How to Test a Fuel Sender

Experiencing a malfunction in your aircraft’s fuel sending unit can be a major safety risk, especially during flight. It could lead to incorrect fuel readings, which might force you into emergency procedures you hoped to avoid.

Imagine taking off for a long cross-country flight only to find yourself low on fuel much sooner than anticipated! With the right tools and know-how, you can test your aviation fuel sending unit at the shop.

You may have learned “don’t trust your fuel gauge” or “fly by your watch.” However, these statements only relate to perfect conditions without other factors. Your watch will never indicate a leak in your fuel tank or that your fuel filler cap has fallen off. Proper fuel level indication is paramount in ensuring the safety of the flight crew and passengers.

This guide will show you how to test the resistance output of an aviation fuel sending unit, ensuring accurate fuel level readings to enhance the safety of both crew and passengers.

Understanding the Problem

Here are some common signs of issues with your fuel level sensor, observable from the cockpit:

1. The fuel quantity indicator oscillates back and forth during normal flight.
2. The fuel quantity reading is stable until it suddenly drops to a lower level.
3. The fuel quantity indicator shows readings that don’t match the fuel flow indicator.
4. The fuel quantity indicator consistently shows either full or empty.

These issues suggest a problem with the fuel level sender. This guide focuses on testing a resistance-based sensor using a test bench. It does not cover the removal of the sensor from the aircraft.

Legal Note: Before proceeding, consult FAR 43.3. Any disassembly may require supervision by a certified A&P mechanic unless it is classified as Preventative Maintenance under FAR Part 43 Appendix A.

Required Tools

• Digital Multimeter
• Aircraft Maintenance Manual
• Resistive Fuel Sensor

Testing Steps

Important: Your resistive fuel level sender must be removed from the aircraft. The unit should be on a test bench and cleaned of any debris or excess fuel.

1. Setup Your Multimeter:
   • Connect the black test lead to the common (ground) terminal.
   • Connect the red test lead to the reference terminal (marked with a “V”).
   • Set your multimeter to measure ohms. Refer to your specific model’s manual for instructions.
2. Connect to the Fuel Sender:
   • Attach the black lead to the sender’s flange or ground terminal.
   • Attach the red lead to the center stud of the sender.
3. Read and Adjust:
   • Check the multimeter for a resistance reading (ohms). If no value is displayed, adjust the lead positions until you get a reading. Ensure the multimeter is set correctly.
4. Test the Float Arm:
   • Slowly move the float arm from its lowest to highest position. Observe the resistance value; it should change consistently. Check your maintenance manual for expected values and directions of change.

Analyzing Results

• Inconsistent Ohm Values: This may indicate the need for cleaning or refurbishing the sender due to issues like metal corrosion, residue buildup, or foreign object debris (FOD) intrusion.
• No Change in Resistance: This typically suggests a problem with the resistance wiper, such as a bent wiper or one that has worn through the resistance card.

Replacement Options

If your tests indicate the need for a new unit, consider options like those offered by CiES. CiES manufactures fuel level senders as a physical bolt-in replacement for Cessna, Beechcraft, Cirrus, Mooney, Piper and many other makes and models. In fact, there are nearly 700 makes and models covered by our FAA STC. Here you can view all covered models. CiES is the leading manufacturer of fuel senders for OEM installations.

CiES also supports 30+ fuel gauges and instrument panels. These configurations are covered by the same STC. Occasionally, CiES can connect to your existing gauge (aircraft must have a tank and gauge map), please contact us for details before ordering.

With over 120,000 CiES fuel quantity sending units in the field and 900,000+ hours of trouble-free operation, we are a trusted ally of general aircraft owners and quality aircraft manufacturers. CiES senders exceed 90,000 hours mean time to failure (MTTF), making our fuel quantity solution one of the most reliable aviation systems in existence.