Chevrolet Blazer/Jimmy 1969-1982 Repair Guide

Transmission Controlled Spark (TCS) System



Introduced in 1970, this system 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 two-way transmission switch is activated by the shifter shaft on manual transmissions, and by fluid 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 fifteen 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 (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. Two throttle settings are necessary; one for curb idle and one for emission control on coast.

The 1972 six cylinder system is similar to that used in 1971, except that an idle stop solenoid was added to the system and the name was changed back to TCS. 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 cylinder 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 twenty seconds after the transmission is shifted into High gear. 1972 V8 engines use a vacuum advance solenoid similar to that used in 1970. The solenoid controls distributor vacuum advance and performs no throttle positioning function. The idle stop solenoid used on V8s operates in the same manner as the one on six cylinder engines. Air conditioned trucks 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 on the six cylinder engine is identical to that on the 1972 six, except for recalibration of the temperature switch. The system used on 1973 V8 engines changed slightly from 1972. In place of the CEC solenoid on the six, the V8 continues to use a vacuum advance solenoid. The other differences are: the upshift delay relay, previously located under the instrument panel had been done away with; a 20 second time delay relay identical to the one on six cylinder engines is used; V8s use manifold vacuum with TCS and ported (above the throttle plates) vacuum without TCS.

The six cylinder TCS system was revised for 1974-75 by replacing the CEC solenoid with a vacuum advance solenoid. Otherwise the system remains the same as 1973.


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:

  1. Blown fuse.
  3. Disconnected wire at solenoid-operated vacuum valve.
  5. Disconnected wire at transmission switch.
  7. Temperature override switch energized due to low engine temperature.
  9. Solenoid failure.

If no vacuum is available in any gear, check the following:

  1. Solenoid valve vacuum lines switched.
  3. Clogged solenoid vacuum valve.
  5. Distributor or manifold vacuum lines leaking or disconnected.
  7. Transmission switch or wire grounded.

Tests for individual components are as follows:

Idle Stop Solenoid

This unit may be checked simply by observing it while an assistant switches the ignition ON and OFF. It should extend further with the current switched on. The unit is not repairable.

Solenoid Vacuum Valve

Check that proper manifold vacuum is available. Connect the vacuum gauge in the line between the solenoid valve and the distributor. Apply 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.

  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 (no vacuum) if the relay is satisfactory.
  3. With the solenoid energized as in Step 1, connect a jumper from the relay terminal with the green/white stripe lead to ground. The solenoid should de-energize (vacuum available) 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 de-energized (vacuum availale) with the engine cold. If it is not, ground the green/white stripe wire from the switch. If the solenoid now de-energizes, replace the switch. 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, the vacuum valve solenoid should be energized (no vacuum). If not, remove and ground the switch electrical lead. If the solenoid energizes, replace the switch.