V 3500 P/U 4WD V8-379 6.2L DSL (1988)
Fuel Economy Warning System: Description and Operation
This system actually monitors the engine vacuum just like the vacuum gauge, but registers only low vacuum readings. The light on the instrument
panel warns the vehicle operator when engine manifold vacuum drops below the economical limit. Switch operation is similar to that of the oil pressure
indicating light, except that the switch opens when vacuum pressure, rather than oil pressure, is applied.