Chevrolet 2009 Suburban Owner's Manual page 141

The vehicle has a shift stabilization feature that adjusts
the transmission shifting to the current driving conditions
in order to reduce rapid upshifts and downshifts. This shift
stabilization feature is designed to determine, before
making an upshift, if the engine is able to maintain vehicle
speed by analyzing things such as vehicle speed, throttle
position, and vehicle load. If the shift stabilization feature
determines that a current vehicle speed cannot be
maintained, the transmission does not upshift and instead
holds the current gear. In some cases, this could appear
to be a delayed shift, however the transmission is
operating normally.
The vehicle's transmission uses adaptive shift controls.
Adaptive shift controls continually compares key shift
parameters to pre-programmed ideal shifts stored in the
transmissions computer. The transmission constantly
makes adjustments to improve vehicle performance
according to how the vehicle is being used, such as with
a heavy load or when the temperature changes. During
this adaptive shift control process, shifting might feel
different as the transmission determines the best
settings.
When temperatures are very cold, the Hydra-Matic
6-Speed transmission's gear shifting could be delayed
providing more stable shifts until the engine warms
up. Shifts could be more noticeable with a cold
transmission. This difference in shifting is normal.
M (Manual Mode): This position is available on
vehicles with the Hydra-Matic
It lets drivers select the range of gears appropriate
for current driving conditions. If the vehicle has
this feature, see Range Select Mode (Hydra-Matic
6-Speed transmission) later in this section.
3 (Third): This position is also used for normal driving.
It reduces vehicle speed more than D (Drive) without
using the brakes. You might choose 3 (Third) instead of
D (Drive) when driving on hilly, winding roads, when
towing a trailer, so there is less shifting between gears
and when going down a steep hill.
2 (Second): This position reduces vehicle speed
even more than 3 (Third) without using the brakes.
You can use 2 (Second) on hills. It can help control
vehicle speed as you go down steep mountain roads,
but then you would also want to use the brakes off
and on.
If you manually select 2 (Second) in an automatic
transmission, the transmission will start in second gear.
You can use this feature for reducing the speed of
®
the rear wheels when you are trying to start the vehicle
from a stop on slippery road surfaces.
®
6-Speed transmission.
®
2-35