2004 GMC Envoy Owner's Manual - Page 160

Voltmeter Gage When your engine is not running, but the ignition is on (in RUN), this gage shows your battery's state of charge in DC volts. As engine speeds are increased, this condition should correct itself as higher engine speeds allow the charging system to create maximum power. 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. Brake System Warning Light 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 idling for an extended period. This condition is normal since the charging system is not able to provide full power at engine idle. When the ignition is on, the brake system warning light will come on when you set your parking brake. The light will stay on if your parking brake doesn't release fully. If it stays on after your parking brake is fully released, it means you have a brake problem. A chime may also sound when the light comes on. Your vehicle's hydraulic brake system is divided into two parts. If one part isn't working, the other part can still work and stop you. For good braking, though, you need both parts working well. 3-40