least 73 ohms and with the float all the way up it should read around 10 ohms. If it does not, your sending unit is bad. You can click here for a new sending unit.
8. Go forth and fuel accurately. 

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