Electronic spark selection is used on 1978 Cadillac Sevilles along with all 1979 Seville, Eldorado, Limousine, Commercial Chassis and carbureted standard Cadillac models. In 1980 it can be found on most Cadillac models except diesels. The system advances or retards ignition timing according to conditions. Timing is retarded during starting to reduce the load on the starter. By delaying ignition until the piston is nearly at TDC, the piston is not forced downward prematurely. Timing is also retarded on California cars when coolant temperature is below 130°F. This reduces catalyst warm-up time. Spark timing is advanced during high engine vacuum/high engine rpm conditions (highway cruise) to increase efficiency and fuel economy.
Components used in addition to the G.M. HEI system are an electronic decoder and a five-pin distributor module. Some 1980 models use a seven-pin distributor module. The HEI pick-up coil sends its signal to the decoder to provide engine speed and ignition timing information. The decoder signal sends its information on through the connector. The signal either delays or does not delay coil primary current shut down. Coolant temperature on California models is sensed at the EGR solenoid except on 1980 carbureted models, which are equipped with a three-way coolant temperature switch instead. This system is not serviceable without special diagnostic tools. This system was not used after 1980.