Failure phenomenon
A 2007 BMW 325i car (model code E90) equipped with an N52 engine, with a cumulative mileage of approximately 120,000 km. The car was repaired in other repair shops due to engine start difficulties and weak acceleration. The maintenance personnel replaced the valve servo motor and the eccentric shaft position sensor according to the instructions of the fault detector (ISTA). The fault was not eliminated, but even worse. Seriously, the engine cannot be started now, so the car was transferred to our factory for maintenance.
Troubleshooting
After receiving the car, test it out and start the engine. The starter runs normally, but the engine cannot start. Use a fault detector to detect and read a lot of fault codes, and filter out the fault codes that can cause the engine to fail to start, as shown in Figure 1. Analyzing the fault code, it is believed that the fault of the vehicle is caused by the abnormal signal of the eccentric shaft position sensor. Reading the eccentric shaft angle data, it is found that the actual angle of the eccentric shaft is very different from the standard angle, indicating that the signal of the eccentric shaft position sensor is indeed abnormal. It is inferred that the possible causes of the fault are: the eccentric shaft position sensor and its wiring failure; the engine control unit (DME) )damage.
Looking at the maintenance data, it is known that the eccentric shaft position sensor works according to the principle of magnetoresistance effect. There are two independent angle sensors inside, and the angle signal is transmitted to the DME through the serial interface. Check the eccentric shaft position sensor control circuit (Figure 2). It is known that 8 wires are connected to the eccentric shaft position sensor wire connector X60253. Terminal 6 corresponds to the power supply line, terminal 4 corresponds to the shielded wire, and terminal 5 corresponds to the ground wire. The role of the 6 wires is not visible. Further inspection of the maintenance information shows that the eccentric shaft position sensor wire connector X60253 terminal 1 and terminal 3 correspond to the signal line of the angle sensor 1 inside the eccentric shaft position sensor (the two signal lines are marked differently, and terminal 1 is marked as P_CS1S, terminal 3 is marked as T_DAT1S), the wires corresponding to terminal 7 and terminal 9 are the signal lines of the internal angle sensor 2 of the eccentric shaft position sensor (the two signal lines are marked differently, terminal 7 is marked as P_CS2S, and terminal 9 is marked as T_DAT2S), the wire corresponding to terminal 8 is the beat clock signal wire of the eccentric shaft position sensor (the wire is marked as P_CLKS). According to the above information, I still don't understand the working principle of the eccentric shaft position sensor. In order to avoid blind diagnosis, I decided to use an oscilloscope to collect and analyze the eccentric shaft position sensor signal of a normal N52 engine.
First, use an oscilloscope to measure the signals on terminal 1 (corresponding to the P_CS1S signal) and terminal 7 (corresponding to the P_CS2S signal) of the eccentric shaft position sensor of the normal N52 engine (Figure 3). It is found that these two signals are all 0 V~ sent by DME 5 V reference signal, even if the eccentric shaft position sensor wire connector is disconnected, the signal will not change; then measure the eccentric shaft position sensor terminal 3 (corresponding to the T_DAT1S signal) and terminal 9 (corresponding to the T_DAT2S signal) signal (Figure 4) ), it is found that these 2 signals are 0 V~5 V signals fed back by the eccentric shaft position sensor. Once the eccentric shaft position sensor wire connector is disconnected, the signal will disappear, and the interval in which these 2 signals occur is between the P_CS1S signal and When the P_CS2S signal is at low level; then measure the signals on the eccentric shaft position sensor terminal 3 (corresponding to the T_DAT1S signal), terminal 8 (corresponding to the P_CLKS signal) and terminal 9 (corresponding to the T_DAT2S signal) (Figure 5), and find the beat clock The signal is a 0 V~5 V signal sent by DME. Even if the eccentric shaft position sensor wire connector is disconnected, the signal will not change, and the interval in which the T_DAT1S signal and T_DAT2S signal occur corresponds to the beat clock signal.
Summarizing the above measurement results, it can be seen that the eccentric shaft position sensor terminal 1, terminal 7 and terminal 8 are responsible for receiving the input signal of the DME. These three signals will not change when the eccentric shaft position sensor wire connector is disconnected or when accelerating; terminals 3 and Terminal 9 is responsible for sending the eccentric shaft position signal to the DME. These two signals will change when accelerating, and disappear when the eccentric shaft position sensor wire connector is disconnected, and the interval in which these two signals occur should be the same as that of terminal 1, terminal 7. Synchronize with the signal on terminal 8.
