Isuzu Cars and Trucks 1981-1991

Thermostatically Controlled Air (TCA) Cleaner System


The TCA functions to maintain ambient air temperature at an optimum level so that the fuel/air ratio remains constant. This ensures fuel combustion and reduces pollutant emissions.

The TCA system is mounted on the air cleaner. It consists of a vacuum motor, hot air control damper and an inlet temperature compensator (ITC) valve.

When the engine is running, there is no vacuum signal at either the vacuum motor or the ITC valve. In this condition the vacuum motor spring closes off the passage from the hot air duct.

On a cold start, the ITC valve delivers maximum vacuum to the vacuum motor which moves the hot air control damper to the fully open position. This closes the ambient air passage and opens the hot air duct. If the engine speed increases, the system vacuum level will drop allowing the diaphragm spring to overcome the vacuum force and push the hot air control damper to the fully closed position.

When the engine is running under normal conditions, the ITC valve closes the passage to the intake manifold and opens the passage to the from the air cleaner to the vacuum motor. As fresh air is fed to the vacuum motor, the diaphragm spring forces the air control valve to close off the hot air duct and open the ambient air passage.

During conditions of extended idling, hill climbing or high speed driving, there is a substantial increase in engine and engine compartment temperatures. This results in an excessive amount of fuel vapor entering the intake manifold, causing an over-rich mixture. The over-rich mixture causes rough idling and increased CO emissions. To prevent this, the ITC valve opens the passage from the air cleaner to the intake manifold. Fresh air is allowed to enter the intake manifold and lean out the mixture.

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Fig. Typical Thermostatically Controlled Air cleaner (TCA) system