Ignition System: Description and Operation

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 controls the distributor vacuum advance at idle to ensure stable idling and good fuel economy. The system utilizes a water temperature switch, located in the radiator, to monitor coolant temperature, the neutral sensing switch at the transmission, and a vacuum switching valve. The vacuum switching valve receives electrical signals from the water temperature and neutral switches, and controls the vacuum signal to the distributor according to coolant temperature and transmission position. When engine is idling and transmission switch is on, vacuum advance only occurs when coolant temperature is 50-203'F, causing water temperature switch to open. With water temperature switch open, the ground contact in the circuit is broken and the vacuum switching valve is not energized. When coolant temperature is too high or too low, the contacts in the water temperature switch are closed. This allows the transmission switch to energize the vacuum switching valve, which vents the distributor vacuum signal to the atmosphere.