GMC 2003 GMT560 T-Series Owner's Manual page 53

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CAUTION:
Shifting into a drive gear while your engine is
"racing" (running at high speed) is dangerous.
Unless your foot is firmly on the brake pedal,
your vehicle could move very rapidly. You
could lose control and hit people or objects.
Don't shift into a drive gear while your engine
is racing.
Automatic Transmission Third
Gear Hold
If your vehicle has this feature, your range selector has
a 2–3 position. You can't shift up to the next higher
gear when the transmission is in this position unless
your engine-governed speed is exceeded, such as when
going downhill.
This feature is used for certain load and traffic conditions
or when plowing snow. See the Allison
Operator's Manual in your vehicle for more information
about this.
Overdrive Defeat
If your vehicle has this feature, you can select to limit
the transmission to fourth gear. This may be useful when
operating at a higher Gross Vehicle Weight (GVW) or
where operating conditions do not require fifth gear.
The indicator light in the switch will come on whenever
overdrive defeat is active.
®
Transmission
The overdrive defeat
switch is located in the
instrument panel to
the right of the steering
column.
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