This connection must be made properly (screw, washer, terminal, and then probe) and it must be secure A low or nonexistent probe capacitance reading is normally caused by a bad connection of one probe terminal.Fuel can migrate into an older harness and cause shorts in the wires. Meg checks usually verify problems with the tank harness.Only a bad probe reading or failed meg check from the spar plug would be cause for accessing the tank. I will exhaust ALL avenues before opening up a fuel tank for troubleshooting. Fuel quantity indication problems can be located anywhere in the system.Minimum values for meg check readings will also be found in the M.M. Different combinations of shorts can be detected. A major function of the fuel quantity unit is measuring insulation resistance or a “Meg Check”.Total tank and probe capacitance values can be found in the aircraft's Maintenance Manual.The Comp probe is located at the lowest point, if it is still sitting in fuel, the capacitance reading will be high. All tank and probe readings need to be made with the tank completely empty.Tank or probe capacitance readings will not be correct if the ground leads from the fuel quantity tester to the breakout box and from the breakout box to aircraft structure are not connected.This reduces the chances of incorrect setup (if the LoZ and Comp are swapped, you’ll know it). The fuel quantity unit’s TANK UNIT co-ax (BNC) connections are differentiated by gender.Using the proper breakout box is still the preferred method. Tank or probe readings can be done with meter test leads, but the hookup requires having the correct spar pin-outs.If the breakout box is connected at the spar plug, each individual probe can be measured along with total tank readings.If the test setup is connected at the indicator, only total tank and compensator readings can be made.The co-ax connections are HiZ, LoZ, and Comp (numerous breakouts have only HiZ and LoZ, the Comp is included with the rotary switch selections). Breakout box connection configurations will differ for aircraft type, but all will have the same outputs for the fuel quantity tester. Each aircraft type usually has a breakout box with all the matching plugs to connect to either the spar plug or indicator.Tank capacitance checks are run with the mode selection in MEASURE EXT.
The tester shown here is JcAir® “PSD60-2R. All will have similar switches and functions.
Fuel quantity test boxes are made by numerous manufactures.
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