In modern automobile maintenance, the workload of fault diagnosis of electronic control system is increasing. For some beginners of automobile maintenance, it is easy to take detours and spend a lot of time because of incorrect diagnosis steps. The author thinks that correct diagnosis steps are the guarantee of successful and effective diagnosis. With this guarantee, no matter how difficult the fault is, it will be solved. The following is the author's practical experience for colleagues' reference.
Fault diagnosis steps
(1) Preliminary observation
Open the engine compartment cover and observe whether the engine parts are complete, whether the vacuum tube falls off, whether the wire connector is loose, whether there are oil leakage, liquid leakage, air leakage and electric leakage, whether the engine runs smoothly at idle speed, and whether the exhaust pipe emits black smoke or smells of gasoline.
(2) code reading-code clearing-operation-code reading again
Connect the scan tool, query the fault codes, record the read permanent and accidental fault codes, and then clear the fault codes. Start the engine. When the coolant temperature reaches above 80℃, the engine runs at high speed for a few seconds, which creates conditions for fault reproduction. Check the fault code again and make records. % 26lt Home of Auto Repairman% 26gt
(3) Fault code analysis
Use the maintenance manual to check the causes, effects and troubleshooting methods of fault codes. Occasional fault codes cannot be ignored. If the fault code is not stored, it is necessary to consider the parts that the control unit cannot monitor, such as the ignition coil fault of Santana Superman car, and there will be no fault code display. Other methods should be used to determine whether there is a fault.
(4) reading the data stream
The engine should meet the conditions for reading the data stream. For the data beyond the normal value in the data stream, we should refer to the fault causes listed in the maintenance manual for analysis. The data stream can provide real-time data of engine running state, and whether the data stream can be correctly analyzed can fully reflect the technical level of the diagnostic staff.
(5) check the measurement
According to the fault phenomenon, fault code content and related values in the data stream, the measurement items can be determined. Necessary measurements can be made by using multimeter, diode test pen, exhaust gas analyzer, fuel pressure gauge, vacuum gauge, cylinder pressure gauge, oscilloscope, analog signal generator and injector detection cleaner. The principle of selecting instruments is to judge faults quickly and accurately.
(6) Troubleshooting
According to the above work records and referring to the maintenance manual or related materials, the fault is analyzed, and the diagnosis conclusion and repair scheme are obtained, such as cleaning the throttle valve, valve and air inlet, adjusting or replacing parts, peeling off the wiring harness to find the fault point, cleaning the grounding wire, etc.
(7) Completion inspection
Test again with a scan tool, exhaust gas analyzer and other equipment to confirm whether the fault is eliminated. Failures such as engine flameout, accelerated running into the car, and insufficient power must be road tested, and the delivery can only be completed after the failure is completely eliminated. If the fault is not eliminated or not completely eliminated, repeat the above diagnostic steps if necessary. % 26lt Home of Auto Repairman% 26gt
As long as you have strong self-confidence, correct diagnosis steps, careful inspection and measurement, and careful analysis of ideas, any fault will not be difficult for the diagnostician.
Fault 1 Accelerated engine flameout.
Model: Jetta ATi
Fault symptom: Engine flameout during rapid acceleration. After the fault occurs, use the scan tool V. A.G 155 1 to clear the fault code and drive normally. Sometimes the fault occurs once a month 1 time, sometimes twice a day.
Inspection: Connect the diagnostic instrument V.A.G 155 1 for inspection, and the equipment prompts that the engine load signal is wrong. The maintenance manual indicates that the fault is caused by the fault of throttle body, intake pressure sensor (Figure 1) or control unit. After testing, the throttle body, intake pressure sensor and connecting wire are all normal. After that, the waveforms of throttle potentiometer G69 and the throttle potentiometer signal input to the automatic transmission control unit were observed by an oscilloscope. The waveforms of the two signals are so different that the waveform input to the automatic transmission control unit is almost a straight line.
