GMC 2003 C4C042 Owner's Manual page 153

Charging System Indicator Light
It should come on briefly when you turn on the ignition,
before the engine is running, as a check to show that it
is working.
If it stays on, or comes on while you are driving, have it
checked right away. You could have a loose generator
drive belt or some other problem.
Driving while this light is on will drain your battery. If
you must drive a short distance with the light on, turn
off your radio and other accessories.
Sustained driving with a generator failure could result in
-
a lack of back up braking if the engine quits or the
power steering pump should fail.
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The charging system
indicator light may be
located in the instrument
panel cluster, or with the
auxiliary warning lights
in the center of the
instrument panel.
Voltmeter
Readings in either warning zone indicate a possible
problem in the electrical system.
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 at an idle for an extended
period. This condition is normal since the charging
system may not be able to provide full power at engine
idle. As engine speeds are increased, this condition
should correct itself because higher engine speeds allow
the charging system to create maximum power.
You can only drive a short time on the battery. Be sure
to shut off the radio, the fan or any unnecessary
accessories and have the system serviced.
When your engine is
running, the voltmeter
gage, at the right side of
the instrument panel cluster,
shows the charging
system voltage.