what causes the engine temperature light to light up on a 1971 chevy chevelle

A lot of these older engines run warm and benefit from a after market radiator, and shroud with extra flumes and shroud. Running rich. Oil passageways clogged, carbon build up on internal parts, oil pump screen clogged , bends in exhaust, passageways in radiator clogged up, Automatic transmission cooling in the radiator raises temp, may add a external trans oil cooler, thermostat is rated for to high a temp and needs a lower temp rating, May need a new thermostat, Change radiator hoses over to heavy duty hoses and or racing style plumbed pipes with heat risers, put on a new or racing high performance water pump, Make sure air filter is clean, All plugs & wires are good, If PCV is in use make sure it is clean or absolute crankcase manifold, And all vacuum lines are good and engine does have vacuum, Valves need carbon cleaned off, High Idle, Wrong cam for everything else installed, Carb too big, Jets to big, Head & intake gaskets shot, Thick hard carbon built up under the intake manifold, wrong type of fuel, I am sure I could think of some more with more time.

If it has under drive pulleys on it go back to stock size.