S 725-017
(10)
Make sure these indications occur:
(a)
The ohmmeter on the M7847 APU electrical control unit indicates less than 5 ohms.
(b)
The APU DUCT OVERHEAT IND LIGHT L6 on the P83 panel is on.
S 085-018
(11)
Remove the jumper from the thermal switch S969.
S 725-019
(12)
Make sure the ohmmeter reads less than 5 ohms.
S 865-020
(13)
Push the APU DUCT OVERHEAT RESET SW switch S220 on the P83 panel.
S 725-021
(14)
Make sure these indications occur:
(a)
The APU DUCT OVERHEAT IND LIGHT L6 on the P83 panel is off.
(b)
The ohmmeter reads an open circuit.
S 085-022
(15)
Disconnect the ohmmeter.
S 425-023
(16)
Install the M7847 unit in the E8 equipment rack (AMM 49-61-07/401).
I. APU Duct Overheat Relay Check
S 035-024
(1)
Disconnect the electrical connector D40127P on the E8 equipment center.
S 485-025
(2)
Connect pin 11 to ground on the receptacle D40127J.
S 725-026
(3)
Make sure these indications occur:
(a)
The APU DUCT OVERHEAT IND LIGHT L6 on the P83 panel is on.
(b)
The EICAS shows the "APU DUCT LEAK" message.
(4)
Remove pin 11 from the ground on receptacle D40127J.
(5)
Connect the connector D40127P on the E8 equipment center.
(6)
Push the APU DUCT OVERHEAT RESET SW switch S220 on the P83 panel.
S 085-027
S 485-122
S 715-028
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747-400 MAINTENANCE MANUAL
S 725-029
(7) Make sure these indications occur:
(a) The APU DUCT OVERHEAT IND LIGHT L6 on the P83 panel is off.
NOTE: If the APU DUCT OVERHEAT LIGHT L6 is not off and stays
____ off then one of the thermal switches has become inoperable or there is a short wire between the thermal switches and the R686 Duct Overheat relay on the P83 panel
(b) Remove the EICAS status message "APU DUCT LEAK" (AMM 45-10-00/201).
J. Put the Airplane to its usual condition.
S 865-030
(1) Remove electrical power if it is not necessary (AMM 24-22-00/201).
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747-400 MAINTENANCE MANUAL
THERMAL SWITCH - REMOVAL/INSTALLATION
_____________________________________
1. General_______
A. This section contains two tasks. The first task is the removal of thermal switch along the APU air duct. The second task is the installation of the thermal switches.
B. Twenty thermal switches S969-S986, S1173, and S1924 are installed along the APU bleed air duct. Fifteen (S969-S983) are installed in the aft and bulk cargo compartment. Five (S984-S986, S1173 and S1924) are installed in the area aft of the bulk cargo compartment. To get access to the thermal switches, remove the side wall lining panel in the aft and bulk lower cargo compartment, and the access panel, 312AR, behind the bulk cargo compartment. In some cases, such as for the S978 and S983 switches, access may be gained by the removal of the Main Deck floor panels.
C. ALL AIRPLANES; Seven more thermal switches (S2207-S2213) are installed along the APU bleed air duct between the APU check valve and the APU isolation valve. To get access to the switches, open the applicable access panels in the left main wheel well.
D. Each thermal switch monitors the overheat condition of the APU air duct area. If an overheat condition is detected, a thermal switch closes and an EICAS message is shown.
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