GM Metro/Sprint 1985-1993 Repair Guide

Thermostatically Controlled Air Cleaner (TCAC) System



See Figures 1 and 2

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Fig. Fig. 1: Thermostatically controlled air cleaner (TCAC) system

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Fig. Fig. 2: Checking the TCAC thermosensor

This system consists of the Air Control Actuator (ACA), and the thermosensor. The TCAC system controls the temperature of intake air by mixing warm, preheated air, and cold air to help improve fuel vaporization and engine warm-up characteristics. The thermosensor senses the inside temperature of the air cleaner, and opens and closes its valve. The vacuum from the carburetor is transmitted to the ACA diaphragm when the thermosensor's valve is closed and not transmitted when open. A damper is linked to the ACA diaphragm and opens or closes the warm air duct according to the movement of the ACA diaphragm.

When the engine is started during cold weather, the thermosensor is closed because the temperature of the intake air in the air cleaner is low. therefore, the vacuum is transmitted to the ACA diaphragm, which then pulls up the damper to open the warm air duct fully. As the engine is warmed up, the temperature of the intake air coming into the air cleaner from the warm air duct rises and the thermosensor starts opening. As a result, vacuum to the ACA diaphragm decreases, and the damper pushed down by the spring force closes the warm air duct opening. In this state warm air and cold air are mixed together and enter the air cleaner.

When the engine is operating at high rpm and under high load condition, the temperature of the air coming from the warm air duct rises very high, causing the thermosensor opening to become even larger and the damper opening smaller due to the decreased vacuum. That is, the warm air amount decreases and the cold air amount increases. When throttle valve is fully opened, the damper closes the warm air duct due to very small vacuum applied to the ACA diaphragm.


System Inspection
  1. Check vacuum hoses for connection, deterioration or damage. Replace as necessary.
  3. Remove warm air hose from warm air duct, when air cleaner is cool.
  5. Make sure that damper closes warm air duct with engine stopped. Perform this check by touching damper with your finger through warm air duct.
  7. Start and run engine at idle. Damper should fully open warm air duct immediately.

Air Control Actuator (ACA)
  1. Disconnect vacuum hose from ACA.
  3. Make sure that damper opens fully when more than 7 in. H