This Car Can "Eat" The Computer Board. Is It A Ghost diagnosis. 3 types
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This Car Can "Eat" The Computer Board. Is It A Ghost

After the car was repaired, the fault recurred again, which is a problem we often encounter in maintenance; When dealing with such problems, we should jump out of the fault phenomenon and analyze the root cause of the fault. This issue of small rail will bring you such a fault case and diagnosis case.

As for the trouble of engine starting, our diagnosis direction is as follows:

  •  fuel injector failure, such as too small fuel injection at idle point;
    L sensor problems, such as signal drift of water temperature sensor and intake air temperature sensor, resulting in low idle fuel injection;
  •  intake and exhaust pipelines, such as blocked intake and exhaust, resulting in poor intake of engine;
  • tightness of cylinder, such as loose sealing of diagnosis inlet and exhaust valves, failure of exhaust braking in cylinder, failure of piston ring, etc;
    L fuel pipeline problems, such as air leakage in fuel pipeline, resulting in air entry in high-pressure pipeline, etc;
  • synchronization timing problems, such as single speed sensor starting, abnormal synchronization signal, etc;
  •  computer board ECU problems, such as abnormal data and ECU electrical aging.
    The above are the fault points we need to check when we encounter difficulties in starting the engine. Let’s talk about the troubleshooting steps of this case.
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  •  read the fault code with a diagnostic expert diagnosis
    The display system is normal, which proves that it is not the cause of the fault code and the engine cannot be started.
  •  use a diagnostic expert to read the data stream, which is the data stream when starting the engine. From the data stream, it can be preliminarily determined that the synchronization and rail pressure are normal
  • Unplug the water temperature sensor
    Unplug the water temperature sensor and increase the fuel injection quantity during starting; Start the engine again and the engine will not stop.

Tips of diagnosis:

 

  • After unplugging the water temperature sensor, the water temperature sensor will use an alternative value (generally – 40 ℃), and the ECU will increase the fuel injection quantity of the fuel injector; When the water temperature sensor is pulled out and can be started, it can be generally determined that the fault point is the injector, valve clearance, valve seat ring and piston ring.
  • use oscilloscope 
    Use the oscilloscope to collect the waveforms of camshaft and crankshaft. It is found that the waveforms of camshaft and crankshaft are located correctly, which proves that there is no problem with synchronization.
  • Use the cylinder pressure gauge to measure the tightness of the cylinder. Use the cylinder pressure gauge to find that there is no problem with the tightness of the cylinder, and the rest suspect the fuel injector.
  • Use the test lamp to measure whether the fuel injector is powered on. Using the test lamp, it is found that the fuel injector is not powered on when starting the engine. When the synchronization is normal and the rail pressure is normal, the fuel injector does not power up. It is suspected that there is a problem with the ECU.

Tips of diagnosis

  • For normal system, the conditions for fuel injector power on are: synchronous speed and rail pressure above 200bar
  • Replace ECU
    After replacing the ECU, the engine can start normally and the fault is solved.
  • Fault postscript:
    After a month, the car failed to start again. After troubleshooting, it was found that the fault point of the car was that the fuel injector could not be powered on. Replace a new ECU and solve the problem. Another month later, ECU failure occurred again.
  • In just two months, burning three computer versions will certainly not be a problem with the quality of the computer version. It can be preliminarily determined as the problem of the whole vehicle line. Use the oscilloscope to collect the voltage of power supply (K01 / k03 / K05), T15 and T50.
This Car Can "Eat" The Computer Board. Is It A Ghost
  • When collecting T15 harness with oscilloscope, it is found that the harness voltage fluctuation here is abnormal.
  • The highest peak voltage collected is 64V, which is more than twice the normal voltage value (normally not more than 32V), indicating that there must be a problem with the harness here.
  • The line change of this vehicle is serious, and the problem can not be solved after flying the line again. Therefore, it is recommended that the driver replace the engine and chassis harness.
  • After replacing the harness, the fault is solved.

case analysis of diagnosis

  • After the fault case is finished, I believe there is no problem for you to check whether the fuel injector is powered on. However, for the following problems, xiaorail also wants to analyze them with you.
  • case analysis
  • After the fault case is finished, I believe there is no problem for you to check whether the fuel injector is powered on. However, for the following problems, xiaorail also wants to analyze them with you.

How is the voltage fluctuation of T15 harness generated?

  • According to the requirements of Bosch harness, the power supply of ECU is directly from the battery and cannot be connected with other loads, especially high-power loads such as generator and starter. When a high-power load shares the power line with ECU, it may lead to violent voltage fluctuation, which is caused by electrical components such as capacitance and inductance in electrical equipment.
  • Therefore, the fault is because the load of the whole vehicle is connected to the ECU power supply, resulting in the internal burning of the ECU.
  • For this fault, why is there no flying wire, but the method of directly replacing the harness of the whole vehicle?
  • After the harness of the whole vehicle is disordered, it is difficult to restore it. If it is not restored well, it will bring other problems, and the computer version may be burned again. Therefore, in case of such problems, it is recommended to replace the harness of the whole vehicle directly.

Can you see the voltage change from T15 data stream?

