GMC 2010 T-Series Owner's Manual page 58

Press the button to turn on the overdrive defeat. Press
the button again to turn off the overdrive defeat. The
indicator light in the button will come on whenever
overdrive defeat is active.
Automatic Transmission
Third-Gear Hold
If the vehicle has this feature, the range selector has a
2-3 position. You cannot shift up to the next higher gear
when the transmission is in this position unless the
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
Transmission Operator's Manual in the vehicle for more
information about this.
2-18
®
If the vehicle is equipped
with a push button shift
selector, refer to the
®
Allison Transmission
Operator's Manual for
shift selector operating
instructions.
Manual Transmission Operation
Using the Clutch
When you are starting to move the vehicle, it is
important to begin with the engine speed at idle. Then
start to engage the clutch and listen for an engine
speed drop of about 100 rpm. At this point, the clutch is
engaging, so you should increase the engine speed and
fully engage the clutch.
Notice: If you increase the engine speed before the
clutch is fully engaged, you could damage your
transmission. Always allow the clutch to fully
engage before increasing engine speed.
Six-Speed Transmission
The transmission has gears that automatically
synchronize when you shift up or down. Choose the
gear which will maintain the road speed you want while
keeping the engine above two-thirds of the governed
speed. When the engine speed drops below two-thirds
of the governed speed, shift into the next lower gear
before your engine begins to become sluggish. When
you shift down, be sure to double-clutch if required.
See Double Clutching later in this section.