GMC Sierra Owners Manual
This gauge indicates the battery voltage when the ignition is turned on.
When the ignition is on, this gauge indicates the battery voltage.
When the engine is running, this gauge shows the condition of the charging system. The gauge can transition from a higher to lower or a lower to higher reading. This is normal. If the vehicle is operating outside the normal operating range, the charging system light comes on.
See Charging System Light on page 5‑24 for more information. The voltmeter gauge may also read lower when in fuel economy mode.
This is normal.
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.