This system is installed on 1973 models manufactured after March 15, 1973 and on many 1974 models to control distributor spark advance. It is basically a refinement of the spark control systems with the temperature switch relocated in the air cleaner and a latching relay added to maintain a strong vacuum signal to the distributor, whether it be EGR port or spark port carburetor vacuum, and to keep the system from intermittently switching vacuum signals when the intake air is 49-60ºF (9-16ºC).
When the temperature switch closes at 60ºF (16ºC), the latching relay (normally off) is energized and stays on until the ignition switch is turned OFF . The latching relay then overrides the temperature switch and forces the solenoid valve to keep the spark port vacuum system closed and open the EGR port vacuum system. This prevents full vacuum advance, once the engine is warmed-up, thereby lowering emissions.