2006 GMC Sierra 1500 Pickup Owner's Manual - Page 149

Shifting Out of Park (P) (Automatic Transmission) Your vehicle has an automatic transmission shift lock control system which locks the shift lever in PARK (P) when the ignition is in the LOCK position. You have to fully apply your regular brakes before you can shift from PARK (P) when the ignition is in RUN. See Automatic Transmission Operation on page 2-26 and "Torque Lock" under Shifting Out of Park (P) (Automatic Transmission) on page 2-55. If your vehicle has an Allison Transmission®, the transmission is equipped with a park pawl designed to hold the higher weight of commercial medium duty trucks. Therefore, the Allison Transmission® uses larger parts than other automatic transmissions. These larger parts may sometimes make a clunk noise when the selector lever is moved from DRIVE (D) or REVERSE (R) to PARK (P). This is normal and there is no damage occurring during these shifts. If you cannot shift out of PARK (P), ease pressure on the shift lever and push the shift lever all the way up into PARK (P) as you maintain brake application. Then, move the shift lever into the gear you want. If you ever hold the brake pedal down but still can't shift out of PARK (P), try this: 1. Turn the key to ACCESSORY. There is no shift interlock in this key position. 2. Apply and hold the brake until the end of Step 4. 3. Shift the transmission to NEUTRAL (N). 4. Start the vehicle and then shift to the gear you want. 5. Have the system fixed as soon as you can. 2-55