In the electronically controlled gasoline injection engine, the oxygen sensor for closed-loop control of the fuel system is an important electronic component, used to monitor the oxygen content in the exhaust gas, and fed back to the ECU with a voltage signal to keep the air-fuel ratio at 14.7. At the same time, it is also an endorsement alarm component for multiple fault signals.
The common failure of the zirconia sensor is that the surface is covered with lead or carbide, which results in gas impermeability and oxygen ion diffusion and failure. When the fault lamp alarms and the sensor fails to read, it is necessary to diagnose it, because the oxygen sensor alarm is not necessarily a sensor failure, and its alarm signal is also affected by the following factors.
1. Working condition of ignition system;
2. Sealing performance of intake system;
3. Whether the exhaust system is blocked;
4. The working condition of the fuel injector;
5. The oil pressure of the oil supply system.
Therefore, in the engine maintenance, once the alarm signal of the oxygen sensor appears, the computer and the human brain should be used to comprehensively analyze, judge, and exchange the faulty parts for reasonable maintenance.
1. Fault diagnosis of the oxygen sensor
According to the characteristic curve of the zirconia sensor: when the air-fuel ratio is maintained at 14.7, the signal reference voltage is 0.4-0.5V, and when the air-fuel ratio is less than 14.7, the voltage gradually increases to 0.8-lV, indicating that the mixture is too rich. When the air-fuel ratio is greater than 14.7, its voltage gradually drops to about 0.2V. This indicates that the mixture is too lean. This is an important basis for oxygen sensor diagnosis, and its diagnosis method is:
1. Run the engine at 2500r / min for 2min, preheat the sensor, unplug the sensor plug-in wire (note the position of the plug-in sensor for the sensor with a heating wire), measure the feedback voltage with a multimeter, and check the number of swings of the voltmeter pointer within 10S. If it is less than 8 times, warm up the sensor again and check the number of times the pointer swings within 10S. If the swing is more than 8 times, it is normal. If it is still less than 8 times, then proceed as follows:
2. Disconnect the sensor harness connector and measure the feedback voltage.
1) When it is greater than 0.45V, disconnect the vacuum tube somewhere on the intake pipe. If the voltage is still greater than 0.45V, it means that the sensor is damaged. If the boy is 0.45V, it means that the mixture is too rich. Check the fuel, intake, or control system.
2) When it is less than 0.45V, unplug the water temperature sensor and connect a 4-8KΩ resistor. If the voltage is still less than 0.45V, it means the sensor is damaged. If it is higher than 0.45V, it means the mixture is too lean.
Second, the ignition system working condition detection
Carry out routine inspection on the computer-controlled ignition system with or without distributor. The inspection items are spark energy, spark plug, high voltage line, ignition timing, ignition advance angle, etc. When using the ignition timing lamp to check the ignition advance angle, the red fish clip is connected to the positive electrode of the battery, the black fish clip is connected to the negative electrode of the battery, and the high-pressure sensor clamps the high-voltage line of a cylinder. When increasing, the ignition advance angle should increase. Use hand hammer or wrench to hit the knocking screws of the knock sensor or around the cylinder head. The ignition advance angle should be obviously delayed.
Third, the air intake system sealing performance check
Connect a vacuum gauge to the appropriate part of the intake pipe. When the engine is at idle speed (5OO-600r / min), based on sea level, the vacuum degree of the intake pipe should be within the range of 57.33-70.66Kpa, otherwise, repair the leak of the intake system. At idle speed, the vacuum gauge pointer gradually drops to zero, indicating that the exhaust system is blocked. Changes in vacuum gauge pointers can also detect valve sealing and ignition performance.
1. Several important concepts about oxygen sensors
Detection method: The oxygen sensor detects the concentration of the mixed gas, but it does not directly detect the mixed gas, but detects the content of oxygen molecules in the exhaust gas after the mixed gas is burned, so as to indirectly obtain the current mixed gas concentration.
Signal characteristics: The oxygen sensor is actually a small battery with low voltage and low current. When the angles of the oxygen molecules on its inner and outer surfaces are different, a potential difference is formed. Its outer surface extends into the exhaust pipe directly with the engine Exhaust gas contacts, its inner surface is in contact with the atmosphere, and the concentration of oxygen molecules in the atmosphere is constant. The concentration of oxygen molecules in the exhaust gas changes with the concentration of the mixed gas. When the actual air-fuel ratio of the mixture is higher than the theoretical air-fuel ratio (14.7, that is, lean mixture), the concentration of remaining oxygen molecules in the exhaust gas is relatively high. At this time, the difference between the concentration of oxygen molecules inside and outside the oxygen sensor is small, and it can only output about 0.1 V voltage; and when the actual air-fuel ratio of the mixture is less than the theoretical air-fuel ratio (ie, the mixture), there are very few oxygen molecules remaining in the exhaust gas. At this time, the concentration of oxygen molecules on the inner and outer surfaces of the oxygen sensor differs greatly, and it can output about 1.0V Around the voltage.
Working characteristics: At present, most models use zirconium type oxygen sensors. These sensors have an important technical indicator during design, that is, the signal rises time and fall time, both require less than 250ms.
If the change time is greater than 250ms, although the maximum voltage when the mixture is rich and the minimum voltage when it is lean are sometimes normal, in actual application, it appears that the oxygen signal is unresponsive and cannot provide real-time mixer information for the engine computer. The fuel feedback system is out of adjustment, and some soft faults are mostly caused by this reason.