Figure 1 intake pressure sensor
Analysis: The automatic transmission control unit mainly upshifts and downshifts according to the throttle potentiometer signal and the vehicle speed signal. If the throttle potentiometer signal is inaccurate, the shift timing will be inaccurate, and even the phenomenon of accelerating flameout will occur. The above tests show that the throttle potentiometer signal output by the engine control unit is wrong, and the problem is caused by the engine control unit.
Troubleshooting: After replacing the engine control unit (Figure 2), troubleshoot.
Fig. 2 Engine control unit
Keywords: electronic control engine maintenance methods
Troubleshooting Methods of Jetta Engine Electronic Control System (2)
Fault 2: The engine can't start after it rains.
Model: Jetta GiF
Fault symptom: the engine can't start, and the user states that the fault occurred after a rain.
Inspection: Using the scan tool V.A.G 155 1, the equipment cannot enter the engine control system. Suspect that there is something wrong with the engine control unit, and prepare to remove the engine control unit for inspection. When the running water board under the windshield was disassembled, it was found that there was water in the running water box. After the sundries blocking the drain hole (Figure 3) are taken out, the accumulated water will flow out.
Fig. 3 The drain hole is blocked.
Analysis: The engine control unit is installed on the left side of the water tank. Because the drainage hole of the water guide tank is blocked by leaves and other sundries, if the water cannot be discharged in time when it rains or washes the car, it will soak the control unit. Therefore, in vehicle maintenance, it is necessary to check the drainage holes on the left and right sides of the water tank and take out the corresponding sundries.
Troubleshooting: After the plug of the engine control unit is disconnected, it is dried by compressed air, and the equipment still cannot enter the engine control system, so it is judged that the engine control unit is damaged. After replacing the new control unit, the query data flow is completely normal and the car is delivered.
Fault 3: Steady-state accelerated working condition method is not exceeded.
Model: Jetta AT
Fault phenomenon: the vehicle is using the steady-state accelerated working condition method to detect that the exhaust emission is unqualified, in which CO and HC are normal and nitrogen oxides exceed the standard.
Inspection: cleaning fuel injector, carbon deposit in combustion chamber and replacing spark plug failed.
Analysis: Check that the part number of the engine control unit of this car is L06A 906 0 18 EL, and there is something wrong. After consulting the data, it is found that although the engine control unit has the same part number, there are four suffixes: G, EL, EK and GE. Suffix G indicates that the system is equipped with a three-way catalytic converter, but the version is lower and it is transitional; EL indicates that the system has no three-way catalytic device; EK indicates that the system is equipped with a three-way catalytic device, and there is no theft prevention; GE means that the system is equipped with a three-way catalytic device with anti-theft. After inspection, the car is equipped with a three-way catalytic device. In addition, although the anti-theft control unit is equipped, it does not have the anti-theft function of the engine control unit, so the engine control unit with the suffix EK should be installed. % 26lt Home of Auto Repairman% 26gt
Troubleshooting: After replacing the control unit with the original suffix EL with the control unit of EK (Figure 4), the tail gas emission is qualified after testing.
Fig. 4 Control unit with correct suffix
Fault 4 The engine cannot be started.
Model: Jetta GiX
Symptoms: When the vehicle is started, the speed of the starter is normal and the engine cannot be started. I have been to the roadside repair department and advised the user to replace the engine harness.
Inspection: Connect the scan tool V.A.G 1552 to enter the engine control system, and the equipment shows that the engine ECU cannot communicate. When the vehicle is started, check that there is no high pressure fire in the spark plug.
