Abstract: An air conditioner in a vehicle is driven by the vehicle engine through an electrically controlled clutch. An upright, miniaturized mercury U-tube is mounted on the vehicle with the legs of the U-tube in upright position. Electrical contacts positioned in one of the legs are variously washed by the mercury depending upon the workload placed upon the engine. The contacts are in circuit through the mercury with the vehicle battery and the clutch. Driving of the air conditioner is automatically and temporarily interrupted when more engine power is needed. Several contacts may be used to afford a selection of levels of sensitivity. Acceleration disconnects the air conditioner compressor. Deceleration, idling and consistent driving speed permits the air conditioner compressor to operate normally.