Emission Control Systems: Description and Operation

Fig. 35 High altitude emission control system:

This factory installed system, Fig. 35, consists of an altitude switch, solenoid valve and electronic control module (ECM). The altitude switch senses atmospheric pressure and energizes the solenoid valve and ECM at high altitude. Solenoid valve opens and leans air/fuel mixture by adding air to the carburetor altitude compensation passage. ECM changes the open duty cycle, also leaning air/fuel ratio at high altitude and reducing hydrocarbon and carbon monoxide emissions. At low altitude, electric current from the altitude switch is cut off and altitude compensating system does not operate.
The altitude switch consists of bellows and a micro switch. The bellows expand or contract in response to changes in atmospheric pressure. As atmospheric pressure rises above 26-27 inches Hg, the bellows expand energizing the micro switch. Electrical current is then supplied to the solenoid valve and ECM, causing the solenoid valve to open, sending air from air cleaner to 3 altitude compensation air passages provided in the carburetor.