01/21/25
Message Overview:
Message Description:
The PACK LOOP B is detected shorted to ground by the IASC-2 during the system self-test. Check the loop and wiring isolation to ground.
Possible Causes:
- Pack Duct Leaking
- Bleed Loop Routing
- Pack Loop Sensor 1B (MT141)
- Associated Wiring
Troubleshooting Tips:
Advisory Wire/Service Bulletin: None
Full Throttle Blog/Forum Articles/Infoservice/Newsletter: None
Flight Operation Notifications Manual (FONM): None
NOTES:
- Bleed Loop Routing and Visual Aid Guide
- Troubleshooting Guide for Bleed Loops
- Pack Channel B Bleed Loop Information
- If the bleed loop resistance value is within limits try cleaning the bleed loop connections per SPM 51-25-00-110-801 and then reapply lubrication to the connections per SPM 20-20-00. Lubrication part number is called out in each bleed loop installation procedure. Run the aircraft after the cleaning and applying lubing procedure and see if the BLEED LOOP FAULT clears.
Quick Links:
| Standard Aircraft Configuration for Maintenance | AMM 12-00-00-867-803 |
| Electrical/Electronic Safety Precautions | AMM 24-00-00-910-801 |
| Electrostatic-Discharge Safety Precautions | AMM 24-00-00-910-802 |
| Connect Electrical Power to the Aircraft | AMM 24-00-00-861-801 |
| Remove the Electrical Power from the Aircraft | AMM 24-00-00-861-802 |
| Component Location | AMM 36-21-00-992-801 |
| Bleed-Air Leak-Detection and Warning-System Safety Precautions | AMM 36-21-00-910-801 |
| Operational Test of the Bleed-Air Leak-Detection and Warning System | AMM 36-21-00-710-801 |
| Initiation Test of the Bleed-Air Leak-Detection Loops | AMM 36-21-00-740-801 |
| Resistance Check of the Bleed-Air Leak-Detection Loops | AMM 36-21-00-760-801 |
| Removal of the Left Bleed-Loop Element | AMM 36-21-01-000-801 |
| Installation of the Left Bleed-Loop Element | AMM 36-21-01-400-801 |
Troubleshooting Recommendations:
- Gain access to IASC #2 located behind the lavatory sink just forward of the water pallet.
- Disconnect connector A56P4 from the IASC and check resistance between pins 10 and 11. Resistance should be approximately 15.4 Ω at room temperature.
- Isolate a system short through standard practices by isolating the different loop and wiring from the IASC. Repair faulty wire segment or replace loop as required.
NOTE: Measure Insulation resistance and line resistance of the detection loops as per the Leak Detection Elements Troubleshooting Sheet. Make sure line resistance & insulation values are within the specified limits.
Perform resistance checks on the applicable loop per AMM 36-21-00-760-801.
NOTE: Before replacing a loop always look at the loop for routing, ensure loop is not too close to the plumbing or the bleed hole on a valve. Review AMM 36-21-00-992-801 for component location. See individual loops for specific routing information.
NOTE: Often while troubleshooting this discrepancy the CAS is not usually present. However what you may find is higher than normal resistance such as 10 Ω or higher above nominal. It is not high enough to trip a bleed loop fault CAS message but high enough to assist in troubleshooting. When you get the resistance value down to nominal values then it is safe to say you've corrected the discrepancy.