  • Due to the different frequency of decoder data acquisition and ECU data acquisition, the voltage change will not be seen on T15

Knowledge expansion of diagnosis

 

  • In this issue, the small rail will tell you about the control logic of rail pressure in the knowledge expansion.
  • Firstly, determine the current fuel injection target pressure according to the current working condition of the diesel engine, then compare it with the actual pressure in the common rail, adopt the corresponding rail pressure control strategy and control method according to the current working condition of the engine, and determine the pump oil volume of the current high-pressure oil pump through the accurate control of the fuel metering unit, so as to make the oil pressure in the common rail reach the target pressure, so as to form a closed-loop control.
  • Rail pressure setting value calculation
    The rail pressure setting determines the target injection pressure of the injector. High injection pressure will promote fuel granulation, improve emission performance and fuel efficiency, but will increase engine noise and torque consumption of high-pressure oil pump. Therefore, the set value of rail pressure should change with the change of engine working conditions.
  • Calculation of initial value of rail pressure setting
  • The calculation of the initial value of rail pressure setting is shown in the figure below. First, check the pulse spectrum of the basic value of rail pressure setting based on the engine speed and the current fuel injection quantity to obtain the basic value of common rail pressure setting. On this basis, add the environmental correction quantity to obtain the initial value of rail pressure setting. The environmental correction mainly considers the correction of the set value of common rail pressure by four physical quantities: atmospheric pressure, intake air temperature, fuel temperature and engine temperature. Its calculation method is to multiply the set basic value of rail pressure by the correction coefficient obtained by checking the corresponding pulse spectrum, and add the four to obtain the correction amount of common rail pressure.
This Car Can "Eat" The Computer Board. Is It A Ghost
  • Set rail pressure limit calculation
  • If the set value of rail pressure is too high, it will cause overshoot of rail pressure and damage to mechanical components such as fuel injector, common rail and high-pressure oil pump. If the set value is too low, it will cause that the fuel injector cannot be opened and cannot be injected. Therefore, it is necessary to limit the set value range of common rail pressure.
  • According to the working principle of the fuel injector, when the battery voltage is low, the current through the solenoid valve will be small and the magnetic force will be reduced. In order to open the fuel injector, a relatively high common rail pressure is required. Therefore, in order to ensure that the fuel injector can be opened normally when the battery voltage is low, the set value of rail pressure should not be too small, and there should be a lower limit value, which is calibrated by the characteristics of the fuel injector. At the same time, the rail pressure range during normal operation of the engine is determined according to the current working conditions of the engine, so as to avoid wrong calculation of the set value of rail pressure
  • Rail pressure control strategy
    Diesel engines under different working conditions have different requirements and objectives for rail pressure control, so different control strategies should be adopted, either under open-loop control or closed-loop control. According to different working conditions of diesel engine, the rail pressure control strategy migrates the state machine, as shown in the figure.
  • Starting mode.
  • Mode entered after system initialization. In order to realize the rapid start of diesel engine, it is very important to quickly establish and reach the starting rail pressure. Therefore, the open-loop control is used to supply as much high-pressure fuel to the common rail as possible to promote the rapid establishment of common rail oil pressure. Since the oil pumping efficiency of the high-pressure oil pump varies greatly at low speed and low temperature of the diesel engine, in order to prevent excessive fluctuation of common rail pressure, the oil pumping volume of the high-pressure oil pump shall be obtained through actual calibration according to the physical characteristics of the equipped high-pressure oil pump and the current diesel engine speed and diesel engine temperature. When the speed of the diesel engine is greater than the low-pressure starting threshold of the diesel engine and the rail pressure fluctuation is within a certain amplitude range, that is, condition (1), it shifts from the starting mode to the transition mode.
  • Transition mode
    At this time, the rail pressure control is unstable, and the fault diagnosis management system does not monitor the rail pressure control effect. The value used is the volume flow calculated by the current method. When the speed of the diesel engine is less than the low-pressure starting threshold of the diesel engine, or the rail pressure fluctuation exceeds a certain amplitude range, that is, condition (2), it will be transferred from the transition mode to the starting mode. After waiting for a certain number of crankshaft revolutions in the transition mode, enter condition (3) and transfer from the transition mode to the normal mode.
  • Normal mode
    When the diesel engine is under normal working condition, the closed-loop control is adopted. At this time, the rail pressure control precision is high and the stability is good. The fault diagnosis system monitors the fault according to the effect of rail pressure control. However, when the conditions are met (2), this mode will directly migrate to the starting mode.
  • Shutdown mode
    In any mode, switch to this mode immediately after receiving the shutdown command. The shutdown command may be sent by the driver or by the fault diagnosis system. At this time, the rail pressure decreases rapidly, the high-pressure oil pump stops oil supply, and the open-loop control is adopted. The system can maintain a minimum rail pressure if necessary. If the diesel engine speed is greater than the minimum speed of closed-loop control, i.e. condition (4), move to the transition mode. When the speed of diesel engine is less than the minimum speed of closed-loop control, i.e. the condition, move to the starting mode.
Failure mode

In case of failure of rail pressure control unit, such as rail pressure sensor failure, a safe alternative value is given according to the characteristics of oil pump in order to ensure “slope home”.

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