Ignition System: Description and Operation

OPERATION
All of these vehicles use only manifold vacuum for distributor spark advance. When coolant temperature is below 120°F, a delay valve maintains the vacuum advance during sudden throttle openings. When coolant temperature is above 120°F, the delay valve is bypassed by the spark (or 1982 vehicles, vacuum advance) coolant temperature override (CTO) valve.

In addition 1980 Calif. vehicles incorporate a thermal vacuum spark control valve located in the air cleaner. When air cleaner intake air is below 63°F, the valve directs manifold vacuum to port 1 of the CTO valve and distributor vacuum advance mechanism.