This system is equipped only on 1975-76 models.
This system is used on all 1975-76 (manual transmission) models, except California models. The system consists of a transmission mounted electrical switch and a vacuum switching valve. The switch functions to limit vacuum advance to operate in 4th gear only. The vacuum switching valve is energized in all gears except 4th and vents vacuum from the throttle chamber into the atmosphere. When the transmission is shifted into 4th gear, the switch breaks the circuit and the vacuum valve is de-energized and vacuum reaches the advance unit on the distributor.