2009 GMC Acadia Owner's Manual - Page 160

3-38 Instrument Panel 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. If there is a problem with the battery charging system, a SERVICE BATTERY CHARGING SYSTEM message will appear in the Driver Information Center (DIC) and/or the charging system light will come on. See DIC Warnings and Messages on page 3-58 and Charging System Light on page 3-37 for more information. However, readings in either warning zone may indicate a possible problem in the electrical system. Have the vehicle serviced as soon as possible. Voltmeter Gage Brake System Warning Light This 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. If the warning light comes on, there is a brake problem. Have the brake system inspected right away. When the engine is not running, but the ignition is turned on, this gage shows the battery's state of charge in DC volts. When the engine is running, this gage shows the condition of the charging system. The vehicle's charging system regulates voltage based on the state of charge of the battery. The voltmeter may fluctuate. This is normal. Readings between the low and high warning zones indicate the normal operating range. United States Canada This light should come on briefly when the ignition key is turned to ON/RUN. If it does not come on then, have it fixed so it will be ready to warn if there is a problem.