GM Corvette 1963-1982 Repair Guide

Transmission Controlled Spark (TCS) System

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OPERATION



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

1970 Vehicles See Figure 1

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Fig. Fig. 1: Transmission Controlled Spark (TCS) system schematic-1970 vehicles

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 energized in the lower gears via the transmission switch and closes off distributor vacuum. The two-way transmission switch is activated by the shifter shaft on manual transmissions, and by oil pressure on automatic transmissions. The switch deenergizes 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 when engine temperature is below 63° or above 232°. This allows vacuum advance in all gears. A time delay relay opens 15 seconds after the ignition is switched on. Full vacuum advance during this delay eliminates the possibility of stalling.

1971 Vehicles See Figure 2

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Fig. Fig. 2: Schematic of the TCS system-1971 vehicles

The 1971 system is similar, except that the vacuum solenoid (now called a Combination Emissions Control solenoid) 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. The CEC solenoid is controlled by a temperature switch, a transmission switch, and a 20 second time delay relay. This system also contains a reversing relay, which energizes the solenoid when the transmission switch, temperature switch or time delay completes the CEC circuit to ground. This system is directly opposite the 1970 system in operation. The 1970 vacuum solenoid was normally open to allow vacuum advance and when energized, closed to block vacuum. The 1971 system is normally closed blocking vacuum advance and when energized, opens to allow vacuum advance. The temperature switch completes the CEC circuit to ground when engine temperature is below 82°. Corvettes also have a high temperature terminal on the switch to complete the CEC circuit when coolant temperature reaches 232°. The time delay relay allows vacuum advance (and raised idle speed) for 20 seconds after the ignition key is turned to the ON position. Models with an automatic transmission and air conditioning also have a solid state timing device which engages the air conditioning compressor for three seconds after the ignition key is turned to the OFF position to prevent the engine from running-on. Two throttle settings are necessary; one for curb idle and one for emission control on coast. Both settings are described in the Tune-Up information.

1972-74 Vehicles See Figures 3, 4 and 5

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Fig. Fig. 3: Transmission Controlled Spark (TCS) system schematic-1972 350 engines with hot override on



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Fig. Fig. 4: TCS system schematic-1972 454 engines with hot override on



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Fig. Fig. 5: Schematic of the 1973 4-speed TCS system

A vacuum advance solenoid similar to that used in 1970 is used. The CEC valve is not used. This relay is normally closed to block vacuum and opens when energized to allow vacuum advance. The solenoid controls distributor vacuum advance and performs no throttle positioning function. The 1973-74 TCS system differs from the 1972 system in three ways. The 23 second uplift 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°, and the switch that 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 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.

All 1973-74 Corvette models are equipped with a Thermo-Override system instead of the normal TCS system. This system consists of a three-position temperature switch, which is mounted in the right cylinder head and a two-position vacuum advance solenoid. Three vacuum lines are connected to the solenoid, a ported vacuum line from the carburetor, a vacuum line from the intake manifold, and a vacuum line that runs to the distributor vacuum advance unit. When the engine temperature is between 92°F and 232°F, the temperature switch contacts are open and the vacuum solenoid is deenergized. This causes carburetor-ported vacuum to control the operation of the distributor vacuum advance unit. When the engine temperature is below 93°F or above 232°F, the temperature switch contacts are closed and the vacuum solenoid is energized. This moves the plunger in the solenoid to block the ported vacuum opening and connect manifold vacuum to the distributor. When the engine reaches normal temperature, the temperature switch contacts open and ported vacuum is restored to the distributor. TCS is not used on 1975 and later models.

 
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