Abstract: A user-adjustable control system monitors an electrical signal indicative of manifold pressure in an internal combustion engine that powers a wheeled vehicle. The engine simultaneously powers the drive train of the vehicle and, when a thermostat so dictates, an air conditioning compressor. This system improves vehicle driving performance by selectively restricting the diversion of engine power from the drive train to operate the air conditioning compressor, based on threshold changes in a monitored electrical signal. With electronic circuitry the system selectively disengages power to the compressor when the difference between manifold and atmospheric pressure drops below a preset value. The system includes a transducer connected to provide an electrical output signal that is indicative of pressure changes in the engine manifold. The manifold pressure changes are, in turn, indicative of changes in the engine's load conditions.