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