2003 GMC Sierra 1500 Pickup Owner's Manual - Page 194

Voltmeter Gage You can only drive for a short time with the reading in either warning zone. If you must drive, turn off all unnecessary accessories. Readings in either warning zone indicate a possible problem in the electrical system. Have the vehicle serviced as soon as possible. Up-Shift Light This light is used on some models with manual transmissions. When your engine is not running, but the ignition is in RUN, this gage shows your battery's state of charge in DC volts. When the engine is running, the gage shows the condition of the charging system. Readings between the low and high warning zones indicate the normal operating range. Readings in the low warning zone may occur when a large number of electrical accessories are operating in the vehicle and the engine is left at an idle for an extended period. This condition is normal since the charging system is not able to provide full power at engine idle. As engine speeds are increased, this condition should correct itself as higher engine speeds allow the charging systemto create maximum power. The shift indicator light will help you get the best fuel economy. 3-39