Barometric Pressure Switch: Description and Operation

The Ignition Barometric Pressure Switch, is used to control spark timing and/or other electrical devices in response to changes in barometric pressure (i.e., altitude). When controlling spark timing, the ignition module (12A244) is made to vary the spark timing by an amount determined by calibration resistors in the switch assembly. In normal operation, spark timing is increased for vehicle operation above the switching point (increasing altitude) and retarded for vehicle operation below the switching point (decreasing altitude). When controlling other electrical devices, only On/Off control is provided; with On (switch closed) above the switching point and Off (switch open) below the switching point. Some switch assemblies control both spark timing and another device (dual switch assembly) and other switch assemblies control only one or the other (single switch assembly).