Analysis: According to the above two inspection results, it can be judged that the engine control unit is not working. There are two reasons why the general engine control unit does not work: one is that the control unit is not powered on, and the other is that the control unit is damaged. Jetta GiX car is equipped with Siemens SIMOS 3PW engine management system. Engine control unit is 12 1 pin, 1 pin and 2 pins are grounding wires, 6 1 pin is ignition switch live wire, and 62 pins are normal live wires. Pull off the plug of the engine control unit and measure the wire harness end of the plug. 1, the grounding resistance of pin 2 is zero, the grounding voltage of pin 6 1 is 12 V when the ignition switch is turned on, and the grounding voltage of pin 62 is zero. According to the analysis of the circuit diagram, the No.62 pin of the control unit is connected with a red wire with a cross-sectional area of 0.35mm2, and the other end of the wire is connected with the No.9 pin of the relay box plug G2 (G2 is a white 9-pin plug). When measuring this wire, there is no open circuit fault in the middle of the wire. The voltage at pin 9 of plug G2 of relay box is 12 V, but there is no voltage at pin 62 of measurement control unit. The above measurement results are unreasonable, but if you think about it carefully, it should be explained as follows: when measuring the ninth pin of G2 plug, insert the stylus from the back of the ninth pin of the plug, so that the ninth pin of the plug can be pressed firmly. After the stylus is removed, the ninth pin is disconnected, so there is no voltage on the pin of the measurement control unit 62.
Troubleshooting: Remove the relay box, and find that the 9th pin of the G2 plug of the relay box is only slightly retracted, and the plug can't touch the pin. Replace relay box and connect all plugs. Connect V.A.G 1552 for smooth communication with the engine control unit. After clearing the fault code, start the engine normally.
Fault 5 Engine starting is difficult.
Model: Jetta GTX
Symptom: The engine will not start.
Inspection: Connect the scan tool V.A.G 155 1 to detect the engine control system without fault codes. When starting the engine, it was found that the electric fuel pump did not work and there was no high-pressure fire. Later, it was found that the engine control unit could not communicate sometimes, so it was decided to check the power cord and grounding wire of the engine control unit. Other grounding points are ok, but it is found that there is a layer of paint on the grounding point column (Figure 5) next to the control unit.
Figure 5 Ground point of control unit
Analysis: The car was repaired by car body panel and painting because of an accident. At that time, the paint of this grounding point was not removed, and there was no fault for some time after it was repaired. After a long time, the grounding here is poor, which eventually leads to the car not starting.
Troubleshooting: Test after grinding and fastening, and start it successfully once each time. % 26lt Home of Auto Repairman% 26gt
Keywords: electronic control engine maintenance methods
Troubleshooting Methods of Jetta Engine Electronic Control System (2)
Fault 2: The engine can't start after it rains.
Model: Jetta GiF
Fault symptom: the engine can't start, and the user states that the fault occurred after a rain.
Inspection: Using the scan tool V.A.G 155 1, the equipment cannot enter the engine control system. Suspect that there is something wrong with the engine control unit, and prepare to remove the engine control unit for inspection. When removing the running water board under the windshield, I found that there was water in the running water box. After the sundries blocking the drain hole (Figure 3) are taken out, the accumulated water will flow out.
Fig. 3 The drain hole is blocked.
Analysis: The engine control unit is installed on the left side of the water tank. Because the drainage hole of the water guide tank is blocked by leaves and other sundries, if the water cannot be discharged in time when it rains or washes the car, it will soak the control unit. Therefore, in vehicle maintenance, it is necessary to check the drainage holes on the left and right sides of the water tank and take out the corresponding sundries.
Troubleshooting: After the plug of the engine control unit is disconnected, it is dried by compressed air, and the equipment still cannot enter the engine control system, so it is judged that the engine control unit is damaged. After replacing the new control unit, the query data flow is completely normal and the car is delivered.
Fault 3: Steady-state accelerated working condition method is not exceeded.
Model: Jetta AT
Fault phenomenon: the vehicle is using the steady-state accelerated working condition method to detect that the exhaust emission is unqualified, in which CO and HC are normal and nitrogen oxides exceed the standard.
Inspection: cleaning fuel injector, carbon deposit in combustion chamber and replacing spark plug are all unqualified.
