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Strange Fault, A Small Bolt Stumps The Service Station!

Strange Fault, A Small Bolt Stumps The Service Station!

Recently, I received a master’s help, saying that I encountered a very strange fault case. I went to the service station to repair it, but it was not repaired, so I came to the small rail for help. In the process of maintenance, many obstacles were encountered. In this issue, the small rail will show you this “strange” fault case.

Strange Fault, A Small Bolt Stumps The Service Station! (2)
Strange Fault, A Small Bolt Stumps The Service Station! (3)

The Troubleshooting Steps Are As Follows Of Service Station:

1. Use A Diagnostic Expert To Read The Fault Code

Strange Fault, A Small Bolt Stumps The Service Station! (4)

When the decoder is used to enter the system, it is found that the decoder cannot enter normally; It indicates that there may be a fault in the can network. Check the can line of OBD port of service station.

2. Measure Obd Port Canh And Canl

Strange Fault, A Small Bolt Stumps The Service Station! (6)

Measure that the canh of OBD port is + 8.5V and canl is + 8.5V, and the can line is abnormal; Under normal conditions, the sum of two groups of can line values is about 5V. It is suspected that there is a problem with the can network cable. I intend to fly the cable directly first, read the engine fault code, and first solve the problem that the engine cannot start the car of service station.

3. Fly Wire Directly From Ecu To Obd Port

After flying the line, the voltage of canh and canl lines is still + 8.5V. It is suspected that there is a problem with the computer board. The next step is to replace the computer board of service station.

Tips:

For Bosch’s computer board, there is a 120 ohm resistance inside its can network. After power connection with the decoder (there is also a 120 ohm resistance inside the decoder), the resistance after parallel connection is 60 ohm; Therefore, the Bosch computer board can fly directly to the OBD port without adding 120 ohm resistance.

4. Replace Engine Ecu

Strange Fault, A Small Bolt Stumps The Service Station! (7)

After replacing the ECU, the engine can be started normally, the decoder is used to read the fault code, and the display system is normal.

Strange Fault, A Small Bolt Stumps The Service Station! (8)

Let the driver go out of the sports car. The next day, the car failed to start again. Follow the driver to check.

Strange Fault, A Small Bolt Stumps The Service Station! (9)

Check again and find that the voltage of OBD port is still 8.5V.

Now it is preliminarily suspected that the ECU on the computer board is damaged again. Before replacing the computer board, check the circuit first. If there is a problem with the circuit, replace the ECU, and the ECU will be burned again.

Tips:

For this kind of ECU damage for many times, we generally suspect that it is caused by abnormal loads on T15, power supply and grounding.

Strange Fault, A Small Bolt Stumps The Service Station! (11)

In this case, in step 5, we check the power supply, grounding and T15; Finally, after a round, the fault point still fell on the grounding wire. At this time, we were “hoodwinked” by the fault phenomenon.

Fault phenomena can help us analyze problems, but fault phenomena sometimes lead us to take a lot of detours. Take this fault as an example. In step 4, after replacing the ECU, the fault disappears. We think it is the problem of the ECU. In fact, when replacing the ECU, we touched the harness to connect the virtual contact, and the fault disappears. However, with the driving vibration, the fault reappears;

Another point is that when the ECU is close to the girder, it will also make people misjudge the fault. There are several special points about the fault case. Let me tell you about it.

Strange Fault, A Small Bolt Stumps The Service Station! (12)
  • 1. In step 7, we measured that the resistance between the ground wire of ECU and the girder is 0 ohm. Why can’t we measure the problem of virtual connection of grounding wire.
  • Virtual connection is a common fault in the circuit, but it is also a fault that is very difficult to check. Sometimes it is difficult to judge whether it is on or off only by multimeter. For determining the possible virtual contact, it can be determined by pulling the wire harness back and forth; For this fault, it was uncertain whether the ground wire was falsely connected in the early stage, so this method was not adopted during troubleshooting, resulting in no fault point was detected in the early stage.
  • 2. When the ECU ground wire is broken or falsely connected, what is the general voltage?
  • When the ground wire of ECU is broken, the output voltage of all pins of ECU will be a virtual 24V; However, when the ground wire of ECU is only falsely connected and not completely disconnected, all pins of ECU can output any voltage value of 1-24v, which is also obtained from the test after the small rail handles this fault.
  • ‘strange ‘fault, a small bolt stumps the service station!
  • In this issue, we will expand the instructions and tell you about the fuel injection principle of the fuel injector.
  • The core of diesel engine high-pressure common rail system is the control of fuel injection, which mainly solves the problems of fuel injection pressure, fuel volume, times and angle of each working cycle of each cylinder under certain working conditions. The advantages and disadvantages of these problems directly determine the emission, power, fuel economy and working noise of diesel engine system, It is a very key link in the electronic control system of diesel engine.
  • ‘strange ‘fault, a small bolt stumps the service station!
  • The engine mainly works through the combustion of fuel to output torque. As the diesel engine system becomes more and more complex, it carries more and more auxiliary components. In addition to outputting power torque, it needs to consume more and more torque. The more torque the engine needs to output, the more oil it needs to inject, so there is a certain relationship between the output torque and the injected oil. Therefore, the calculation of the injected oil can be calculated by the output torque.
  • The output torque can be calculated by the sum of various torque requirements, including the torque consumed by various auxiliary components and the torque requiring power output. The torque consumed by each auxiliary component can be calculated separately according to the physical characteristics of the component, and the torque of power output can be calibrated by the opening of accelerator pedal.
  • Therefore, the output torque can be completed by adding various demand torques, and the independence between each module is very strong. If a component is replaced, just recalculate the demand torque of the component. If a new component is added, just add the demand torque of the component to the total demand torque, and the expansibility of the control algorithm is very strong.
  • For different working conditions of the engine, only the torque that needs to be consumed under this working condition needs to be calculated. The calculation strategy of injection oil quantity based on torque coordination is shown in the figure below.
  • The demand for torque mainly includes four aspects. The internal working condition torque mainly refers to the torque that needs to be consumed under different working conditions of the engine, such as the starting torque required to start the engine, the idle torque to maintain the idle speed of the engine, and the limiting torque for overheating protection and limiting engine smoke in case of engine failure, The correction torque is mainly introduced to prevent excessive fluctuation of engine speed or demand torque, so as to ensure the smooth operation of the engine. The consumed torque, such as the engine torque and output torque consumed by accessories such as generator, air conditioner, gear pump and high-pressure oil pump, is the torque that the driver controls the output of the engine through the opening of accelerator pedal.
  • Finally, through the calculated total torque, according to the fuel economy of the engine, the required injection oil amount is calculated through the torque to oil module. The following focuses on the torque calculation under internal working conditions, such as starting, idling and normal working conditions.
Strange Fault, A Small Bolt Stumps The Service Station! (14)
Strange Fault, A Small Bolt Stumps The Service Station! (13)

