GMC 2008 Envoy - Manual (Page 171)

Voltmeter Gage
When your engine is not
running, but the ignition is
on, this gage displays the
battery voltage in DC volts.
When the engine is running, the gage shows the
condition of the charging system. The gage may
transition from a higher to lower or a lower to higher
reading, this is normal. Readings between the low
and high warning zones indicate the normal operating
range. The voltmeter gage may also read lower when
in fuel economy mode. This is normal.
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. 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.
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