Speed Controlled Spark: Description

Road Speed Controlled Spark Advance (SCS) System was first introduced in 1971 California light duty vehicles. In 1972 all states incorporated SCS system. Purpose of system is to reduce level of carbon monoxide (CO) and oxides of nitrogen (NOx) by delaying vacuum spark advance until a preset vehicle speed is reached (34±2 MPH). SCS system consists of solenoid vacuum valve, speed sensor unit (centrifugal switch), low temperature vacuum control valve, high temperature vacuum control valve (all V8's except 400" V8).