Fault phenomenon: a 2005 production of FAW Mazda 6 2 L sedan, mileage of 60 thousand km. The owner reflected that the engine of the car was operating at idle speed, and the idle time and space adjustment system was working normally. However, when the vehicle is running, the engine is not strong enough to drive. When the vehicle accelerates, the air conditioning compressor is repeatedly engaged and separated, and the air conditioning system is not refrigerated. When the air conditioner is closed, the engine is back to normal. Inspection and analysis: as the engine turned back to normal after closing the air-conditioning, I think the cause of the trouble is in the air conditioning system. The Mazda six air conditioning compressor is a constant displacement compressor. The suction and disconnection of the air conditioning compressor is realized by the control unit of the engine (PCM) by controlling the air conditioning relay. I decided to start from the circuit to check. Normally, when the voltage between the pressure switch B and the C terminal is 12 V, the compressor is disconnected; when the pressure switch B and the C terminal voltage are 0 V, the compressor is connected. This shows that when the automatic air conditioner controls the PCM ground signal, the compressor is connected. Start the engine of the car and run idle. Measure the B terminal (PCM) or the C terminal of the pressure switch line with the oscilloscope. When the compressor is connected, the voltage of the pressure switch B and C is 0 V; when the compressor is disconnected, the voltage switch B and C terminal voltage is 12 V, indicating that the system is normal. When the engine speed rises to 3000 r/min, the compressor is disconnected, at this time the B terminal voltage is 13.8 V, and the C terminal voltage is 0 V, indicating that the high and low voltage switch has been disconnected at this time. Because the voltage of the C terminal is 0 V, the fault of the line and the automatic air conditioning controller is basically eliminated. The reason for the malfunction should be in the loop of the refrigeration system. The principle of PCM control air conditioning compressor relay execution action is: first, when the driver presses the air conditioning (A/C) switch, the application signal is transferred to the PCM by the automatic air conditioning control unit through the pressure switch, that is, the application signal of the automatic air conditioning system is received by the PCM. Only the pressure of the air conditioning system is normal to ensure the normal connection of the pressure switch, when the PCM can receive the instructions that make the air conditioning system work, and then the PCM controls the operation of the air conditioning compressor.