Servicing a harsh GM shift
A harsh 1-2 shift following transmission service may be experienced in GM vehicles equipped with the 4L60E, 4L65E or 4L70E transmissions.
Vehicle applications include 2007-2012 Chevy Avalanche, Colorado, Express and Silverado, 2007-2009 Chevy Suburban, Tahoe and Trailblazer, 2007-2012 GMC Canyon, Savana, Sierra and Sierra Denali, 2007-2009 GMC Envoy, Yukon, Yukon XL and Yukon XL Denali, 2007-2010 Hummer H3 and H3T, and 2007-2009 Saab 9-7X and 9-7X Aero.
The issue involves the 1-2 accumulator spring. When inspecting the 1-2 accumulator assembly while diagnosing a condition of “Harsh 1-2 Shift,” it is important to note that all transmission models use an inner spring (104), but many models do not use an outer spring (54).
The 1-2 accumulator piston / spring assembly is transmission-model-specific and may make use of only an inner spring or may use both an inner and outer spring. The most accurate and up-to-date source of information on how the accumulator assembly is built is available under Section Transmission/Transaxle Automatic Transmission – 4L60-E/4L65-E/4L70-E - Specifications - 1-2 Accumulator Piston Spring Selection in SI.
NOTE: Do Not install an outer spring (54) in a transmission built with only an inner spring (104). Adding the outer spring may cause additional harsh 1-2 shifts and/or delayed 2-1 downshifts.