One model year 2003renaultScenic commercial vehicle, equipped with FranceCitroenThe user said that the AL4 4-speed electronic controlled automatic transmission produced by the company has been repaired elsewhere for a long time, but the fault has not been resolved. The test run fault is: when the shift lever is set to position 1, the transmission will enter the failsafe mode immediately after the accelerator exceeds the medium load, and the transmission will not enter the safety protection mode if the low accelerator continues;When the shift lever is set to position 2, the accelerator with medium load or above will have the impact of shifting from gear 1 to gear 2. After continuing to refuel, the accelerator will not shift into gear 3 and will immediately enter the failsafe mode;When the gear shift lever is set to D position, test drive with the accelerator above medium load. The fault phenomenon is that the first gear rises to the second gear and impacts. After continuing to refuel, it will enter the third gear of safety protection mode. The electronic control system of the transmission is detected with a fault diagnosis instrument, and no fault code is found.Repeat the test run again and again. It is found that when the accelerator is very small, there is still an impact when shifting from gear 1 to gear 2. It is normal when shifting from gear 2 to gear 3. The impact occurs after slipping 800 r/min when shifting from gear 3 to gear 4.Since there is no special diagnostic instrument, this fault can only be roughly analyzed based on experience.The change of large and small loads will directly affect the shift of the transmission and the working pressure of the hydraulic systemengineThe load is small, and the shift process can be completed when the shift and working oil pressure are about 200 kPa;Due to heavy loadengineWhen the load increases, the shift and working oil pressure cannot be met, and the transmission control unit passes the oil pressuresensorAfter monitoring, it will enter the security protection mode. Because this transmission has some common faults: for example, the transmission filter screen is easy to get dirty, especially the filter screen with poor quality is easier to get dirty;The two pulse controlled oil pressure solenoid valves (Fig. 1) are usually prone to wear.So we decided to carry out maintenance from these two points first, so we replaced the new filter screen and solenoid valve, and carried out a careful inspection of the internal part of the transmission. After installation, the fault remained.Since the valve body of the car has been replaced when it was repaired elsewhere, we decided to conduct input and outputsensorThe resistance of the flow solenoid valve and the oil pressure sensor is tested, and the test results are normal.Next, a valve body with solenoid valve (Fig. 2) was replaced, and the fault still exists.After that, the throttle valve was adjusted, but the fault symptom remained unchanged.
Although the transmission control unit also has the possibility of failure, there is no relevant failure found in the control unit through the preliminary detection of the diagnostic apparatus. At the same time, the control unit and the control unit of the vehicle use the CAN data bus for communication, and no relevant failure found in the transmission control unit in other systems,So we focused on the components around the transmission.In general, the abnormal operation of the oil pressure sensor will give an error signal to the control unit, which will make the transmission enter the safety protection mode. The abnormal adjustment of the flow solenoid valve will also cause the low operating oil pressure of the system to enter the safety protection mode.For this reason, we first tested the oil pressure sensor and found no abnormality.After that, when the resistance value is 1.8?When testing the redundant flow solenoid valve, it is occasionally found that the solenoid valve is stuck during power on and power off.Find a 2.5?The redundant bulb replaces the resistance value of the solenoid valve and supplies the solenoid valve directlyBatteryDuring the test run with voltage, it was found that except for the occasional impact when shifting from gear 1 to gear 2, other gears were in good condition.Repeated test runs found that the transmission occasionally enters the safety protection mode.Because the control mode of this solenoid valve is duty ratio controlBatteryThe voltage replacement was unreasonable, so the flow solenoid valve was removed and cleaned carefully.Test drive the car again after the line is restored. When shifting from gear 1 to gear 2, there is still occasional impact. Everything else is normal.At this time, the distance to completion is getting closer and closer. If the impact is less, the car can be delivered. At this stage of maintenance, there is no plan to carry out, so we calmed down and summarized the problem of occasional impact from gear 1 to gear 2.The possibility of failure of external components of the transmission has been eliminated one after another. At this time, the possibility of mechanical, hydraulic and electronic control of the transmission is extremely low. Therefore, it is necessary to find a good road surface for careful test run to find the source of the occasional impact from gear 1 to gear 2.When we test the vehicle on a road with few vehicles and good road conditions, we found that the impact of the transmission from gear 1 to gear 2 occurred immediately after the cylinder knocking sound of the vehicle;When the engine has no cylinder knocking sound, the transmission responds well from gear 1 to gear 2.At this time, the problem has suddenly become clear. The engine transmits the wrong working condition information to the transmission control unit through the CAN bus. The transmission control unit gives the wrong shift oil pressure for this reason. At the same time, the abnormal operation of the engine also affects the engine's ability to reduce torque when shifting. After a 2.5h road test with the user to confirm that there are no other problems, we conducted a comprehensive inspection of the engine with the user's consent.findcoolingThe liquid temperature is a little higher than the normal temperature. Considering that the engine has been repaired and there is a cylinder knocking sound, we suspect that there is a problem with the valve timing of the engine. Therefore, we focus on checking the valve mechanism.Through careful observation of the timing marks, it is found that the timing belt of the gas distribution has a difference of 1 tooth when it is correctly assembled. So far, it can be sure that this is the cause of the failure of the vehicle. After reassembling the valve distribution mechanism correctly, we disassembled and cleaned the water tank, and the failure was completely eliminated after commissioning. Through the maintenance of this vehicle's fault, we have deeply realized that any vehicle model should be thoroughly tested before maintenance, because the "road test" is the most important link in diagnosing and repairing the fault of today's new automatic transmission.For vehicles equipped with automatic transmission and stepless transmission with high degree of electronic control, it is necessary to test drive the vehicle on a good road, and understand the common faults of the repaired vehicle model clearly, because sometimes it is difficult to find the fault point through principle analysis, but it is not difficult to explain the fault with principle after troubleshooting.In addition, as the relationship between the control units on the vehicle becomes closer and closer, and the interaction becomes more frequent, especially after the application of CAN bus, the requirements for the comprehensive ability of relevant maintenance personnel are higher.
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