C 3500 Truck 2WD V8-393 6.5L DSL VIN P (1995)
Vacuum Gauge/Economy Meter: 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 the switch opens when vacuum pressure, rather than oil pressure, is applied.