1. The main fault and cause of the electronic control unit (ECU)
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The engine electronic control unit (ECU) itself and the poor line will cause the engine to start difficult or unable to start, the idle speed is unstable or even extinguished, the acceleration is poor, and the exhaust pipe emits black smoke and other faults. The main reason is that the electronic control unit (ECU) has poor contact with the line, and the joint is oxidized or detached; the external moisture enters the electronic control unit, causing damage to the electronic control unit; and the electronic control unit is burnt out due to improper operation during maintenance.
2. Inspection of electronic control system
(1) Inspection of electronic control unit (ECU)
The failure of the electronic control unit and its control line can be checked by the electronic control unit detector of the vehicle model or the vehicle electronic control unit decoder commonly used for each vehicle type. If you do not have these instruments, you can use a multimeter to measure the voltage or resistance of each terminal on the socket on the side of the electronic control unit to determine whether the electronic control unit and its control circuit are faulty. To detect the failure of the electronic control unit and the control line by this method, it must be based on the detailed maintenance technical data of the tested vehicle. These data include: which terminals of the control unit harness connector of the vehicle are connected with which sensors and actuators in the control system; and the standard voltage values ​​of the terminals under different working conditions of the engine. If an abnormality is found during the test, it indicates that there is a fault: if the connection with the actuator is abnormal, it indicates that the electronic control unit is faulty; if the connection with the sensor is abnormal, the sensor line may be faulty.
The usual methods for checking the ECU are as follows.
1) Voltage measurement method. According to the ECU connector diagram and the normal voltage data and measurement conditions of each ECU connection point, check with a multi-input multi-impedance multimeter.
1 The battery voltage should be above 11V.
2 Remove the ECU harness connector, but keep the connector in the connected state for voltage check.
3 should make the ignition switch in the ON position.
4 The multimeter should be inserted laterally from the harness connector, or inserted with a pin to measure the voltage between the terminals of the ECU and the ground.
5 The measurement result should be compared with the standard value. If it differs greatly from the standard value, it indicates that the ECU or control harness is faulty.
2) Resistance measurement method
1 Unplug the ECU cable and measure the normal resistance value of the connector diagram and the ECU connection points.
2 Use a high-impedance digital multimeter and try to measure with high ohms to prevent the measurement current from damaging the internal components of the ECU and expanding the fault.
3 The ECU connector diagrams of various models are different, but the symbols are used in the same vehicle system: the ECU's contact voltage and resistance value can only be referenced for other models.
(2) Inspection of the sensor
1) Coolant temperature sensor
When the coolant temperature sensor itself or the line is not working properly, the following faults may occur: 1 engine starting is difficult; 2 idle speed is unstable; 3 is easy to extinguish.
The main causes of the coolant temperature sensor failure are as follows.
1 Scale and grease are the main causes of misalignment of the coolant temperature sensor. Misalignment of the coolant temperature sensor causes the air-fuel ratio (A/F) to be out of alignment, which may result in difficulty in cold start, unstable idle speed, poor acceleration, or black smoke from the exhaust pipe.
2 failure refers to open circuit or conduction. When the circuit is broken, its resistance is infinite, causing the fuel injection to increase and the idle speed to be too high. When the conduction is made, the resistance is zero, the mixture is no longer thickened, the cold start is difficult, and there is no fast idle speed during the hot start. For this reason, some ECUs have a cut-off function, which is corrected to the normal water temperature once it fails, causing the injector to inject oil at normal water temperature.
The precision of the coolant temperature sensor has a large effect on the amount of fuel injected. When the mixture is too rich or too lean, the coolant temperature sensor should be removed. The detection method is as follows.
1 Measure the resistance of the coolant temperature sensor wiring. When the temperature of the coolant is 20. ° C, the resistance should be 2 to 3 kΩ: at 80 ° C, it should be 0.2 to 0.4 kΩ. If the measurement does not meet the specified requirements, the coolant temperature sensor should be replaced.
2 ignition switch. When the coolant temperature is 80 °C, measure the ignition switch THW and E, the voltage should be 0.2 ~ 1.0V, if it does not match, it should be further checked.
2) Intake air temperature sensor
When the intake air temperature sensor itself or the line is not working properly, the following faults may occur: 1 the engine performance is poor; 2 the idle speed is unstable; 3 is easy to extinguish; 4 the fuel consumption is increased or rich.
The main causes of the intake air temperature sensor failure are as follows.
1 Use a bad air filter.
2 Electrical maintenance personnel are not operating properly (use the fire test method to find the fault).
When checking the intake air temperature sensor whose structure is similar to the coolant temperature sensor, the method of checking the coolant temperature sensor can be used. Under normal conditions, when the temperature is 20 ° C, the resistance is about 2 to 3 kΩ; at 60 ° C, the resistance is about 0.4 to 0.7 kΩ. If the measurement results are not in compliance, the sensor should be replaced. When the helium temperature sensor installed in the air flow meter is damaged, the air flow meter should be replaced.
3) Oxygen sensor
An abnormality in the oxygen sensor signal will cause an increase in engine fuel consumption. The oxygen sensor signal line must be in good contact and well insulated because its output power is weak and the energy is extremely small.
The oxygen sensor is available in both heated (three-wire) and non-heated (single-wire). For heating, the heater resistance should be checked. The oxygen sensor inspection method is as follows.
1 voltage check method. The oxygen sensor voltage is measured by a digital multimeter with a high input impedance.
Start the engine and measure the voltage at the Ox and E1 terminals on the diagnostic box or on the ECU. Normally around 0.451.
Remove the gasoline pressure regulator hose from the intake manifold to allow the upper part of the pressure regulator to communicate with the atmosphere. After the manifold joint is blocked, the engine is started, and Ox and E are measured at normal idle speed, and the voltage at the terminal should be above .5V.
2 Oxygen sensor electric wire cold resistance resistance is about 4 ~ 40kΩ. If it does not meet the requirements, the oxygen sensor should be replaced.
Other sensors include a throttle position sensor, a crank position sensor, a camshaft position sensor, a knock sensor, a vehicle speed sensor, a Hall sync signal sensor, and the like.
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