2009 GMC Sierra 1500 Crew Cab Owner's Manual - Page 225

Charging System Light This light comes on briefly when the ignition key is turned to START, but the engine is not running, as a check to show it is working. Voltmeter Gage (US-Canada) For vehicles with a voltmeter gage, this gage indicates the battery voltage when the ignition is turned on. If it does not, have the vehicle serviced by your dealer/retailer. The light should go out once the engine starts. If it stays on, or comes on while driving, there could be a problem with the charging system. A charging system message in the Driver Information Center (DIC) can also appear. See DIC Warnings and Messages on page 3-66 for more information. This light could indicate that there are problems with a generator drive belt, or that there is an electrical problem. Have it checked right away. If the vehicle must be driven a short distance with the light on, turn off accessories, such as the radio and air conditioner. When the ignition is on, this gage indicates the battery voltage. When the engine is running, this gage shows the condition of the charging system. The gage can transition from a higher to lower or a lower to higher reading. This is normal. If the vehicle is operating outside the normal operating range, the charging system light comes on. See Charging System Light on page 3-41 for more information. The voltmeter gage may also read lower when in fuel economy mode. This is normal. 3-41