Control principle The engine computer understands the slight deviation of the current mixture concentration from the theoretical value through the signal output by the oxygen sensor and accordingly adjusts the energization time of the injector according to this signal to compensate for this slight deviation, thereby improving the control accuracy. This is the so-called closed-loop control.
Second, the common faults of the oxygen sensor
Heating unit failure (generally open circuit);
The minimum voltage (failure) when the signal voltage is always lean;
The signal is unresponsive (the change time of rich and lean voltage is greater than 250ms).
3. The general detection method of the oxygen sensor
1. Check the heater resistance of the oxygen sensor. Unplug the oxygen sensor plug and use a multimeter resistance file to measure the resistance value between the 1st and 2nd plugs on the sensor side. The specific standard should refer to the maintenance manual of the specific model, but generally, it should be between 4 and 40, if it does not meet the standard Value, the oxygen sensor should be replaced.
2. Check the feedback voltage of the oxygen sensor. Consult the maintenance manual of the tested model, find the signal wire of the oxygen sensor, and insert the copper wire in the wire into the corresponding surgical jack. Then plug in the connector and measure the voltage of the copper wire to the negative electrode with a multimeter DC voltage. Note that a digital multimeter must be used, and the copper wire must not be grounded, otherwise, the oxygen sensor will be irreversibly damaged. At this time, start the engine and make the water temperature reach at least 80 ℃. After the engine reaches 2500r / min-multiple times, keep the engine speed at 2500r / min, and observe the voltage displayed by the multimeter. The voltage value should jump rapidly between 0.1-1.0V. Within 10S, the voltage should change at least 8 times between 0.1-1.0V. If the voltage changes slowly, it is not necessarily a malfunction of the oxygen sensor or feedback control system. It may be that the surface of the oxygen sensor is covered by carbon deposits and the sensitivity is reduced. At this time, the engine can be operated at high speed for a few minutes to remove carbon deposits, and then observe whether the signal voltage of the oxygen sensor meets the regulations. If it still does not meet the regulations, conduct a comprehensive characteristic analysis check.
4. Limitations of traditional detection methods
For many years, the industry's congressmen have used the above methods to perform in-vehicle tests on oxygen sensors, or use a good oxygen sensor for comparison tests. Under normal circumstances, you can also determine the fault. However, the above methods have the following problems, and these factors often restrict the accuracy of the detection:
1. It is required that the engine condition of the tested vehicle is good in order to achieve the required test results. If the engine operating conditions are not good and the mixture is rich or lean, the oxygen sensor cannot accurately achieve the change between the rich and lean signals according to the given test conditions;
2. The change time of the oxygen signal rich voltage cannot be directly detected, but the change time of the oxygen signal rich voltage can be evaluated by such an indirect way that the signal voltage should change at least 8 times between 0.1-1.0V within 10S. In practice, it is more random;
3. Since the off-vehicle test cannot be achieved, the above method is very limited, and some more complicated hidden faults cannot be effectively located. Even if the "comparison method" is used to solve the fault, it is often "knowing it, not knowing it".
5. The latest comprehensive and accurate detection methods
In view of the above situation, we have proposed a more comprehensive new scheme for oxygen sensor detection. This scheme has two methods: on-vehicle and off-vehicle, which provides an extremely convenient means for automotive technicians to diagnose engine failures related to oxygen sensors.
1. Features and functions of the on-board detection method:
Use traditional testing methods combined with accurate oxygen signal concentration change time testing to quickly evaluate the performance of oxygen sensors.
Oxygen sensor signal simulation-simulating the rich and lean mixture signal for the engine computer;
Oxygen sensor concentration signal detection-accurately detect the output voltage of the oxygen sensor;
Low oxygen sensor signal detection-accurately detect whether the output voltage of the oxygen sensor is less than 0.1V;
Detection of the number of changes of the oxygen sensor signal-detection of the number of changes of the voltage signal within 10S;
Oxygen sensor output characteristic analysis —— During a period of oxygen signal change, the change time of the mixed gas signal from rich to lean or from lean to rich is measured with a resolution of 50ms.
2. Features and functions of off-car detection method:
A carefully designed oxygen sensor working condition simulation device makes it possible to accurately test the oxygen sensor on the workbench. This method is a practical solution to comprehensively analyze the working performance of oxygen sensors and is also the best choice for auto repair teaching units and oxygen sensor production and management organizations.
Oxygen sensor signal simulation-simulating the rich and lean mixture signal for the engine computer;
Oxygen sensor concentration signal detection-accurately detect the output voltage of the oxygen sensor;
Oxygen sensor actual working condition simulation —— Simulate oxygen sensor oxygen enrichment and hypoxia working conditions on the workbench;
Detection of the initial response time of the oxygen signal —— provides a detection method for whether the oxygen sensor complies with Euro III or Euro IV standards;
Low oxygen sensor signal detection-accurately detect whether the output voltage of the oxygen sensor is less than 0.1V;
Detection of the number of changes of the oxygen sensor signal-detection of the number of changes of the voltage signal within 10S;
Analysis of the operating characteristics of the oxygen sensor-Measure the change time of the mixed gas signal from rich to lean or from lean to rich within a period of oxygen signal change with a resolution of 50ms.
Post time: 2020-05-06
