2004 GMC Yukon Owner's Manual - Page 202

Battery Warning Light If this light is displayed when the engine is running, you may have a problem with your charging system. Voltmeter Gage When your engine is not running, but the ignition is in RUN, this gage shows your battery's state of charge in DC volts. The battery display will also stay on while the key is in RUN until the engine is started. If the light stays on after starting the engine it could indicate a problem with the generator drive belt, or some other charging system problem. Have it checked right away. Driving with this light displayed could drain your battery. If you must drive a short distance with this light displayed, it helps to turn off all your accessories, such as the radio and the air conditioner. 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 system to create maximum power. 3-44