Ignition System: Description and Operation

Fig. 411 - Spark Timing Control System:

Fig. 412 - Spark Timing Control System:

System is designed to reduce HC and NOx emissions by controlling distributor vacuum advance.
- Venturi vacuum is routed--to a thermal vacuum valve--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.