The lock cylinder in a GMC truck is one of the main components of the ignition system, and it is sometimes confused with the ignition switch. The cylinder, which is connected to the actual ignition switch within the steering column, can be removed and replaced, but changing the ignition switch will mean replacing the key that starts the vehicle. The exact method of changing the lock cylinder can differ depending on the exact year and model of your GMC vehicle.
Disconnect the vehicle's negative battery cable; it is usually clamped into place, so you must loosen the clamp nut with a wrench.
Detach the trim covers on the steering column by removing the screws on the lower cover with a screwdriver. Remove the driver's side knee bolster by removing its screws.
Insert the ignition key into the lock cylinder and turn it to the "Run" or "Start" position.
Push a small Phillips head screwdriver or similar tool into the retaining hole at the top of the lock cylinder housing. Pull the ignition key and lock cylinder out of the housing together.
Install the replacement cylinder with its key inserted into the housing, making sure its retainer aligns with the retainer slot in the housing; the cylinder needs to be installed in the "Start/Run" position. Push in the cylinder so the retainer snaps into position.
Turn the cylinder into the "Lock" position and remove the key.
Reconnect the trim covers and the negative battery cable.
