(a)
APU indication problems before the APU start
(b)
APU start problems
(c)
APU indication problems during the APU operation
(d)
APU indication problems during the APU shutdown
(e)
Additional APU problems for airplanes with the ETC.
(2)
The fault index will refer you to a figure to correct the problem.
(a) Each figure has a troubleshooting procedure. The troubleshooting procedure has two sections: 1) Possible Causes
a) The list of possible causes can be used if there is not much time. The possible causes are given in a sequence with the most possible and easiest cause at the top. The least possible and most difficult causes are at the bottom. The list of possible causes does not include defective wiring.
2) Fault Isolation a) The fault isolation is a diagram which gives the procedure to find the problem.
2. APU Control and Indicators__________________________
A. BAT Switch
(1) The BAT switch supplies the power to the APU control system.
B. APU Master Switch
(1) When the APU master switch is moved to the START position, the start relay R5 is energized and the APU start sequence begins. In the ON position, the APU master switch supplies the power to the APU fuel valve and the air inlet door.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
////////////////////////A / GTCP85-129 SERIES /737-300/400/500/ ENGINES/MAINTENANCE MANUAL ////////////////////////
(2)
In the ON position, the APU master switch supplies the power to the APU fuel valve and the air inlet door.
(3)
When the APU master switch is moved to the OFF position, power is supplied to the overspeed oscillator in the electronic speed switch to give a signal like an APU engine overspeed.
C. DC AMPS Meter
(1) This meter will move in the negative direction when the APU starter engages. The DC AMPS meter will be a little negative during the starter operation.
D. DC VOLTS Meter
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