The GMC Sierra has an alternator that sends electrical current to various devices in the vehicle while the engine is running. Most alternators made today include a voltage regulator that ensures the voltage level remains constant. You may need to replace the alternator if your battery keeps running down or your headlights become dimmer while driving.
Determine which alternator your GMC Sierra will need. You'll want to be sure the new alternator is compatible with your Sierra's engine. Models are equipped with a Vortec 4.3 L, 4.8 L, 5.3 L or 6.0 L engine, depending on the exact model.
Disconnect the negative battery cable and remove the accessory belt(s) with a socket wrench. Use a socket wrench to remove the nut on the alternator that attaches the battery feed wire to the alternator and remove the feed wire.
Remove the mounting bolts from the alternator with a socket wrench. You may also need to remove a mounting bracket with a socket wrench on some models. This will allow you to remove the alternator assembly.
Install the new alternator assembly by reversing the removal procedure. Tighten the bolt on the bracket (if present) and the right mounting bolt to 18 foot pounds with a torque wrench. Torque the left mounting bolt to 37 foot pounds and attach the battery feed wire to the stud on the alternator. Torque the nut for the battery feed wire to 71 inch pounds.
Install the accessory belt(s) and connect the negative battery cable with a socket wrench.
