It is very unusual for the new 8 speeds to have slippage and if so it will set error codes showing exactly which clutch packs are involved. These are clutch to clutch architecture transmissions, not anything like the older band type automatics which could have destructive slipping bands. The same clutch packs used in combination in the first three gears are also used in other ranges so slippage should also be occurring there if a clutch pack or its actuator is defective. This same is true of the prior 6L series used in earlier years of the ATS along with many other GM vehicles. I believe Chrysler was the last major manufacturer to use band type automatics.
These clutch to clutch architecture transmissions work by very accurately controlling apply/release pressure for the various clutch packs when shifting ranges and speed is accurately measured at the input, intermediate, and output shafts during shifts. If a range mismatch occurs (which happens if a pack is slipping), the apply pressure will be greatly increased which you will feel and if that doesn't prevent the slippage, an error code will be set and in severe cases the transmission will lock into its safe range in drive rather than trying to shift gears.
If this diagnosis wasn't done by a GM dealership, it is likely the diagnosis wasn't done properly.
The feeling of slippage in multiple ranges of the 8L45/8L90 transmission is most commonly due to either a seriously overfilled transmission leading to foaming of the fluid which won't allow proper clutch pack activator function or some undesirable transmission additive is in place. But in any case where the transmission had sustained slippage not correctable by the TCM algorithm, then the check engine light will be on and a P0700 code will be set indicating that further DTCs are stored within the transmission controller.
A newly installed transmission will be in learning mode as it adapts to operation. Solenoids within the transmission control timing and force of the clutch pack actuators and it has to "learn" proper timing and force by comparing actual to desired shift characteristics and will adjust quickly so that actual coincides with desired.
Rodger
These clutch to clutch architecture transmissions work by very accurately controlling apply/release pressure for the various clutch packs when shifting ranges and speed is accurately measured at the input, intermediate, and output shafts during shifts. If a range mismatch occurs (which happens if a pack is slipping), the apply pressure will be greatly increased which you will feel and if that doesn't prevent the slippage, an error code will be set and in severe cases the transmission will lock into its safe range in drive rather than trying to shift gears.
If this diagnosis wasn't done by a GM dealership, it is likely the diagnosis wasn't done properly.
The feeling of slippage in multiple ranges of the 8L45/8L90 transmission is most commonly due to either a seriously overfilled transmission leading to foaming of the fluid which won't allow proper clutch pack activator function or some undesirable transmission additive is in place. But in any case where the transmission had sustained slippage not correctable by the TCM algorithm, then the check engine light will be on and a P0700 code will be set indicating that further DTCs are stored within the transmission controller.
A newly installed transmission will be in learning mode as it adapts to operation. Solenoids within the transmission control timing and force of the clutch pack actuators and it has to "learn" proper timing and force by comparing actual to desired shift characteristics and will adjust quickly so that actual coincides with desired.
Rodger
