GM builds vehicles with five and six-speed automatic transmissions. This part of the manual covers the basics of these. With these transmissions, you will find an Allison® Transmission Operator's Manual in your vehicle that goes into more detail. On the headliner, in front of and above the driver, or in some other place near the driver, you will see a label that describes important operating facts about the automatic transmission in your vehicle. Make sure you follow the instructions on this label.

Caution: It can be dangerous to get out of the vehicle without the parking brake firmly set. The vehicle can roll. If you have left the engine running, the vehicle can move suddenly. You or others could be injured. To be sure the vehicle will not move, even when you are on fairly level ground, always set the parking brake.

Caution: Shifting into a drive gear while the engine is running at high speed is dangerous. Unless your foot is firmly on the brake pedal, the vehicle could move very rapidly. You could lose control and hit people or objects. Do not shift into a drive gear while the engine is running at high speed.

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 overdrive defeat button is located in the instrument panel to the right of the steering column. See Instrument Panel Overview for more information.

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.