Section III. TECHNICAL PRINCIPLES OF OPERATION

TM 55-4920-431-13

Section III.

TECHNICAL PRINCIPLES OF OPERATION

1-16. General. Refer to figure 2-1 and table 2-1. The electronic sequence unit

(ESU) is a part of the APU system in the aircraft. The ESU controls the start

sequence of the APU, monitors its operation, and provides Built-In Test Equipment

(BITE) indications of system condition and failure modes. The tester is used for

troubleshooting the ESU and electrical components of the APU, either in the air-

craft or in a test cell.

1-17. Power. When used in the aircraft, 28 VDC power to the tester is provided

by the aircraft through Harness-J2, connected to the aircraft harness. MASTER

switch (3) controls 28 VDC power to both the Tester and ESU. When power is on,

MAIN POWER indicator (12b) is illuminated.

1-18. Function Switches. In addition to MASTER SWITCH (3), there are six

function switches. Three of these, BLEED AIR OPEN/CLOSE switch (5), MAX.

AIR switch (7), and ECONO AIR switch (8), are used only for testing systems

equipped with a modulating bleed air valve. Harness-J2 provided with the tester,

has no connector for a bleed valve, and these three switches are not used.

1-19. LOCAL/REMOTE switch (9) is provided for selecting either the analog con-

verter in an aircraft or the analog converter in the tester. Harness-J2 provided

with the tester has no connector for a local analog converter; therefore, the tester

is used with LOCAL/REMOTE switch (9) in the UP or REMOTE position. This

energizes a relay to apply speed and temperature signals from the ESU to the analog

converter in the tester. The conditioned signals are then applied to meters (1 and

2) on the tester panel.

1-20. When placed in the START/RUN position, START/RUN/STOP switch (4)

applies a signal to the ESU through J2-1 to initiate the start sequence. The ESU

in turn provides a start signal to the tester. This signal is routed through fused

and display circuitry and then outputed through HarnessJ2 to the APU start relay.

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