Airbus pneumatic leak detection systems require the use of an LCR meter for troubleshooting. The meter uses an alternating current (A/C) signal to measure inductance (L), capacitance (C), and resistance (R). Standard VOM’s use direct current to measure resistance and cannot be used to measure the impedance of inductors and capacitors. Also, the use of a standard VOM will produce incorrect loop resistance readings and the possibility of damaging the loop.
Troubleshooting a faulty loop system is fairly simple. With the LCR meter selected to resistance, a reading from either loop end to aircraft ground is made from the controller interface plug. Properly operating loops should have readings that are higher than 3K ohms for the wing and APU loops or more than 17K ohms for the pylons.
If a pylon loop is reading low from the controller, the reading must be taken again directly at the loop itself. This will isolate it from the aircraft wiring. Unlike the wing and APU loop systems, the pylon overheat system uses only one loop section.
Low readings from the controller for the wing and APU loops require a “halving” troubleshooting method. The loop system is split and the faulty side identified. The splitting process continues in steps until the actual faulty loop section is verified.
The aircraft wiring “to” the loops can also cause faults. With both loop ends disconnected, any reading to ground would be cause for further troubleshooting. Wiring shorts can be read with a regular VOM or possibly a megger.
The use of an LCR meter is required for troubleshooting of an “open” loop system. Also note that a “good” ground must be used when using the LCR meter. Most loops run close to aircraft structure which can be used for the meter ground (black) test lead. The BK PRECISION meter, like the one shown here, power up in the “L” mode. Pressing the “L/C/R” button twice will place the meter in resistance mode.
A300-600 ATA 36-22-00
Maintenance Manual 36-22-00