04/27/21
Message Overview:
Message Description:
The APU does not shutdown when commanded via cockpit APU Master Switch (S89)) or the External Power Control Panel AUP Stop Switch (S88).
Possible Causes:
- Fuel and APU Control Panel (PL35) / APU Master Switch (S89)
- External Power Control Panel (PL40) / APU Stop Switch (S88)
- APU Electronic Control Unit (ECU) (A39)
- Associated Wiring
Troubleshooting Tips:
Advisory Wire/Service Bulletin: None
Forum Articles/Infoservice/Newsletter: None
Quick Links:
| Removal of the External-Power Control Panel | AMM 24-41-05-000-801 |
| Installation of the External-Power Control Panel | AMM 24-41-05-400-801 |
| APU Safety Precautions | AMM 49-00-00-910-801 |
| Auxiliary Power Unit (APU) Pre-start Check | AMM 49-11-00-740-801 |
| Starting the Auxiliary Power Unit (APU) | AMM 49-11-00-866-801 |
| Auxiliary Power Unit (APU) Shutdown (Normal) | AMM 49-11-00-866-802 |
| Auxiliary Power Unit (APU) Shutdown (Remote) | AMM 49-11-00-866-803 |
| Removal of the APU Control Panel | AMM 49-42-05-000-801 |
| Installation of the APU Control Panel | AMM 49-42-05-400-801 |
| Removal of the Auxiliary Power Unit (APU) Electronic Control Unit (ECU) | AMM 49-61-01-000-801 |
| Installation of the Auxiliary Power Unit (APU) Electronic Control Unit (ECU) | AMM 49-61-01-400-801 |
| Electrical Wiring - Continuity Test - Adjustment /Test - ALL | SPM 20-12-01-05 |
| Wire Repair - Maintenance Practices - ALL | SPM 20-12-10-02 |
Troubleshooting Recommendations:
- Attempt to perform both a normal APU shutdown and a remote APU shutdown.
- If fault is observed using both methods, go to step 7.
- If only the normal APU shutdown fails, continue with next step
- If only the remote APU shutdown fails, go to step 5.
- Remove the cockpit Fuel and APU Control Panel (PL35) APU Master Switch (S89) connector S89J1. Peform a continuity check on the panel side, cycling the APU Master Switch in all three positions as follows:
From To Expected Result Result Connector/Contact Connector/Contact OFF RUN START S89J1-H S89J1-E Open Open Continuity S89J1-H S89J1-F Open Continuity Open S89J1-T S89J1-A Open Open Continuity S89J1-T S89J1-R Open Continuity Open S89J1-N S89J1-K Open Open Continuity S89J1-N S89J1-M Continuity Open Open - If results are correct, continue with next step.
- If results are not correct, replace the APU Master Switch and do close out.
NOTE: You may need to test the switch positions several times to account for intermittent contacts.
- Perform a continuity check on the aircraft connector as follows:
From To
Ground StudExpected Result Result S89P1-K GS2136 Continuity S89P1-M GS2136 Continuity S89P1-E GS2136 Continuity S89P1-F GS2136 Continuity S89P1-A GS2370 Continuity S89P1-R GS2370 Continuity - If wiring defects are found, repair defective wiring as required and do close out.
- If no defects are found, continue with next step.
- Perform a continuity check between the APU Master Switch and the APU ECU as follows:
From To Expected result Result S89P1-N A39AP1-C12 Continuity - If wiring defects are found, repair defective wiring as required and do close out.
- If no defects are found, go to step 7.
- Access and remove the External Power Control Panel (PL40). Perform a continuity check on the panel side while switch is activated as follows:
From To Expected result Result PL40J1-12 PL40J1-13 Continuity - If there is no continuity, replace the External Power Control Panel (PL40) or APU Stop switch as required.
- If there is continuity, continue with next step.
- Perform a continuity check between the External Power Control Panel (PL40) and the APU ECU as follows:
From To Expected result Result PL40P1-13 A39AP1-B13 Continuity - If wiring defects are found, repair defective wiring as required and do close out.
- If no defects are found, continue with next step.
- Remove the APU ECU (A39) and inspect connectors for contamination or damage.
- If inspection results are good, continue with next step.
- If defects are found, repair as required.
- Replace the APU ECU (A39).
- Do close out.