1979 Models
When transmission is shifted into high gear, the top gear detecting switch and overdrive detecting switch (if equipped) on the transmission send an electrical signal to the vacuum switching valve. This signal activates the vacuum switching valve, closing the vent passage and allowing full vacuum to the distributor vacuum advance unit.
In all other gear positions, the vacuum switching valve interrupts the vacuum normally directed to the vacuum advance unit and, at the same time, opens the vent passage so that vacuum can be released. On 210 (except 5-Speed Hatchback) and 310 models, a thermal vacuum valve is added to this system to allow full vacuum advance when engine coolant temperature is below 41-64°F (5-18°C), or above 122-145°F (50-63°C).