GMC C series 2002 Owner's Manual page 230

Surge Tank (Caterpillar Diesel Engines)
When your engine is cold, the coolant level should be
at the COLD level, or a little higher, on the coolant
recovery tank with the gasoline engine, or at the COLD
FULL mark, visible through the surge tank, with the
diesel engine.
See "Lights, Low Coolant Warning Light" in the Index
for further information.
If this light comes on and
stays on, it means you're
low on engine coolant.
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