GMC 1999 Sierra 1500 Pickup Owner's Manual page 150

Features & Controls
"Engine Coolant'' in the Index and have your vehicle serviced as soon as
you can.
Charging System Light
K2328
The red charging system light is located at the upper right hand corner of
your instrument cluster. It should come on briefly when you turn on the
ignition, before starting the engine, as a check to show you it is working.
After the engine starts, the light should go out. If it stays on, or comes on
while you are driving, you may have a problem with your electrical charging
system. It could indicate that you have a loose alternator drive belt, or that
you have some other electrical problem. Have it checked right away. Driving
while this light is on could drain your battery.
If you must drive a short distance with the light on, be certain to turn off all
your accessories, such as the radio and air conditioner.
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