Abstract: A a testing device for testing aircraft auxiliary power units. A special multiwire cable system electrically imposes certain test circuits in between the APU control station and the ESCU and imposes other test circuits in between the ESCU and the LRU's. LRU electrical circuits are subjected to short electrical pulses and the resulting current flow is monitored. Zero or very low current indicates an open or defective circuit and excess current flow indicates a short circuit. Circuit selector equipment permits easy monitoring of circuit resistance and signal characteristics using external standard test equipment. In a preferred embodiment, signal buffer circuits and signal latch circuits provide a record indicating if signals are transmitted to the APU LRU's. In this preferred embodiment the APU battery provides power for the testing device. This preferred embodiment does not include standard monitoring equipment such as voltmeters and oscilloscopes which would require calibration.