Description & Operation: TCS/EGR

When ignition is "ON", voltage is supplied through time delay relay switch to distributor vacuum solenoid and to one terminal of EGR solenoid. Both solenoids are now ready to receive ground signal. Signal is provided either by thermal override switch or transmission (controlled spark) switch. If signal is from transmission switch, time delay relay waits 33-55 seconds (depending on engine) to complete circuit to both solenoids. This provides full vacuum advance and no EGR. On typical cold start (coolant temperature below 71 °F and cylinder head temperature below 125°F), thermal override switch grounds both solenoids, resulting in no EGR and full vacuum advance. When coolant temperature reaches 71 °F, thermal override switch de-energizes both solenoids, resulting in full EGR and no vacuum advance. Both solenoids remain de-energized until cylinder head temperature reaches 125°F. Thermal delay switch then grounds, completing circuit between transmission switch and time delay relay. Both solenoids become energized 33-55 seconds (depending on engine) after transmission shifts into second gear or intermediate gear range, resulting in no EGR and normal vacuum advance. When coolant temperature reaches 235°F (hot idle), thermal override switch energizes both solenoids, resulting in no EGR and full vacuum advance for maximum cooling.