Nissan Pick-ups and Pathfinder 1989-1995

Automatic Temperature Controlled (ATC) Air Cleaner



See Figures 1 and 2

Z24i and VG30i Engines Only

The rate of fuel atomization is dependent upon the temperature of the air being taken into the engine and the temperature of the engine itself (which can have the effect of heating intake air). The air/fuel ratio cannot be held constant (during engine warm-up) for efficient fuel combustion if air temperatures vary. For example, when cold air is drawn into a cold engine, the fuel does not atomize as efficiently as it would if the mixture were heated. The end result is more hydrocarbons are emitted in the exhaust gas. In contrast, as warmer air is drawn into a cold engine, the air/fuel mixture undergoes a more efficient atomization, creating better combustion and less hydrocarbons from the exhaust.

The automatic temperature controlled air cleaner is designed so that the temperature of the ambient air being drawn into the engine is automatically controlled during cold engine operation. This holds the temperature of the air and consequently, the air/fuel ratio, at a more constant rate for better combustion.

A temperature sensing switch controls the vacuum applied to a vacuum motor operating a valve in the intake snorkel of the air cleaner. This valve determines whether the engine receives cold or heated air. When the engine is cold, the switch causes the vacuum motor to open the valve, allowing air heated by the exhaust manifold to be drawn into the engine. As the engine approaches normal operating temperature (and the supplemental heat is no longer necessary), the switch triggers the vacuum motor to close, shutting off the heated air and allowing cooler ambient air to be drawn in.

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Fig. Fig. 1: Air control valve-open

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Fig. Fig. 2: Air control valve-closed


See Figures 3, 4 and 5

Vacuum Motor

When the air temperature around the sensor is below 100F (38C), the sensor should allow vacuum to pass through the air valve vacuum motor, thus blocking off the air cleaner snorkel, making the engine take in heated air from the exhaust manifold. When the air temperature around the sensor reaches 100F (38C), the sensor should trigger the blockage of vacuum to the air control valve vacuum motor. This will allow the engine to take in cooler unheated air, through the air snorkel.

When the air temperature around the sensor is about 118F (48C), the air control valve should be completely open to unheated air.

When the engine is operating under a heavy load (wide open throttle acceleration), the air control valve fully opens to underhood air to obtain full power no matter what the temperature is around the temperature sensor.

If the air cleaner fails to operate correctly, check for loose or broken vacuum hoses. If the hoses are not the cause, replace the vacuum motor in the air cleaner.

Temperature Sensor

While the engine is cool and at idle, disconnect the hose at the vacuum motor and check that there is vacuum present at the end of the hose. If there is little or no vacuum, check the condition of the hose. If its good, replace the temperature sensor.

After the engine has warmed up, check the hose again. There should be no vacuum felt; if there is vacuum, replace the sensor.

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Fig. Fig. 3: ATC air cleaner-Z24i engine

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Fig. Fig. 4: Checking the ATC vacuum motor

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Fig. Fig. 5: Checking the ATC temperature sensor