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Info - Description, Operating Characteristics and Diagnostic Information of New NVG Transfer Case (RPO NP8) for New Style Pickup and Utility Models

Subject:Description, Operating Characteristics and Diagnostic Information of New NVG Transfer Case (RPO NP8) for New Style Pickup and Utility Models

Models:2007 Chevrolet Avalanche, Silverado, Suburban, Tahoe
2007 GMC Sierra, Yukon, Yukon XL
with Active Transfer Case (RPOs NP8, NQF, NQH)


This bulletin is being revised to add models and additional information (in italics). Please discard Corporate Bulletin Number 06-04-21-006 (Section 04 -- Driveline/Axle).

The operation of the new style pickup and utility models NP8 transfer case controls is significantly different from the previous pickup and utility models. This document is an overview of the changes, with the intent of helping troubleshoot the new transfer case system in the field.

This information applies to the 2007 New Style Pickup and Utility models equipped with transfer case RPOs NP8, NQF and NQH.

The unusual operations described below are as intended. The operations listed are associated with a system failure. The normal troubleshooting tree for each DTC should be completed. In the majority of cases, this will correct the concern.

    • While operating in AWD mode, any diagnostic fault that disables the AWD system will now result in a default shift to 4HI. A shift to 4HI is preferred. It provides for improved traction for safety on low friction surfaces such as ice and snow. In cases where default is 4HI, a shift to 2HI can be chosen. Only AWD will be disabled. This is true for the following diagnostic trouble codes (DTC):
       - C0300 Rear Propshaft Speed Sensor / System Malfunction
       - C0305 Front Propshaft Speed Sensor / System Malfunction
       - C0327 Encoder Circuit Malfunction
    • The new system is designed to proactively send a proportion of torque to the front wheels in AWD mode. This relies on communication from the ECM and TCM to send the proper amount of torque. If the data from the ECM or TCM is determined to be missing or invalid, then a default shift out of AWD to 4HI will occur. In the TCCM, the following codes may be set:
       - U0100 Lost Communication with ECM/PCM
       - U0101 Lost Communication with TCM
       - U0073 Control Module Communication Bus Off
    • It is possible for a default shift from AWD to 4HI to occur and no DTCs will set in the TCCM. If this occurs, check for codes in the ECM and TCM. If the conditions that set the codes in the TCM or ECM are corrected, this will correct the transfer case issue. If a perceived "Phantom Shift" occurs, check all of these modules, not just the TCCM.
    • A default shift out of AWD to 2HI is also possible. In this circumstance, the TCCM will stay in 2HI and no mode changes will be available until the next ignition cycle after the condition is corrected. In the TCCM, the following codes may be set:
       - C0306 Motor A/B Circuit Malfunction
       - C0321 Transfer Case Lock Circuit Malfunction
       - C0326 Transfer Case Lock Malfunction
       - C0374 General System Malfunction
       - C0550 ECU Malfunction (No mode lamp will light)
    • The C0378 - Front Axle Switch Malfunction and C0379 - Front Axle Control Circuit Malfunction DTCs operation is the same as the previous model utilities C0379 fault. If the Front Axle Switch input shows that the front axle is not engaged, the 2HI lamp will light solid, and the requested 4WD mode lamp (4HI, AWD or 4LO) will flash. The shift will be blocked.
        If this condition occurs in 4LO, the transfer case will be in low range without the front axle engaged. If it occurs in AWD, AWD will not operate.
    • The C0327 - Encoder Circuit Malfunction DTC will cause a default shift to 4HI as mentioned above. This can occur on any shift request or from AWD.
        In this mode, 2HI and 4HI are available. Placing the mode switch in 2HI will result in the front axle output turning off and the 2HI lamp lighting (the motor will not operate). Placing the mode switch in 4HI, 4LO or AWD will result in the 4HI lamp lighting and the front axle engaging. This operation will continue until the next ignition cycle after the condition is corrected.
    • While in the AWD mode, the shift motor will now activate while driving. Because the system is constantly adjusting for changes in torque, motor activations may be observed on the Tech 2 during normal driving with no slip present. This is normal operation.
        On the 2006 and prior truck models, this would have been cause for repair. On 2007 NP8 Tahoe/Yukon models it is not.
    • Another new feature on the 2007 new style pickup/utility models is that the vehicle is speed limited to 72 km/h (45 mph) when operating in 4LO. This is normal operation.
    • DTC C0550 - An early issue with the software, P/N 24237154, caused a potential for the false setting of the C0550 DTC. If the C0550 type 37 code is set in this version of the software, the resolution is to reflash the module wit the latest version of software. Module replacement for this issue is not necessary.
        If a vehicle comes in with a DTC C0550 set, check to see which software version is in the TCCM module. If software P/N 24247154 is present, then simply reflash the module.
        On the 2006 and prior truck models, a C0550 DTC would have been cause for module replacement. On 2007 NP8 Tahoe/Yukon models if C0550 is set without the early software version mentioned above, then a TCCM replacement may be necessary.
    • Missing Wheel Speed Sensor Signals - Default shifts to 4High while in Auto. This may generate a DTC U0121. Also check for DTCs C0035 or C0040 (front sensors) or C0045 or C0050 in the EBCM.
    • Missing Transmission Output Speed Sensor Signal - Default shifts to 4High while in Auto. This may generate a DTC U0101. Also check for DTCs P0721 (performance), P0722 (no signal) or P0723 (intermittent signal).

OTHER RECOMMENDATIONS

 

    • All DTCs now include a 3rd "Failure Subtype." Please include this byte when recording the DTC # on the documentation. This byte helps to minimize troubleshooting, and documenting it will help Engineering in identifying trends in the failures.
        Please include this information on all repair orders where DTCs are set. Have this information ready before calling TAC.
    • In some instances, rebooting the module will enable it to function and eliminate the need for replacement. Pull the TCCM fuse and reinstall it. If the module starts to function, do not replace it. Report the occurrence to Technical Assistance. Set-up a TAC case.