Chevrolet Nova/ChevyII 1962-1979

Transmission Controlled Spark System



Click image to see an enlarged view

Fig. Fig. 1 Transmission Controlled Spark (TCS) system schematic

Click image to see an enlarged view

Fig. Fig. 2 Common TCS system installation and related components

Click image to see an enlarged view

Fig. Fig. 3 Common Combination Emission Control (CEC) system

This system, introduced in 1970, controls exhaust emissions by eliminating vacuum advance in the lower forward gears.

The 1970 system consists of a transmission switch, solenoid vacuum switch, time delay relay, and a thermostatic water temperature switch. The solenoid vacuum switch is de-energized in the lower gears via the transmission switch and closes off distributor vacuum. The transmission switch is activated by the shifter shaft, on manual transmissions, or by oil pressure, on automatic transmissions. The switch energizes the solenoid in high gear, the plunger extends and uncovers the vacuum port, and the distributor receives full vacuum. The temperature switch overrides the system until the engine temperature reaches 82°F. This allows vacuum advance in all gears, thereby preventing stalling after starting. A time delay relay opens 15 seconds after the ignition is switched on. Full vacuum advance during this delay eliminates the possibility of stalling.

The 1971 system is similar, except that the vacuum solenoid, now called a Combination Emissions Control solenoid, (CEC) serves two functions. One function is to control distributor vacuum; the added function is to act as a deceleration throttle stop in high gear. This cuts down on emissions when the vehicle is coming to a stop in high gear. Two throttle settings are necessary; one for curb idle and one for emission control on coast. Both settings are described in the tune-up section.

The 1972 6-cylinder system is similar to that used in 1971, except that an idle stop solenoid has been added to the system. In the energized position, the solenoid maintains engine speed at a predetermined fast idle. When de-energized, the solenoid allows the throttle plates to close beyond the normal idle position; thus cutting off the air supply and preventing engine run-on. The six is the only 1972 engine with a CEC valve, which serves the same deceleration function as in 1971. The time delay relay delays full vacuum 20 seconds after the transmission is shifted into high gear. A vacuum advance solenoid, similar to that used in 1970, is used on V8 engines. The solenoid controls distributor vacuum advance and performs no throttle positioning function. The idle stop solenoid used operates in the same manner as the one on 6-cylinder engines. All air-conditioned cars have an additional anti-diesel (run-on) solenoid which engages the compressor clutch for three seconds after the ignition is switched off. The 1973 TCS system differs from the 1972 system in three ways. The 23 second upshift delay has been replaced by a 20 second starting relay. This relay closes to complete the TCS circuit and open the TCS solenoid, allowing vacuum advance, for 20 seconds after the key is turned to the ON position. The operating temperature of the temperature override switch has been raised to 93°F, and the switch which was used to engage the A/C compressor when the key was turned OFF has been eliminated. All models are equipped with an electric throttle control solenoid to prevent run-on. The 1973 TCS system is used on all models equipped with a 307 cu in. V8 engine and all V8 models equipped with a manual transmission.

The 1974 TCS system is used only on manual transmission models. System components remain unchanged from 1973. The vacuum advance solenoid is located on the coil bracket. The TCS system is not used on 1975 and later models.