GM Chevy Mid-Size Cars 1964-1988 Repair Guide

Transmission Controlled Spark


Introduced in 1970 and used on vehicles up into the 1974 model year, this system controls exhaust emissions by eliminating vacuum advance in the lower forward gears. With the introduction of High Energy Ignition (HEI) systems, the TCS system fell out of favor and was not used in 1975 and later cars.

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 de-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 when engine temperature is below 63°F or above 232°F. 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.

The 1971 system is similar, except that the vacuum solenoid (now called a Combination Emissions Control or CEC 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 delay, 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°F. The time delay relay allows vacuum advance (and raised idle speed) for 200 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.

The 1972 6-cylinder system is similar to that used on 1971, except that an idle stop (anti-dieseling) solenoid has been added to the system. In the energized position, the solenoid maintains engine speed at a predetermined fast idle. When the solenoid is de-energized by turning off the ignition, the solenoid allows the throttle plates to close beyond the normal idle position; thus cutting off their air supply and preventing engine run-on. The 6-cylinder is the only 1972 engine with a C.E.C. valve, which serves the same deceleration function as in 1971. The 1972 time delay relay delays full vacuum 20 seconds after the transmission is shifted into High gear. V8 engines use a vacuum advance solenoid similar to that used in 1970. 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 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 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.


See Figures 1 and 2

If there is a TCS system malfunction, first connect a vacuum gauge in the hose between the solenoid valve and the distributor vacuum unit. Drive the vehicle or raise it on a frame lift and observe the vacuum gauge. If full vacuum is available in all gears, check for the following:

Blown fuse.
Disconnected wire at the solenoid-operated vacuum valve.
Disconnected wire at the transmission switch.
Temperature override switch energized due to low engine temperature.
Solenoid failure.If no vacuum is available in any gear, check the following:
Solenoid valve vacuum lines switched.
Clogged solenoid vacuum valve.
Distributor or manifold vacuum lines leaking or disconnected.
Transmission switch or wire grounded.

The individual components may be tested as follows:

Click image to see an enlarged view

Fig. Fig. 1: Small Block V8 TCS system in the engine OFF mode (Big block V8 and inline systems similar)

Click image to see an enlarged view

Fig. Fig. 2: TCS switch location-turbo hydro-matic 350 shown (most transmissions similar, though side may vary)

Idle Stop Solenoid

This unit may be checked simply by observing it while an assistant switches the ignition on and off. The solenoid's piston should extend further with the current switched on. The unit is not repairable and must be replaced if defective. Before replacing a suspected bad solenoid, make sure it is not the fault of the wiring. A voltmeter may be used to check for voltage at the solenoid connector when the ignition is switched on.

Solenoid Vacuum Valve

Check that proper manifold vacuum is available. Connect the vacuum gauge in the line between the solenoid valve and the distributor. Check the system descriptions above to determine if the valve should be energized or de-energized to allow vacuum. Simulate the proper control conditions by applying or denying 12 volts to the solenoid. If vacuum is still not available, the valve is defective, either mechanically or electrically. The unit is not repairable. If the valve is satisfactory, check the relay next.

A valve of this type may also be checked by attempting to blow are through its passages while the solenoid is energized and while it is de-energized. Air should be allowed to blow through under only 1 of these conditions. If air passes is allowed through or blocked under both conditions, the solenoid valve is faulty.

  1. With the engine at normal operating temperature and the ignition on, ground the solenoid vacuum valve terminal with the black lead. The solenoid should energize (vacuum/no vacuum depending upon application) if the relay is satisfactory.
  3. With the solenoid energized (allowing or denying vacuum, as applicable) as in Step 1, connect a jumper from the relay terminal with the green/white stripe lead to ground. The solenoid should de-energize if the relay is satisfactory.
  5. If the relay worked properly in Steps 1 and 2, check the temperature switch. The relay unit is not repairable.

Temperature Switch

The vacuum valve solenoid should be either de-energized or energized (depending upon application) in order to allow vacuum for distributor advance when the engine is cold. The easiest way to check switch conditions is through continuity tests with the engine cold and the engine warmed. The switch should open or close (again, depending upon application) once a certain temperature is reached. First check switch continuity with the engine cold (coolant temperature below 63°F for 1970 engines, 82°F for 1971 engines or 93°F for 1973-74 engines). Then start and run the engine until normal operating temperature is reached. At a coolant temperature above these listed values, recheck for switch continuity. The switch should have opened or closed once these temperatures were surpassed. If the switch was satisfactory, check the transmission switch.

Transmission Switch

With the engine at normal operating temperature and the transmission in one of the no-vacuum gears (check the system description for year applications), the vacuum valve solenoid should be energized (no vacuum). If not, remove the ground switch electrical lead. If the solenoid energizes, replace the switch.