GMC 2009 Sierra - Manual (Page 279)

Your vehicle is much more likely to stall when going
uphill. But if it happens when going downhill:
1. Stop your vehicle by applying the regular brakes.
Apply the parking brake.
2. Shift to PARK (P) and, while still braking, restart the
engine.
3. Shift back to a low gear, release the parking brake,
and drive straight down.
4. If the engine will not start, get out and get help.
Driving Across an Incline
Sooner or later, an off-road trail will probably go across
the incline of a hill. If this happens, you have to
decide whether to try to drive across the incline. Here
are some things to consider:
{ CAUTION:
Driving across an incline that is too steep will
make your vehicle roll over. You could be
seriously injured or killed. If you have any
doubt about the steepness of the incline, do
not drive across it. Find another route instead.
• A hill that can be driven straight up or down
may be too steep to drive across. When you
go straight up or down a hill, the length of the
wheel base — the distance from the front wheels
to the rear wheels — reduces the likelihood the
vehicle will tumble end over end. But when
you drive across an incline, the much more narrow
track width — the distance between the left and
right wheels — may not prevent the vehicle
from tilting and rolling over. Also, driving across
an incline puts more weight on the downhill wheels.
This could cause a downhill slide or a rollover.
• Surface conditions can be a problem when you
drive across a hill. Loose gravel, muddy spots,
or even wet grass can cause the tires to slip
sideways, downhill. If the vehicle slips sideways,
it can hit something that will trip it — a rock, a rut,
etc. — and roll over.
• Hidden obstacles can make the steepness of the
incline even worse. If you drive across a rock with the
uphill wheels, or if the downhill wheels drop into a rut
or depression, your vehicle can tilt even more.
For reasons like these, you need to decide carefully
whether to try to drive across an incline. Just because the
trail goes across the incline does not mean you have to
drive it. The last vehicle to try it might have rolled over.
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