SCU Fault Code:

'5'

Associated CAS:

NWS FAIL
(Caution)

A '5' on the 7-segment display indicates a fault in the Nosewheel Feedback RVT or associated wiring.

NOTE: An SCU Fault Code '3' and an SCU Fault Code '5' on the 7-segment display indicates that the SCU is not able to isolate the fault to a specific LRU. Use the combined '3' & '5' troubleshooting section.
NOTE: If SCU Fault Code '7' is flashing with another code, carry out SCU Fault Code '7' troubleshooting before addressing any other fault.

• Electro-Hydraulic Servo Valve (EHSV)
• Rotary Variable Transducer (RVT)
• Steering Control Unit (SCU) (A73)
• Associated Wiring

Advisory Wire/Service Bulletin:

• SB100-32-25 - Special Check/Modification - Nosewheel-Steering Control System - Replacement of the Steering Manifold

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 '5' is displayed, go to the next step.

4. Verify the RVT connector and receptacle for contamination/oxidation. Clean or repair the connector as required.
5. Measure resistance at the RVT receptacle per the following table:
   

   FROM (RVT)	TO (RVT)	EXPECTED RESULTS	ACTUAL RESULTS
   Pin 1	Pin 2	Between 125Ω - 155Ω
   Pin 3	Pin 4	Between 125Ω - 155Ω
   Pin 5	Pin 8	Between 180Ω - 218Ω
   Pin 6	Pin 7	Between 180Ω - 218Ω
   Pin 1	Pin 6	OPEN (∞Ω)
   Pin 3	Pin 5	OPEN (∞Ω)

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

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

   FROM (RVT)	TO A73P2 (SCU)	RESULTS
   Pin 1	Pin S
   Pin 2	Pin P
   Pin 3	Pin A
   Pin 4	Pin R
   Pin 5	Pin K
   Pin 6	Pin M
   Pin 7	Pin Z
   Pin 8	Pin J

   1. Repair wiring as required.
   2. If No Fault Found, go to the next step.
      NOTE: The reason for verifying the EHSV with an SCU Fault Code '5' is that the excitation voltage comes from the same oscillator for both, the EHSV and the RVT, and the SCU cannot always determine which component has failed.

7. Verify the EHSV connector and receptacle are clear of contamination/oxidation.
8. Measure resistance at the EVSV receptacle per the following table:
   

   FROM (EHSV)	TO (EHSV)	EXPECTED RESULTS	ACTUAL RESULTS
   Pin 3	Pin 4	Between 160Ω - 220Ω
   Pin 3	Pin  5	OPEN (∞Ω)

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

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

   FROM (EHSV)	TO A73P2 (SCU)	RESULTS
   Pin 3	Pin a
   Pin 4	Pin N

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

10. 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.
11. Replace RVT.
12. If fault continues, replace SCU.