Prince Haima's instrument is not installed.

Prince hippo is not equipped. To solve the problem, we must first find out what the real problem is. Repairing a car is like curing a disease. You can't blindly dismantle and repair just by the superficial phenomenon of the fault. First, we must reproduce the fault to determine the existence of the fault phenomenon, find out the root fault by combining Chinese and western, make a preliminary diagnosis according to experience, diagnose with instruments and equipment, distinguish between true and false, find out the real problem hidden under the fault phenomenon, and get rid of the dilemma of repairing the car like fortune telling. Repairing a car depends on data, and every maintenance process must be supported by data. Identify the authenticity of the problem with data, and verify the maintenance effect with data. Why are the maintenance processes and methods different for the same high-temperature fault phenomenon? The coolant temperature sensor has a negative temperature coefficient, and its resistance is high when the temperature is low. Therefore, the engine coolant temperature can be determined by detecting the voltage of the coolant temperature sensor in the circuit.

As shown in fig. 7, the basic principle of temperature monitoring of coolant temperature sensor is to use the partial pressure principle of series circuit and combine the physical characteristics of negative temperature coefficient. In the circuit diagram, first, the detection circuit gives a standard reference voltage signal VCC, which is generally 12V or 5V, and different control circuits are slightly different. When the negative temperature coefficient voltage is high, the detection circuit thinks that the measured water temperature is low; On the contrary, when the negative temperature coefficient voltage is low, the detection circuit thinks that the temperature is high.

Therefore, when the standard reference voltage signal VCC, grounding point, line and its load are normal, the coolant temperature can be correctly monitored.

In the first case of this paper, because the fixed bolt of the gearbox grounding wire is not tightened, the potential on the machine body is not equal to the negative potential of the battery, which leads to poor grounding of the negative electrode of the coolant temperature sensor, which is equivalent to connecting an additional resistor in series in the water temperature monitoring series circuit. Compared with the normal situation of the coolant temperature detection circuit, there is an additional partial pressure load in the circuit. Therefore, the negative temperature coefficient directly detecting the coolant temperature can share a relatively small voltage. In this way, the detection circuit thinks that the current temperature has increased. When the additional resistance caused by poor grounding is large enough, the voltage shared by the negative temperature coefficient will be small enough, which leads the water temperature detection circuit to think that the coolant temperature is too high, that is, the fault phenomenon occurs in this case.

In the coolant temperature monitoring circuit, there is a very important parameter that can correctly respond to the coolant temperature, and that is the standard reference voltage signal VCC. Because the power supply of the vehicle has a generator and a battery, the voltage of these two power supplies can not be kept stable continuously during operation. Therefore, no matter whether the reference voltage signal of the vehicle is 12V or 5V, it is necessary to process the vehicle power supply voltage through the power control module to obtain a stable voltage as the standard sensor reference voltage signal.

If the stability and accuracy of the reference voltage signal cannot be guaranteed for other reasons during the voltage processing, the detection circuit of the coolant temperature sensor based on the processed voltage cannot truly reflect the actual situation of the coolant temperature. In the second case of this paper, due to the poor grounding of the instrument itself and the abnormal power processing circuit, the standard reference voltage signal is abnormal and cannot be used as a real standard reference voltage signal. When the instrument itself is poorly grounded, it is equivalent to connecting a resistor in series in the circuit. Because the standard reference voltage is equivalent to the power supply of the detection circuit of the coolant temperature sensor, when the grounding is poor, it is equivalent to the increase of the internal resistance of the power supply. When the internal resistance of the power supply increases, the terminal voltage that can effectively output will decrease. In case 2, that is, when the standard reference voltage decreases, the standard reference voltage VCC decreases.