GM Service Manual Online
For 1990-2009 cars only

All column shift equipped vehicles contain a mechanical and an electrical neutral safety start system. The mechanical block and the electrical switch prevent the starting of the engine in any gear other than PARK or NEUTRAL. The mechanical block is a wedge-shaped lock bolt added to the ignition switch actuator rod. The wedge-shaped lock bolt passes through slots in the shift bowl and allows the lock cylinder to turn and start the engine.