Hummer 2008 H2 Owner's Manual page 116

SECOND (2): This position reduces vehicle speed
without using the brakes. Use SECOND (2) on hills.
It can help control your speed as you go down steep
mountain roads, but then you may also want to use the
brakes off and on.
FIRST (1): This position reduces vehicle speed without
using the brakes. Use it for major/severe downgrades
and off-road driving where the vehicle would otherwise
accelerate due to steepness of grade.
When you shift to SECOND (2) or FIRST (1) it provides
the lowest gear appropriate to your current road
speed and continues to downshift as the vehicle slows,
eventually downshifting to the selected gear.
Your 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
will be able to maintain vehicle speed by analyzing
things such as vehicle speed, throttle position and
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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 may appear to be
a delayed shift, however the transmission is operating
normally.
Your vehicle's transmission uses adaptive shift controls
that 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 temperature changes. During this adaptive
shift controls process, shifting may feel different as the
transmission determines the best settings.
When temperatures are very cold, the Hydra-Matic
6L80 automatic transmission's gear shifting may
be delayed providing more stable shifts until the engine
warms up. Shifts may be more noticeable with a cold
transmission. This difference in shifting is normal.
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