Engine repair is not believe “evil”!

Time: 2018-10-01

Let’s take a look at the story of this almost “hedge”:

An inter-provincial passenger bus with a four-cylinder engine travels between the two places, 280 kilometers apart. The engine’s lack of power is also a common fault, but the fault conditions that occur in this vehicle are not the same as the normal situation, and even some people find it strange – according to the driver feedback, every day is from When the county went to the 80km location, there was a power failure. After the failure, the engine had only 1800 rpm and the fuel did not respond. This failure situation continued until returning to the county. However, the next day when the car again, the front 80 km power is normal, but also the same failure to 80 KM position. This phenomenon has continued for almost a year! The company’s maintenance staff are helpless in this failure, have speculated: Do you have any “hedge” thing in the 80KM position? !

When the maintenance personnel received the repair call for the vehicle fault, he expressed that he did not believe this “fair” and rushed to the fault site.

On-site processing

First, confirm the phenomenon of failure:

After reaching the site, the diagnostic equipment was connected to check the non-history code. The empty car was tested in the urban area. The boost pressure and fuel injection were normal, and the engine was determined to be fault-free and the power was normal.

The driver started the car normally and collected data with the car. The facts were as true as the driver said. The driving power of the 80KM engine in front was normal. When the vehicle arrived at the location of 80KM on the daily driving route, there was no response to the accelerator pedal. The maximum speed was only 1800 rpm. The phenomenon of failure, turn off the restart, hung up to the start of a stall on the throttle can only be up to 1800 rpm. The data is as shown below:

In the figure, the red curve is the throttle opening, the blue curve is the engine speed, and the gray curve is the vehicle speed. As can be seen from the figure, the driver started at step 1 and stepped on the throttle, and the speed increased from 0. When the engine speed reached 2000, it fell back to 1800 and could no longer be improved. At this time, the speed was only 12km/h and could no longer be improved.

Is… really scary

(My heart is almost collapsed)……

Second, data analysis:

Do not forget, we maintenance personnel technology is not covered, how can we hear this strange rumors? The maintenance personnel then carefully analyzed the data:

According to common sense, the first-speed engine of the vehicle can definitely reach the highest speed, but the faulty car is limited to 1800 rpm. Take the data in the figure 846-860, and add the cyclic fuel injection amount for further analysis, as shown in the following figure:

In the figure, the red curve is the throttle opening degree, the blue curve is the engine speed, the gray curve is the smoke limit oil quantity, and the yellow line is the cycle fuel injection quantity. From the figure, after the start of 1st gear, the throttle is full and the engine speed rises. The actual cycle fuel injection is about 75mg/cyc, close to the smoke limit the amount of oil, but when the speed is increased to about 1747, the actual cycle fuel injection suddenly dropped out of the smoke limit, the oil volume rapidly decreased, and finally stabilized at 32mg/ Cyc (shown in the green circle in the figure), the throttle opening continues to increase at this time, but the engine speed is stable at 1800 rpm and no longer rises.

It can be judged from this that this situation is caused by the engine actively reducing the circulating fuel injection and actively reducing the power. There are two situations in which the engine actively reduces power. One is a failure strategy, and the other is a vehicle speed limit.

Third, troubleshooting:

1, fault strategy

When this fault occurs, the ECU reports a current fault code: P0704 Clutch signal error – unreasonable. Check the clutch switch status at this time to 129. After arriving in Meitan County, the driver was asked to drive the car to the repair shop. The clutch switch No. 166 was cut off from the 48 terminal and the clutch switch status changed to 1. The P0704 fault code was not reported and the test run was still ineffective. . Therefore, the focus turned to the vehicle speed limit.

  1. Vehicle speed limit:

The vehicle’s vehicle speed limit is achieved through a smart drive module of the vehicle. The smart drive module sends the TSC1 torque limit command according to the vehicle speed to limit the torque and speed of the engine to limit the speed. The smart drive module is installed on the left side of the cab, as shown by the red circle in the following figure:

Unplug the power supply harness below the smart drive module (pointed by the arrow in the figure) and test the vehicle. The power of the vehicle will return to normal and the test run will no longer occur.

Fourth, analysis conclusion:

The cause of the power shortage is the malfunction of the smart drive module that executes the speed limit. The torque limit command is sent incorrectly, limiting the torque and speed of the engine.


The engine failures are all due to be followed. Do not be confused by the surface phenomena that have not been dealt with before. Carefully observe, carefully analyze, and troubleshoot problems can be solved.

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