The battery on a GMC Sierra is responsible for starting the engine. Once the engine is running the alternator takes over, providing enough electrical power to maintain the battery and power the Sierra's lights, air conditioning and any other electrically powered component. If the alternator fails, the vehicle will be running on battery power alone. This will not last very long, especially if you are running the headlights. If you have a charging problem, check the charging circuit components and wiring before you check the alternator.
Open the hood and locate the battery in the front driver's side of the engine compartment. Check the battery terminals and cable clamps. Make sure they are tight, clean and free of any white, flaky corrosion, which can be removed with a wire brush or emery paper.
Check all the external wiring on the alternator and make sure the connections are tight and clean.
Check the alternator mounting bolts. They should be tight and secure.
Check the condition of the drive-belt. Cracks on the "V" portion of the belt are normal, but if the belt looks very worn or is missing ribs, replace it. The belt is tensioned by a spring-loaded tension assembly. If the indexing arrow is lined up with the indexing mark on the tension assembly, replace the belt.
Start the engine and check the alternator for abnormal noise.
With the engine off, attach the probes of a voltmeter to the positive and negative terminals on the battery. The red probe goes to the red (positive) terminal and the black probe to the black (negative) terminal.
Take a reading from the voltmeter. If the battery is fully charged it should register between 12.4 and 12.6 volts.
Start the engine and perform the test again in the same way. This time the reading should be higher. Up to 14.7 volts is normal. Higher or lower readings could indicate a malfunction on the alternator.
Take the vehicle to a dealer service department. They will attach a scanner to the on-board computer which will generate a problem code to diagnose the problem more specifically.
