V 3500 P/U 4WD V8-350 5.7L VIN M 4-bbl (1987)
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Air Conditioning Switch: Description and Operation
The air conditioning "On" signal informs the ECM the A/C selector switch is turned on, and the A/C pressure cycling switch is closed. Using this
information, the ECM adjusts the engine idle speed as necessary to ensure a smooth idle.