See Figures 1, 2 and 3
Fresh air supplied to the air cleaner comes either from the normal snorkel, or from a tube connected to an exhaust manifold stove. A door in the snorkel regulates the source of incoming air so that a warm engine always takes in warm air, approximately 100°F. The door may be controlled by a thermostatic spring or expansion bulb, or it may be vacuum operated. The vacuum operated designs use a thermostatic bimetal switch inside the air cleaner that bleeds off vacuum as the engine warms up, and regulates the position of the air door. On all late models, the snorkel is connected to a long tube so it takes in cooler air from outside the engine compartment. In hot climates the cool air tube is necessary because underhood air can easily reach 200°F.
Vacuum operated air doors are all designed so that the air cleaner takes in cold air when there is no vacuum. This means that an air door in the hot air position will switch to the cold position at wide open throttle because of the loss of manifold vacuum. The sudden switching of the door from hot to cold may cause a tumble or misfire in the engine, so some designs include a modulator valve mounted on the side of the air cleaner to block the vacuum and hold the door in the hot air position. A small thermostat inside the modulator opens it when the underhood temperatures reach normal. Other designs use a delay valve that allows the air door to move to the cold position slowly, to prevent stumble.
To test the non-vacuum type of heated air cleaner, start with an engine that is cold enough to have the air door in the hot air position. Remove the top of the air cleaner and put a thermometer inside the cleaner, then replace the cover without the nuts. Start the engine and watch the air door through the end of the air cleaner. You may have to remove some air ducting to be able to see the air door. As soon as the air door starts to move from the hot air position, lift the top off the air cleaner and read the temperature. If the temperature is between 130 and 150°F., the thermostat is working correctly. If not, replace the thermostat.
To test the vacuum type of heated air cleaner, inspect the air door with the engine off. It should be in the cold air position. Start the engine. If the engine is cold, the air door should move to the hot air position. As the engine warms up, the air door should move to a mid position, depending on the outside air temperature.
If the outside air is extremely cold, the air door may stay in the hot air position indefinitely. On a warm day, after the engine warms up the air door should move to the cold air position. If it doesn't, the temperature sensor inside the air cleaner might be faulty, or the air door itself might be hanging up. Check the air door by running a hose from manifold vacuum to the vacuum motor. Connect and disconnect the hose to see if the air door moves freely. If the air door is free, check out the hoses for leaks or blockage. If the hoses are okay, the trouble must be in the temperature sensor, and it should be replaced.
Modulators are used in the air cleaner vacuum line on some engines. The modulator mounts on the side of the air cleaner and has two hose connections, one to the air cleaner temperature sensor, and the other to the vacuum motor. Below 50-80°F. the modulator is a one-way check valve, which allows vacuum to move the air door to the hot air position, but traps the vacuum so the door will not jump back to the cold air position during acceleration. This prevents a stumble.
After the modulator warms up, the check valve unseats so that the vacuum can pass freely in either direction, and the air door then operates normally. The connections for the modulator are important. The connection in the center goes to the vacuum motor, and the connection on the edge goes to the vacuum source, which is the temperature sensor.
To test the modulator on a cold engine, apply enough vacuum to the edge port to move the air door to the hot position. Then remove the hose from the port, and the air door should stay in the hot position. Make the same test when the engine is warmed up, and the air door should move to the cold position when you pull off the hose.