Since the fault code of the car indicates that the signal of the eccentric shaft position sensor is abnormal, and now that the working principle of the eccentric shaft position sensor has been understood, the next step is to use an oscilloscope to measure the signal of the eccentric shaft position sensor of the car. Connect the probe to the eccentric shaft position sensor wire connector (Figure 6), and use an oscilloscope to measure the signals of terminals 1, terminal 3, terminal 7 and terminal 9 of the eccentric shaft position sensor of the faulty car (Figure 7). No abnormality is found; The Allen wrench rotates the valve servo motor (Figure 8) and changes the position of the eccentric shaft. It is found that the signal of terminal 9 has changed, but the signal of terminal 3 has not changed (Figure 9). It is abnormal. Under normal circumstances, both signals should occur. It can be seen that the angle sensor 1 inside the eccentric shaft position sensor is damaged, which indicates that the quality of the eccentric shaft position sensor parts replaced by the maintenance personnel is defective. Install the eccentric shaft position sensor of the original vehicle replaced by the maintenance personnel and measure again. It is found that the signal starting voltages of terminals 3 and 9 are both about 4 V (Figure 10), which is abnormal. The signal starting voltage under normal conditions should be All are 0 V. So far, the diagnosis shows that the eccentric shaft position sensor of the original car and the eccentric shaft position sensor replaced by the maintenance personnel are both damaged (Figure 11).
Troubleshooting
Replace the eccentric shaft position sensor again, and the signal of the eccentric shaft position sensor returns to normal, indicating that the replaced eccentric shaft position sensor is normal. After learning the limit of the electronic valve control system with the fault detector, the engine starts normally and runs normally. The fault is eliminated.
Fault summary
If you use a multimeter to diagnose the fault of the car, the diagnosis process may be like this: use the multimeter to measure the power supply and grounding of the eccentric shaft position sensor, all are normal; measure the continuity of the eccentric shaft position sensor signal line, all are normal; replace the eccentric shaft After testing the position sensor (assuming the quality of the parts is defective), the fault remains; after replacing the DME, the fault remains the same. Because the multimeter cannot measure the signal of the eccentric shaft position sensor, and the current auto parts market is mixed, if the maintenance personnel only use the "fault detector + multimeter + replacement" maintenance diagnosis method, sometimes they feel very helpless. When the fault detector prompts that the signal of a sensor is abnormal, it involves the signal collector (the sensor itself), the signal receiver and processor (control unit) and the signal transmission line. In general, a multimeter can only measure the power supply, grounding and resistance of the sensor, but cannot measure the signal of the sensor. Therefore, after eliminating the possibility of the sensor's peripheral circuit failure, choose to replace the sensor or control unit.
Because this maintenance diagnosis method belongs to the elimination method, once the quality of the replaced sensor is defective, the maintenance personnel will not be able to detect it, and then will replace the associated parts again, if the fault is still not eliminated, then will he realize the replacement The parts may have quality problems, and then replace them one by one. Most maintenance personnel should have experienced the experience of troubleshooting only after replacing the same accessory many times. Although maintenance personnel with rich maintenance experience have been wary of the quality of parts, this diagnostic method still makes them feel psychologically unsure.
Using an oscilloscope can collect the working signals of sensors, actuators and communication lines, allowing maintenance personnel to diagnose faults from the perspective of the control unit, and use data to speak, so the diagnosis is no longer blind. The difficulty of using an oscilloscope to diagnose faults lies in the analysis of the signal. Everything is difficult at the beginning. As long as the maintenance personnel collect and analyze the signal more often, the ability to analyze the signal will naturally become stronger over time. After the working signal of the device, the "second kill" can be realized when diagnosing related faults.
About half a month after the failure of the car was diagnosed, a 2011 BMW 730Li car (equipped with an N52 engine) came into the shop for maintenance. Customers reported that the car occasionally had difficulty starting, and the engine fault light was turned on abnormally. Use a fault detector to detect and read the fault code as shown in Figure 12. Analyzing the fault code, the author believes that the fault of the car is also caused by the abnormal signal of the eccentric shaft position sensor. Use an oscilloscope to measure the signals of terminal 1 (P_CS1S signal terminal), terminal 7 (P_CS2S signal terminal), terminal 8 (P_CLKS signal terminal) and terminal 9 (T_DAT2S signal terminal) of the vehicle's eccentric shaft position sensor (Figure 13) at the same time, and find the terminals The signal of 9 is abnormal, and it is determined that the eccentric shaft position sensor is damaged. Test drive after replacing the eccentric shaft position sensor and learning the electronic valve control system to limit the position, troubleshooting.
Watch the video below
Auto disassembly-free diagnosis of BMW 325I eccentric shaft failure
Brief introduction of BMW variable valve lift system
BMW's variable valve lift system, also known as the electronic valve (Valvetronic) system, mainly changes the valve lift by adding eccentric shafts, valve servo motors, intermediate push rods and other components to its valve train (Figure 14). DME calculates valve opening time and valve lift based on signals such as camshaft position sensor, accelerator pedal position sensor, crankshaft position sensor and air flow sensor, and then controls the operation of the valve servo motor through the duty cycle; when the valve servo motor is working, the worm gear The mechanism will drive the eccentric shaft to rotate, and then push the valve through the middle push rod and rocker arm. The rotation angle of the eccentric is different, and the lift generated by the camshaft pushing the valve through the intermediate push rod and rocker arm is also different, so as to realize the control of the valve lift.
