Mileage of about 50 thousand km, equipped with 3 VZ - FE engine and automatic transmission TOYOTA Camry car. User response: after the car starts the engine, the cooling fan at the back of the radiator runs at high speed regardless of the temperature of the coolant or whether the air conditioning system is turned on. Troubleshooting: after inspection, it is found that the car is equipped with an electronically controlled hydraulic fan control system. The fan ECU is the control center of the system, and the speed of the fan is controlled by the hydraulic flow rate. To confirm the fault symptoms, it is found that the cooling fan has been running at high speed when the air conditioning cooling air has been closed and the engine temperature is not more than 80 C, thus it seems that the system circuit has problems. From the working principle of the system, the cooling fan ECU needs to control the cooling fan according to the engine speed signal (from the igniter), the temperature signal of the coolant, the refrigerant pressure signal and so on, so the related components are checked. Observe the engine tachometer of the dashboard and work normally. It can be judged that the fan ECU receives the engine speed signal. A coolant temperature sensor wire harness plug on the pipe is pulled out. The fan speed has no change, but the engine warning light on the dashboard is lit. It shows that the cooling fluid temperature sensor is not used by the cooling fan ECU, but is used by the engine control module. As a result, the cooling fluid temperature sensor used by the cooling fan ECU system is found near the pump. The sensor's operating voltage is 5V, and the sensor's resistance is about 230 omega (the actual temperature of the coolant is 90). The above results are within the standard range. Check the air conditioning system and find that the air conditioning pipeline has been changed. The air conditioning pressure switch (refrigerant pressure sensor) is not on the liquid storage dryer, and it is changed to the air conditioning line under the windowglass. The air conditioning pressure switch is composed of a low voltage switch and a high voltage switch. The terminals of the two switches are arranged in the form of cross pairs, and the wiring harness plug is in the form of 4 pins. Turn off the engine, open the wire harness of one of the switches, and measure the conductance of the two terminals to 0 Omega, no problem; the wire harness of the other switch is opened and the conductance between the two terminals is 600 Omega, indicating that the contact of the switch is in bad contact and the two terminals are connected together to start the engine test and heat dissipation fan. The speed dropped and the noise disappeared. Replace the air conditioning pressure switch, the fault is completely eliminated. If the air conditioning refrigeration mode is not enabled or the engine is at low temperature, if the fan has been running at high speed, the fan ECU recognizes the failure of the system circuit, and enables the failure protection mode to make the cooling fan run at high speed (1100r/min), resulting in larger operation noise and acceleration. Wear of the machine parts. The failure protection mode is conditional, and the related signals include the coolant temperature sensor signal, the high voltage switch signal of the air conditioner, the engine speed signal from the igniter, the air conditioning request signal of the A/C control assembly and the load signal from the engine module. In addition, the electromagnetic valve blocking (hydraulic fluid not circulating) of the electrically controlled hydraulic cooling fan control system will also cause the heat dissipation fan to run at high speed. This case is caused by the bad contact of the high voltage switch of the air conditioner. When the air conditioning is not opened, the low voltage switch of the air conditioning pressure switch and the high voltage switch of the air conditioning should be closed. When the air conditioning is opened, the low voltage switch is still closed, and the high pressure switch is broken when the refrigerant pressure is above 1600kPa (the engine load is increased). The signal is transmitted to the cooling fan ECU, which is an input signal for increasing the speed of the cooling fan. The cooling fan ECU controls the working current of the solenoid valve, so that more hydraulic oil flows through the hydraulic actuator to drive the cooling fan to run at high speed and keep the engine temperature normal. During the whole control process, the cooling fan can be adjusted continuously.