The Speed Controlled Spark (SCS) system allows either ignition advance or retard depending on vehicle speed. Generally, this system reduces advance during low speed operation but allows full advance at higher speeds. This system operates on the vacuum advance or retard unit. The SCS vacuum solenoid valve is controlled by a temperature sensor mounted in the intake manifold water passage and by an engine speed switch. When coolant temperature is below 133°F (56°C), or if the engine speed is over 3000 rpm, the solenoid valve closed vacuum flow to vacuum control valve and to first stage of the EGR valve. This prevents the EGR system from operating when the engine is cold, improving driveability and cold response.
TESTING THE SPEED SWITCH
With the engine running and at operating temperature, connect a test lamp to the disconnected plug of the solenoid valve. The test lamp should be on at engine speeds over 3000 rpm. If not, replace the speed switch.