Message Name:

AIR DATA SENSOR/PROBE HEATERS ON WHEN ENGINES ARE OFF CAS LESS THAN 50 KNOTS AND WOW SENSES ON GROUND

It is unusual that the Air Data Sensor/Probe Heaters are on when the engines are off, the airspeed is less than 50 knots and the aircraft is on the ground. The Heater/Brake-Temperature Monitoring Unit (HBMU) receives ARINC data about the engines on/off status and the airspeed, and the aircraft air/ground status Weight-On-Wheels (WOW) from the Landing Gear Electronic Control Unit (LGECU). If the HBMU does not receive any one of these data, the Sensor/Probe/Actuator Heaters can become hot on the ground.

If the HBMU is de-energized or the Data Acquisition Unit (DAU) is unserviceable, all of the related Sensor/Probe Heaters will become hot.

The HBMU is a CAIMS member and reports on other defective LRU.

• Data Acquisition Unit (DAU) 1 (A17)
• Data Acquisition Unit (DAU) 2 (A18)
• Engine Electronic Controller (EEC) 1
• Engine Electronic Controller (EEC) 2
• Micro-Air Data Computer (MADC) 1 (A27)
• Micro-Air Data Computer (MADC) 2 (A28)
• Electrical Management System-Control and Display Unit (EMS CDU) 1 (A94)
• Electrical Management System-Control and Display Unit (EMS CDU) 2 (A91)
• Heater/Brake-Temperature Monitor Unit (HBMU) 1 (A121)
• Heater/Brake-Temperature Monitor Unit (HBMU) 2 (A93)
• Associate Wiring

Advisory Wire/Service Bulletin: None

Forum Articles/Infoservice/Newsletter: None

1. Check if the following CAS messages are displayed on the EICAS primary display. Select displayed message from menu below:
   Select the CAS MessageDAU 1A FAILDAU 1B FAILDAU 2A FAILDAU 2B FAILADC 1 DEGRADEDADC 1 FAILADC 2 DEGRADEDADC 2 FAILADC 3 DEGRADEDADC 3 FAIL

   1. If any message is displayed, perform the troubleshooting procedure and continue with next step.
   2. If none of these messages are displayed, continue with next step.

2. Check CAIMS active or stored faults for the following messages "LEFT HBMU or RIGHT HBMU".

   1. If fault message is found, replace the defective HBMU and do close out.
   2. If fault message is not found, continue with next step.

3. Check CAIMS active or stored faults if internal faults for the ADC, DAU, EEC or EMS CDU are found.

   1. If an internal fault is found, replace the affected unit and do close out.
   2. If no internal fault is found, continue with next step.

4. Access CAIMS active faults. Select displayed message from the menu below.
   Select the Fault Message27660271SX - AIR DATA CMPTR #3/ BUS #1 FAULT27660272SX - AIR DATA CMPTR #3/ BUS #2 FAULT27660281SX - AIR DATA CMPTR #2/ BUS #1 FAULT27660282SX - AIR DATA CMPTR #1/ BUS #2 FAULT27660291SX - AIR DATA CMPTR #1/ BUS #1 FAULT27660292SX - AIR DATA CMPTR #2/ BUS #2 FAULT

   1. If fault message is found, perform the applicable troubleshooting in the SmartFix Plus "Troubleshooting by CAIMS" section and do close out.
   2. If none of the messages above are displayed, continue with next step.

5. Disconnect HBMU 1 connector (A121P2) and DAU 1 connector (A17CP1) or HBMU 2 connector (A93P2) and DAU 2 connector (A18CP1). Perform a continuity check as follows:

   From	To	Result
   A121P2-5	A17CP1-A9
   A121P2-6	A17CP1-A10
   A121P2-33	A17CP1-K1
   A121P2-34	A17CP1-K2
   A93P2-5	A18CP1-A9
   A93P2-6	A18CP1-A10
   A93P2-33	A18CP1-K1
   A93P2-34	A18CP1-K2

   1. If there is no continuity, repair or replace the defective wiring as required and do close out.
   2. If there is continuity, continue with next step.

6. Perform a continuity check as follows:

   From	To	Result
   A121P2-5	Ground
   A121P2-6	Ground
   A121P2-33	Ground
   A121P2-34	Ground
   A93P2-5	Ground
   A93P2-6	Ground
   A93P2-33	Ground
   A93P2-34	Ground

   1. If there is no continuity, replace the defective HBMU and do close out.
   2. If there is continuity, repair defective wiring as required and continue with next step.