When the ignition key of the engine is turned to the starting position, the starter drives the engine to run, and the engine speed is greater than a certain starting threshold. During this period of time, the engine is in the starting condition. The calculation of starting torque is shown in the figure.

‘strange ‘fault, a small bolt stumps the service station!

Firstly, because the torque required for cold engine and hot engine starting is completely different, check the pulse spectrum determined according to engine speed and engine temperature to obtain the basic starting torque of the engine.

Secondly, the basic starting torque is corrected according to the different inlet pressure.

Thirdly, in the new vehicle emission regulations, the emissions during starting are also included. In order to ensure reliable cold start and avoid smoke emission due to excessive fuel injection and insufficient combustion.

Therefore, the basic starting torque is set relatively small, and the torque is moderately increased according to the starting condition of the engine at that time. If the engine speed is less than a certain threshold within a certain period of time, the basic torque is increased by using the climbing function. The waiting time and the gradient of the climbing function usually adopt values related to the engine temperature, that is, different values are adopted under the condition of cold engine and hot engine. Finally, the starting torque should be less than a certain limit.

1.2 low idle torque calculation

Low idle speed refers to the torque required to overcome friction resistance and other factors when there is no external power torque demand and in order to maintain a certain stable speed of the engine. In order to ensure the stability of speed, the control mode is adopted to calculate the low idle torque. The calculation strategy of low idle torque is shown in the figure.

‘strange ‘fault, a small bolt stumps the service station!

1.3 accelerator pedal torque calculation

The accelerator pedal opening reflects the driver’s driving intention. When it is necessary to output large torque, it can be realized by increasing the accelerator pedal opening. In order to ensure the accuracy and reliability of the collected accelerator pedal signal, first of all, it is necessary to verify its reliability. As shown in the figure, the accelerator pedal signal adopts dual voltage mode, and its voltage ratio is 2:1.

 

‘strange ‘fault, a small bolt stumps the service station!

 

According to the characteristics of the accelerator pedal, the collected voltage value is converted into the accelerator opening. At the same time, combined with the current engine speed, the accelerator pedal torque corresponding to the accelerator opening at the current speed is obtained by querying the calibrated pulse spectrum. Finally, if the accelerator pedal signal is valid, output the accelerator pedal torque, otherwise output the accelerator pedal torque to 0.

 

1.4 torque to oil calculation

 

Through torque coordination, the torque that the starting engine needs to output under the current working condition can be calculated. For the engine, the output torque is controlled by controlling its fuel injection quantity. Therefore, it is necessary to convert the current torque to be output into the amount of oil to be injected according to the characteristics of the engine.

 

Because at the time of starting, the speed is low, the fuel combustion is insufficient and the fuel economy is poor, a fixed conversion factor is used to calculate the torque to fuel. Under normal working conditions, the fuel economy of diesel engine is also different at different speeds. Therefore, combined with the engine speed, check the pulse spectrum calibrated according to the engine characteristics to obtain the injection oil amount corresponding to the required torque at different speeds. The conversion process is shown in the figure.

‘strange ‘fault, a small bolt stumps the service station!

‘strange ‘fault, a small bolt stumps the service station!

‘strange ‘fault, a small bolt stumps the service station!

Pilot injection: injection performed in order to form an easy to burn gas mixture in the combustion chamber in advance, which helps to reduce the emission of particles and hydrocarbons.

Pre injection: the injection performed to form seed flames in the combustion chamber before the main injection. Because the air pressure in the pre injection cylinder will rise slightly, the ignition delay period of the main injection will be shortened, the combustion pressure will rise, and the peak value of the combustion pressure will be reduced, resulting in a flexible combustion, so as to reduce the combustion noise and reduce the emission of nitrogen and oxygen compounds.

Main injection: injection performed mainly to generate output torque.

 

Post injection: additional injection for complete combustion of residual fuel, which can reduce the emission of particulate matter.

Post injection: there is no such injection in the original combustion control injection, which is mainly the injection performed to increase the post-treatment effect of exhaust emission, such as the regeneration of the filter body of the particulate trap.

 

Under different engine working conditions, not all the above injections occur, but the combination of some injections. Therefore, these injections need to be coordinated and controlled, and the control strategy is shown in the figure. Firstly, calculate whether a single injection occurs according to the current working condition of the engine, then determine the maximum possible injection times according to the actual working condition of the system, and finally determine those injections according to the priority of injection.

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