If you've ever experienced a fuel gauge that constantly reads "empty" or fluctuates unpredictably, a faulty fuel sending unit might be the culprit. Rather than installing a new unit only to discover your fuel gauge still doesn’t work properly, bench testing allows you to verify its accuracy beforehand. Determining if your sending unit is in working condition can point to problems elsewhere such as a faulty voltage stabilizer or a faulty fuel gauge.

Over time, wear and tear, corrosion, or mechanical failure can lead to inaccurate readings or a completely non-functional unit. These simple diagnostic steps can prevent frustration, save money, and ensure your vehicle provides reliable fuel level readings when you need them most.

 Tools & Materials Needed:
  • Multimeter (with resistance measurement capability)
  • Alligator clips (optional and helpful)
  • Fuel sending unit
  • Service manual (for resistance specifications)
  • Workbench or a flat surface
 Inspect the Sending Unit

If you are testing a used sending unit to deem if purchasing a new unit is necessary, start by visually inspecting the unit for faults. Even brand new fuel sending units can become crushed or damaged during shipping, so here are some things to look out for.

  • Visually inspect for corrosion, damaged housing, broken wires, or mechanical damage.

  • Ensure the float moves freely without obstruction There should not be any grinding or stalling.

  • Plastic housing (aftermarket sensors) may become brittle or crack, restricting float movement.

 Hold it Upright While Testing

Test the unit in the same upright position it will be mounted in the fuel tank. For Air cooled VW's, the sending unit is mounted upright. This orientation is necessary to ensure the float and metal arm move freely in the intended manner without obstruction. Testing the unit sideways may cause additional or reduced pressure between the arm and resistance sensor, which will result in false readings.

 Place Lead on Signal Terminal

It doesn't matter what color (positive or negative) is used when testing ohms. In our drawing, we attach the black negative lead to the signal terminal, but your lead placement can be flipped for this and the following step.

On air cooled VW sending units (both mechanical lever style & cylinder style), the signal terminal is an isolated tab located at the top of the sending unit. You can also attach the lead with alligator clips to free up a hand for testing.

 Position Opposite Lead to Metal Body

Place the opposite lead (positive or negative) to the metal body (ground) at the top of the sending unit. Some sending units will have a stamped metal base while other units have a cast metal base. Base construction does not matter as long as your lead touches anywhere on the metal base to complete the circuit.

 Set multimeter to Ohms

Using a multimeter to measure resistance, set the range to 200 ohms (Ohms, symbol: Ω). This allows you to measure resistance values up to 200 ohms.

Most stock VW fuel level sending units are calibrated for 73+/- Ohms at empty, and to 10+/- Ohms at a full. Note: We have seen brand new fuel sending units bench test at 90 to 0 Ohms which we ran without any issues once the fuel gauge was calibrated.

There will be variance with your Ohm numbers. The Sending unit should not be defective as long as you are getting close to these numbers. A calibrated fuel gauge and voltage stabilizers work together to make up the difference.

 Test for an Empty Fuel Tank

Holding the sending unit upright, let the float arm fall freely to it's lowest position. This simulates an empty fuel tank. At this arm position, our multimeter should read about 73+/- Ohms. This number doesn't have to be exact. There is variance in resistance readings, and your fuel gauge and voltage stabilizer account for the difference after calibration.

 Test for a Full Fuel Tank

Holding the sending unit upright, slowly lift the float arm to it's highest position. This simulates a full fuel tank.

Observe the resistance change as you move the arm. It should change smoothly without sudden jumps or drops. Erratic readings may indicate a worn-out or faulty resistor track inside the sender.

At the highest arm position, our multimeter should read about 10+/- Ohms. (It's perfectly normal for your reading slightly vary.)

 Conclusion: Our Test Results

Different manufacturers will provide different tolerances. You can see in the images below that our brand new fuel gauge didn't exactly hit the 73 Ohm and 10 Ohm numbers we mentioned earlier.

Typically 60(ish) to 80(ish) is normal at the higher end, and around 10(ish) at the lower end. Either way, as long as your getting close to these readings your sending unit should work just fine. Just remember to calibrate your Fuel Gauge "Zero" Adjustment Lever (located behind the fuel gauge) to work with your new Fuel Level Sending Unit. If your sending unit is way out of spec, consider repairing or replacing the unit to ensure accurate fuel level readings in your vehicle.

 75.4 at Fuel Tank Full

In our testing, this sending unit read 75.4 Ohms when the float arm was raised to the "full" position. This is higher than the 73 Ohms we expected, but not something to be concerned about.

 11.2 at Fuel Tank Empty

When the float arm was lowered, (simulating empty) our multimeter read 11.2 Ohms. Not exactly 10 Ohms, but close enough and should work just fine.