Sending Unit and Fuel Gauge Issues

Two of the most important parts on your classic Jeep, truck, and sports utility vehicle is the sending unit and fuel gauge. When there are problems with the sending unit or fuel gauge, this can result in inaccurate fuel readings, causing you to think there is more gas in your tank then there really is. Don’t get stuck out in the middle of nowhere with no gas. If you suspect that you’re not getting accurate readings from your fuel tank, there are several possible causes that need to be identified. Here are some ways to diagnose faulty fuel readings.

Sending Unit Regulator Voltage Output

The first thing to check is whether or not your temperature and oil pressure gauges are also giving you false readings. If they’re affected by the problem, the issue is not with the sending unit or the fuel gauge, it’s likely the instrument voltage regulator or the wiring in the instrument panel.

Remove the instrument panel and use a voltmeter to check the voltage regulator. If readings aren’t within 5 volts of what the owner’s manual specifies, the ground connection could be weak or there is an open connection in the resistor wire. To find the regulator, reference your owner's manual.

Sending Unit Connector

If you’ve determined that the voltage output is reading accurately, the problem is most likely with the sending unit or the fuel gauge itself. To check the sending unit, unplug the unit connector and plug an ohmmeter to the terminals in the unit. Based on your vehicle's specs, the resistance reading should fall within the range specified. If readings aren’t within the minimum and maximum levels, the sending unit is the culprit and it will need to be replaced.

Check the Fuel Levels

Another way to diagnose your sending unit is to check fuel level readings and ensure a proper response. To do this, use a piece of wire to slide down the filler neck to get an approximate fuel reading. You can also drain the entire tank before you start. Once you have your initial reading, fill your tank up with gas and check the resistance readings. If the sending unit isn’t responding correctly and will need to be replaced.

Bench Test

Using the bench test method is another way to check your sending unit. Remove the unit from the fuel tank and use an ohmmeter to bench test. When moving the float between empty and full, the resistance reading should change. Check your vehicle’s manual to see specific ranges,but if the resistance doesn’t change, readings are out of range, or levels skip, it’s likely that you’ll need a new sending unit.

Non-Electrical Sending Unit Problems

If you’ve checked all of the electrical aspects of the sending unit, be sure to check the non-electrical parts of the unit. Leaks in the float or a broken float arm can cause sinking or inaccurate readings. Damage to the fuel tank can also cause inaccurate readings, preventing the float from moving properly.

Use a Test Box on the Fuel Gauge

If you’ve tested and checked the sending unit and you haven’t found the problem, then your fuel gauge is the next step. By using a test box or resistor jumper wires, you can simulate resistance readings that mimic output from the sending unit. A test box will determine if the fuel gauge accurately responds to the simulation, allowing you to pinpoint the problem. If the gauge isn’t accurately responding and you’ve checked to ensure it’s receiving a voltage, then it’s time to replace your old fuel gauge.

Ohmmeter Check

If your old truck or utility vehicle has a magnetic or resistance gauge, you can use an ohmmeter to check the internal resistance. Check your owner's manual for specific readings, but typically a resistance of 10 -15 ohms is normal. If your gauge shows no resistance, it's likely a short in the gauge’s wiring and will need to be replaced.

Replacing Old Sending Units and Fuel Gauges

Once you have diagnosed the problem with either your sending unit or fuel gauge, it’s time to replace them. For old Jeep’s, GMC’s, Ford’s, Willy’s and more, replacement parts can be hard to find at your local automotive store. T-Rex Auto Parts specializes in hard to find replacement parts for vintage trucks and sport utility vehicles including sending units and fuel gauges. Our sending units and fuel gauges can help restore your vintage truck or SUV back to its glory days and keep you on the road for years to come.