Voltmeter Gauge; Brake System Warning Light - GMC 2010 Sierra 1500 Crew Cab Owner's Manual

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
for more information. The voltmeter gauge
may also read lower when in fuel economy mode. This
is normal.
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For vehicles with a
voltmeter gauge, this
gauge indicates the
battery voltage when the
ignition is turned on.
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.
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.