Barometric Pressure Switch: Description and Operation

Fig. 15 Ignition barometric pressure switch:

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The ignition barometric pressure switch, Fig. 6, is used to control spark timing and/or other electrical devices in response to changes in barometric pressure (altitude). When controlling spark timing, the ignition module 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 engine operation above the pre-determined switching point (increasing altitude) and retarded for engine operation below the pre-determined switch point (decreasing altitude).