Barometric Pressure Switch: Description and Operation

Altitude Switch:

The Altitude Switch is an integral component of the high altitude emission control system. The device consists of a bellows that expands and contracts in response to changes in atmospheric pressure occurring with changes in altitude. At the specified low pressure (high altitude), the bellows will expand enough to turn on a microswitch. Current will then energize a solenoid valve that allows extra air to the three altitude compensation passages in the carburetor. The idle-up solenoid is also opened to allow additional air/fuel mixture into the engine. The ECM receives an "ON/OFF" signal from the altitude switch that prevents setting of trouble code 45 (rich indication) when high altitude is the only reason the code might set.