Analysis: Check that the part number of the engine control unit of this car is L06A 906 0 18 EL, and there is something wrong. After consulting the data, it is found that although the engine control unit has the same part number, there are four suffixes: G, EL, EK and GE. Suffix G indicates that the system is equipped with a three-way catalytic converter, but the version is lower and it is transitional; EL indicates that the system has no three-way catalytic device; EK indicates that the system is equipped with a three-way catalytic device, and there is no theft prevention; GE means that the system is equipped with a three-way catalytic device with anti-theft. After inspection, the car is equipped with a three-way catalytic device. In addition, although the anti-theft control unit is equipped, it does not have the anti-theft function of the engine control unit, so the engine control unit with the suffix EK should be installed. % 26lt Home of Auto Repairman% 26gt
Troubleshooting: After replacing the control unit with the original suffix EL with the control unit of EK (Figure 4), the tail gas emission is qualified after testing.
Fig. 4 Control unit with correct suffix
Fault 4 The engine cannot be started.
Model: Jetta GiX
Symptoms: When the vehicle is started, the speed of the starter is normal and the engine cannot be started. I have been to the roadside repair department and advised the user to replace the engine harness.
Inspection: Connect the scan tool V.A.G 1552 to enter the engine control system, and the equipment shows that the engine ECU cannot communicate. When the vehicle is started, check that there is no high pressure fire in the spark plug.
Analysis: According to the above two inspection results, it can be judged that the engine control unit is not working. There are two reasons why the general engine control unit does not work: one is that the control unit is not powered on, and the other is that the control unit is damaged. Jetta GiX car is equipped with Siemens SIMOS 3PW engine management system. Engine control unit is 12 1 pin, 1 pin and 2 pins are grounding wires, 6 1 pin is ignition switch live wire, and 62 pins are normal live wires. Pull off the plug of the engine control unit and measure the wire harness end of the plug. 1, the grounding resistance of pin 2 is zero, the grounding voltage of pin 6 1 is 12 V when the ignition switch is turned on, and the grounding voltage of pin 62 is zero. According to the analysis of the circuit diagram, the No.62 pin of the control unit is connected with a red wire with a cross-sectional area of 0.35mm2, and the other end of the wire is connected with the No.9 pin of the relay box plug G2 (G2 is a white 9-pin plug). When measuring this wire, there is no open circuit fault in the middle of the wire. The voltage at pin 9 of plug G2 of relay box is 12 V, but there is no voltage at pin 62 of measurement control unit. The above measurement results are unreasonable, but if you think about it carefully, it should be explained as follows: when measuring the ninth pin of G2 plug, insert the stylus from the back of the ninth pin of the plug, so that the ninth pin of the plug can be pressed firmly. After the stylus is removed, the ninth pin is disconnected, so there is no voltage on the pin of the measurement control unit 62.
Troubleshooting: Remove the relay box, and find that the 9th pin of the G2 plug of the relay box is only slightly retracted, and the plug can't touch the pin. Replace relay box and connect all plugs. Connect V.A.G 1552 for smooth communication with the engine control unit. After clearing the fault code, start the engine normally.
Fault 5 Engine starting is difficult.
Model: Jetta GTX
Symptom: The engine will not start.
Inspection: Connect the scan tool V.A.G 155 1 to detect the engine control system without fault codes. When starting the engine, it was found that the electric fuel pump did not work and there was no high-pressure fire. Later, it was found that the engine control unit could not communicate sometimes, so it was decided to check the power cord and grounding wire of the engine control unit. Other grounding points are ok, but it is found that there is a layer of paint on the grounding point column (Figure 5) next to the control unit.
Figure 5 Ground point of control unit
Analysis: The car was repaired by car body panel and painting because of an accident. At that time, the paint of this grounding point was not removed, and there was no fault for some time after it was repaired. After a long time, the grounding here is poor, which eventually leads to the car not starting.
Troubleshooting: Test after grinding and fastening, and start it successfully once each time. % 26lt Home of Auto Repairman% 26gt
Hello, I wonder if this can help you ~ ~