SCU Fault Code:

'4'

Associated CAS:

NWS FAIL
(Caution)

A '4' on the 7-segment display indicates a fault in the Steering Control Handwheel or associated wiring.

NOTE: A '4' combined with a '6' on the 7-segment display indicates that the SCU is not able to isolate the fault to a specific LRU. Use the combined '4 & 6' troubleshooting page.

NOTE: If SCU Fault Code '7' is flashing with another code, carry out SCU Fault Code '7' troubleshooting before addressing any other fault.

• Steering Control Handwheel (HW) (MT85)
• Rudder Pedal Transducer (RP) (MT86)
• Steering Control Unit (SCU) (A73)
• Associated Wiring

Advisory Wire/Service Bulletin: None

Forum Articles/Infoservice/Newsletter: None

Troubleshooting Tips:

• ATA 32-51-00 NOSEWHEEL STEERING SYSTEM

Fault Isolation Procedure:

1. Before starting the following Fault Isolation Procedure, make sure the SCU Fault Confirmation Procedure is carried out to ensure the correct fault code is being addressed.
2. Carry out the Rigging of the Nosewheel-Steering Control System (AMM TASK 32-51-00-820-801).
   → If a null voltage is found out of tolerance and cannot be adjusted, replace the applicable component and carry out the rigging procedure again.
3. Carry out the Operational Test of the Nosewheel-Steering Control System (AMM TASK 32-51-00-710-801).
   NOTE: Always establish Left Hydraulic System pressure before switching the NWS System to 'ON' as it may trigger and record a false 'NWS FAIL' CAS message if any NWSS component is out of neutral position.
   

   1. If the 7-segment display indicates '0', no other maintenance action is required.
   2. If an SCU Fault Code '4' is displayed, go to the next step.

4. Disconnect connector MT85P1 and measure the Handwheel resistance per the following table:
   

   FROM MT85P1 (HW)	TO MT85P1 (HW)	EXPECTED RESULTS	ACTUAL RESULTS
   Pin 1	Pin 3	5 ±0.5kΩ
   Pin 4	Pin 6	5 ±0.5kΩ
   Pin 3	Casing	OPEN (∞Ω)
   Pin 6	Casing	OPEN (∞Ω)

   1. If any value is not within expected range, replace Handwheel.
   2. If No Fault Found, go to the next step.

5. At the Handwheel connector, connect an ohmmeter between pins 2 and 3, and between pins 5 and 6.
6. Slowly rotate the Handwheel from one stop to the other while observing the ohmmeter. Make sure applicable circuit does not momentarily open or spike while turning the handle.

   1. If a fault is found, replace the Handwheel.
   2. If No Fault Found, go to the next step.

7. Carry out a wiring check per the following table:
   

   FROM MT85P1 (HW)	TO A73P2 (SCU)	RESULTS
   Pin 1	Pin D
   Pin 2	Pin U
   Pin 3	Pin T
   Pin 4	Pin F
   Pin 5	Pin W
   Pin 6	Pin c

   1. Repair wiring as required.
   2. If No Fault Found, go to the next step.

8. Disconnect connector MT86P1 and measure the rudder Pedal Transducer resistance per the following table:
   

   FROM MT86P1 (RP)	TO MT86P1 (RP)	EXPECTED RESULTS	ACTUAL RESULTS
   Pin 1	Pin 3	5 ±0.5kΩ
   Pin 4	Pin  6	5 ±0.5kΩ
   Pin 3	Casing	OPEN (∞Ω)
   Pin 6	Casing	OPEN (∞Ω)

   1. If any value is not within expected range, replace Rudder Pedal Transducer.
   2. If No Fault Found, go to the next step.

9. At the Rudder Pedal Transducer connector, connect an ohmmeter between pins 2 and 3, and between pins 5 and 6.
10. Slowly action the rudder pedals from one stop to the other while observing the ohmmeter. Make sure applicable circuit does not momentarily open or spike while moving the rudder pedals.

    1. If a fault is found, replace Rudder Pedal Transducer.
    2. If No Fault Found, go to the next step.

11. Carry out a wiring check per the following table:
    

    FROM MT86P1 (RP)	TO A73P2 (SCU)	RESULTS
    Pin 1	Pin E
    Pin 2	Pin V
    Pin 3	Pin b
    Pin 4	Pin G
    Pin 5	Pin H
    Pin 6	Pin X

    1. Repair wiring as required.
    2. If No Fault Found, go to the next step.

12. If No Fault Found at this point, it is recommended to do a download of the SCU NVM and send it to Bombardier CRC for analysis.
13. Replace Handwheel.
14. If fault continues, replace SCU.