Description

Fig. 75 Transmission controlled spark timing control system.:

NOTE: Refer to Fig. 75 for typical spark timing control systems. Complete system schematics are located in the "Vacuum Hose Routing" section.


The spark timing control system is designed to control distributor vacuum advance in order to reduce HC and NOx emissions. Venturi vacuum is routed to a thermal vacuum valve and through a delay valve to the distributor advance unit. The vacuum delay valve provides a delay in the advance when the vacuum signal increases. Due to the one way construction of the valve, the advance unit responds normally when the vacuum signal decreases. The thermal vacuum valve detects engine coolant temperature and during certain temperature ranges, vents the vacuum advance signal to the atmosphere, reducing vacuum advance.