Mitsubishi Pick-ups and Montero 1983-1995 Repair Guide

Secondary Air Supply System

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OPERATION



See Figure 1

The 2.0L and 2.6L engines use a second system to supply fresh air into the exhaust stream. This extra air is rich with oxygen and enhances the conversion process within the catalytic converter. At certain times, the exhaust system pressure is less than the pressure in the secondary air supply system. This is due to the natural power pulses of an engine. When the exhaust pressure is low, the higher pressure of the system pushes the reed open and admits fresh air into the exhaust stream. As the high pressure exhaust pulse passes the reed valve, the high pressure stops the fresh air from entering the exhaust stream and closes the valve itself. The fresh air may be either from the air cleaner or a separate intake system containing its own air cleaner. The entire system is controlled by the secondary air control solenoid, an electrically operated vacuum switch capable of disabling the system when it is not needed.



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Fig. Fig. 1: A diagram of a common secondary air supply system-2.0L and 2.6L engines

TESTING



See Figures 2, 3, 4 and 5

A vacuum pump capable of producing more than 10 in. Hg (33.8 kPa) of vacuum will be needed to perform this test.

Although this system rarely fails, to check the secondary air control valve, remove the valve from the pipes connecting it to the right-hand exhaust manifold. Blow air into the valve from the air filter side to ensure that air does not blow through. Connect a hand vacuum pump to the secondary air control valve nipple. Apply a vacuum of 19.75 in. Hg (67 kPa) and check for air tightness (air does not blow through). Apply a vacuum of 40.75 in. Hg (20 kPa) and blow air into the valve from the air filter side; air should blow through. Still with the vacuum applied, blow through the valve from the exhaust manifold side; air should not blow through. If any fault is found in the above checks, replace the secondary air control valve. Install the secondary air control valve to 37-44 ft. lbs. (50-60 Nm).



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Fig. Fig. 2: The secondary air control valve is a vacuum assisted reed type valve-attach the hand vacuum pump to the top vacuum port to test



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Fig. Fig. 3: Connect the vacuum pump to the port which contained the red stripe and apply a vacuum

To test the secondary air control solenoid valve, label and remove the vacuum hoses, taking note of the location of each. Generally, the hoses are marked with a blue, yellow and white stripe OR a green stripe and a white stripe or a red stripe. Attach the vacuum pump to the port which contained the red striped hose. Use jumper wires to bridge battery voltage to the terminals. The 4-cylinder Pick-up and Montero use a four conductor plug; apply the voltage to terminals 2 and 3.

Draw vacuum with the pump and check that the unit holds vacuum when power is applied and the other hose nipples are blocked or plugged. Remove the other nipple's plug and the vacuum should leak. When the power is removed and the other nipple is unplugged, the vacuum should leak. Use an ohmmeter to check the resistance across the terminals; correct resistance is 36-44 ohms at 68°F (10°C).



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Fig. Fig. 4: Connect a 12 volt direct current power source to the No. 2 (+) and No. 3 (-) terminals-check to see if the vacuum leaks or is held



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Fig. Fig. 5: Measure the resistance between the two terminals

 
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