Spark Timing Control (STC) System

Fig. 416 - Spark Timing Control System:

Vacuum Advance Mechanism
This system, Fig. 416, is controlled in two stages, warm-up and normal driving, to obtain good fuel economy and quick warm-up of the catalyst. The thermal vacuum valve monitors coolant temperature and activates system above 131° F.

1. Disconnect gray harness connector from distributor and check that vacuum hoses are properly connected and distributor vacuum controller functions normally, then connect timing light.
2. Start engine in cold condition and note timing, then allow engine to reach normal operating temperature. Timing should advance.
3. If timing does not change, check that thermal vacuum valve is open below or closed above 131° F.

Fig. 417 - Electrical Advance System:

Electric Advance System, Fig. 417

Note: When coolant temperature is below 95° F., or the throttle valve is closed, spark timing is advanced electrically.

1. Start engine when cold and check timing.
2. Stop engine and disconnect lead wire between terminals No. 24 and No. 30 of throttle valve switch harness connector.
3. Start engine and recheck timing. Timing should read the same as in step 1.
4. Warm up engine. Timing should retard as engine warms up.
5. Stop engine and connect throttle valve switch harness connector. Timing should advance.
6. If timing does not respond as described, check throttle valve switch and water temperature switch.
7. Drain about 1 1/8 quarts of coolant and remove switch.
8. Dip switch in water and check that it has continuity below 95° F and no continuity above 95° F.