Barometric Pressure Switch: Description and Operation

Fig. 15 Ignition barometric pressure switch:

This switch, Fig. 15, controls spark timing and/or other electrical devices according to altitude. Calibration resistors in the switch assembly cause the ignition module to vary the spark timing. Spark timing is advanced for vehicle operation above a predetermined altitude and retarded for vehicle operation below a predetermined altitude. Some switch assemblies control both spark timing and another device while other switch assemblies control only one or the other.