7. Access CAIMS active or stored faults and check if an EEC INTERFACE fault is displayed.

   1. If fault message is not found, go to step 10.
   2. If fault message is found, continue with next step.

8. Disconnect DAU 1 connector (A17CP1) and L ENG FIREWALL connector (1JA2) for HBMU 1 or DAU 2 connector (A18CP1) and L ENG FIREWALL connector (1JB4) for HBMU 2. Perform a continuity check for open circuit or short circuit between the pins of the connectors.

   From	To	Result
   A17CP1-B5	1JA2-6
   A17CP1-C5	1JA2-17
   A17CP1-H9	1JA2-18
   A17CP1-J9	1JA2-36
   A18CP1-B5	1JB4-6
   A18CP1-C5	1JB4-17
   A18CP1-H9	1JB4-18
   A18CP1-J9	1JB4-36

   1. If there is no continuity, repair the defective wiring as required and do close out.
   2. If there is continuity, continue with next step.

9. Perform a continuity check as follows:

   From	To	Result
   A17CP1-B5	Ground
   A17CP1-C5	Ground
   A17CP1-H9	Ground
   A17CP1-J9	Ground
   A18CP1-B5	Ground
   A18CP1-C5	Ground
   A18CP1-H9	Ground
   A18CP1-J9	Ground

   1. If there is continuity, repair defective wiring as required and do close out.
   2. If there is no continuity, replace defective DAU and continue with next step.

10. Access CAIMS active or stored faults and check if an EMS CDU INTERFACE/WRG fault is displayed?

    1. If fault message is not found, go to step 15.
    2. If fault message is found, continue with next step.

11. Perform the operational test of the EMS CDU.

    1. If it is not satisfactory, check CAIMS active or stored faults for EMS CDU related messages and perform the troubleshooting and do close out.
    2. If it is satisfactory, continue with next step.

12. Disconnect DAU 1 connector (A17CP1) and EMS CDU 1 connectors (A94P1) and (A94P2) for HBMU 1 or DAU 2 connector (A18CP1) and EMS CDU 2 connectors (A91P1) and (A91P2) for HBMU 2. Perform a continuity check for open circuit or short circuit between the pins of the connectors as follows:

    From	To	Result
    A17CP1-D9	A94P1-3
    A17CP1-E9	A94P1-4
    A17CP1-H5	A94P2-3
    A17CP1-J5	A94P2-4
    A18CP1-D9	A91P1-3
    A18CP1-E9	A91P1-4
    A18CP1-H5	A91P2-3
    A18CP1-J5	A91P2-4

    1. If there is no continuity, repair or replace the defective wiring as required and do close out.
    2. If there is continuity, continue with next step.

13. Perform a continuity check as follows:

    From	To	Result
    A17CP1-D9	Ground
    A17CP1-E9	Ground
    A17CP1-H5	Ground
    A17CP1-J5	Ground
    A18CP1-D9	Ground
    A18CP1-E9	Ground
    A18CP1-H5	Ground
    A18CP1-J5	Ground

    1. If there is continuity, repair defective wiring as required and do close out.
    2. If there is no continuity, replace the defective DAU and continue with next step.

14. Perform a operational test of the air data probes and sensors anti-ice system. Check if the EMS CDU INTERFACE fault is displayed.

    1. If fault message is not found, do close out.
    2. If fault message is found, replace the defective EMS CDU and do close out.

15. Access CAIMS active or stored faults and check if an ADC INTERFACE/WRG fault is displayed.

    1. If fault message is not found, do close out.
    2. If fault message is found, continue with next step.

16. Disconnect the connectors DAU 1 (A17CP1) and ADC 1 (A27P1) for HBMU 1 or DAU 2 (A18CP1) and ADC 2 (A28P1) for HBMU 2. Perform a continuity check as follows:

    From	To	Result
    A17CP1-G1	A27P1-30
    A17CP1-J1	A27P1-34
    A18CP1-G1	A28P1-30
    A18CP1-J1	A28P1-34

    1. If there is no continuity, repair or replace the defective wiring as required and do close out.
    2. If there is continuity, continue with next step.

17. Perform a continuity check as follows:

    From	To	Result
    A17CP1-G1	Ground
    A17CP1-J1	Ground
    A18CP1-G1	Ground
    A18CP1-J1	Ground

    1. If there is continuity, repair defective wiring as required and do close out.
    2. If there is no continuity, replace the defective DAU and continue with next step.

18. Perform the operational test of the air data probes and sensors anti-ice system. Access CAIMS Active or Stored faults and check if an ADC INTERFACE fault is displayed.

    1. If fault message is not found, do close out.
    2. If fault message is found, continue with next step.

19. Replace the defective MADC.
20. Do close out.