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WHITE MESSAGE Require immediate crew attention. These messages are accompanied by flashing master warning, triple chime attenson with or without voice message or dedicated tone. Require immediate crew awareness and future crew action may be required in the form of alternate system selection or performance limitation. These message are accompanied by flashing master caution and a single chime. Indicate safe or normal system operation which require crew awareness, over and above the dark cockpit philosophy. Indicate minor failures or reduction in systems capability, which require no crew action.

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02/26/21

Message Overview:

Message Name:

SHORT CIRCUIT

Message Code:

B3-008133

Associated CAS:

FIRE SYS FAULT
(Advisory)

Reporting LRU: APU FIRE LOOP B
System Description: 26-13-00
Schematic Diagram: 26-13-00
Wiring Diagram: 26-13-01

Message Description:

Short circuit on APU firewall sensing element (A105) or APU eductor sensing element (A106), wire short, or FIREX Control Unit (A91) internal failure.

Possible Causes:

  • Firewall Sensing Element Loop B (A105)
  • Eductor Sensing Element Loop B (A106)
  • FIREX Control Unit (A91)
  • Associated Wiring

Troubleshooting Tips:

Advisory Wire/Service Bulletin: None

Forum Articles/Infoservice/Newsletter: None

Quick Links:

Removal of the Fire-Detection-and-Extinguishing (FIREX) Control-Unit AMM 26-11-01-000-801
Installation of the Fire-Detection-and-Extinguishing (FIREX) Control-Unit AMM 26-11-01-400-801
Removal of the Eductor Sensing Element AMM 26-13-01-000-801
Installation of the Eductor Sensing Element AMM 26-13-01-400-801
Removal of the Firewall Sensing Element AMM 26-13-05-000-801
Installation of the Firewall Sensing Element AMM 26-13-05-400-801
Kiddie Aerospace CMM 26-11-88
Electrical Bonding - Adjustment/Test SPM 51-80-00-05
Wiring - Maintenance Practices - ALL SPM 20-12-00-02
Wire Repair - Maintenance Practices - ALL SPM 20-12-10-02

Troubleshooting Recommendations:

  1. Check the FIREX Control Unit (A91) circuit breakers CB2-A11 and CB6-A4, pull and reset. Re-check MDC for the fault.

    1. If system checks are good, do close out.
    2. If fault remains, continue with next step.

  2. Check the FIREX APU Loop B for continuity and no shorts to ground, ring from A91P2 pin 12 to A91P2 pin 13. The resistance of the forward firewall (A105) is about 5.5Ω and the resistance of the eductor loop (A106) is about 2.1Ω, with a total resistance about 7.6Ω.

    1. If resistance value is good, go to step 7.
    2. If resistance value is out of tolerance, continue with next step.

  3. Gain access to the loop A105 and check continuity at loop, it should be 5.5Ω.

    1. If resistance value is 5.5Ω, go to step 5.
    2. If resistance value is not 5.5Ω, continue with next step.

  4. Replace loop A105, do close out.
  5. Gain access to the loop A106 and check continuity at loop, it should be 2.1Ω.

    1. If resistance value is 2.1Ω, go to step 7.
    2. If resistance value is not 2.1Ω, continue with next step.

  6. Replace loop A106, do close out.
  7. Inspect the APU fire loops: forward firewall A105 and eductor A106 for proper installation and adjust as required Reset CB2-A11 and CB6-A4 and re-check MDC for fault.

    1. If fault no longer posted in MDC, do close out.
    2. If failure is still posted in MDC, continue with next step.

  8. Replace the FIREX Control Unit.
  9. Do close out.

13_apu_firewall_detection_harnessapu_fire_loop

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