Some 2020-2021 Silverado 2500HD/3500HD and Sierra 2500HD/3500HD models – equipped with the 6.6L Duramax diesel engine (RPO L5P), 10L1000 10-speed automatic transmission (RPO MGM, MGU) and the 2-speed transfer case RPO NQF (Electric Shift Cont, Two Speed Transfer Case) or NQH (Active, Two Speed, Switch Activated Transfer Case) – may have a transfer case noise, vibration and harness (NVH) concern after repairs have been made. If the transfer case was removed and reinstalled to the transmission, a harsh downshift, noise, vibration or a clunk sound may occur during a coast-down event.
The NVH condition may be the result of a misalignment between the transmission output shaft and the transfer case input. The misalignment could cause the transfer case shift collar to slip out of engagement.
If these conditions are found, monitor transmission shift data using GDS 2 during a road test to determine if the noise/clunk/roughness occurs during the downshift or if the condition occurs randomly during deceleration. If the sound or roughness is verified to be from the transfer case, the transmission output shaft may be misaligned. It will be necessary to disconnect and reconnect the transfer case to the transmission. The transfer case does not need to be completely removed from the vehicle.
Misalignment Correction
Follow the appropriate Service Information for Transfer Case Assembly Replacement (with MGM/MGU Automatic Transmission). Loosen the transfer case bolts, but do not remove the bolts, to allow the transfer case to be separated from the transmission. It may be helpful to use a transmission jack or adjustable jack stand to provide support in holding the transfer case to the transmission.
With the bolts loosened, slide the transfer case rearward so there is a minimum gap of 1/2 inch (13 mm), but not more than 1 inch (25 mm). (Fig. 8) Next, slide the transfer case forward until it is contacting the transmission again.
Fig. 8
Tighten the transfer case bolts in sequence following the appropriate Service Information.
Refer to Bulletin #21-NA-120 for additional information.
– Thanks to Mark Gordon
Updated July 28, 2021
Thanks for the note. The article has been updated.
fig 7 picture is of the 19 and below transfer cases and 6 speed allison transmissions,these used a paper gasket,2020,2021 used o ring seal.