2009 GMC Sierra 1500 Crew Cab Owner's Manual - Page 226

Readings outside the normal operating range can also 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. As engine speeds are increased, this condition should correct itself as higher engine speeds allow the charging system to create maximum power. The vehicle can only be driven for a short time with the readings outside the normal operating range. If the vehicle must be driven, turn off all accessories, such as the radio and air conditioner. Readings outside the normal operating range indicate a possible problem in the electrical system. Have the vehicle serviced as soon as possible. Brake System Warning Light With the ignition on, the brake system warning light comes on when the parking brake is set. If the vehicle is driven with the parking brake engaged, a chime sounds when the vehicle speed is greater than 5 mph (8 km/h). The vehicle's hydraulic brake system is divided into two parts. If one part is not working, the other part can still work and stop the vehicle. For good braking, both parts need to be working well. If the warning light comes on and a chime sounds there could be a brake problem. Have the brake system inspected right away. This light can also come on due to low brake fluid. See Brakes on page 5-41 for more information. United States Canada 3-42