Mileage of about 20 thousand km, equipped with 473QE engine, matching the joint electronic 1788 engine control system of BYD speed sharp car (model QCJ7150W). User response: sometimes the vehicle can not be accelerated. Fault diagnosis: after the vehicle is tested, no matter how to step on the accelerator pedal when the fault occurs, the vehicle can not be accelerated. The possible reasons for the failure are that the fuel pump is not working occasionally or the fuel system is blocked, and the acceleration pedal position sensor and its line failure cause the acceleration pedal signal to not be accurately transferred to the engine control unit; the electronic throttle and its line failure cause the throttle opening to be fixed at a certain level A position. It connects the fault detection instrument detection, the fault free code storage, reads the engine data flow, and finds that the acceleration pedal position signal changes with the acceleration pedal position when the failure occurs, but the throttle opening is always about 3%. Check throttle, no clamping phenomenon; check throttle position sensor and throttle actuator circuit, no abnormalities. After replacement of throttle body assembly, the test run is still in trouble. At this point, the fault diagnosis has fallen into a deadlock. In the absence of diagnostic ideas, the engine control unit, the accelerator pedal position sensor, the fuel pump, the crankshaft position sensor and the camshaft position sensor are replaced, but the failure is still unruled. Reorganizes the maintenance thought, thinks that the car failure phenomenon (no matter how to step on the accelerator pedal, the vehicle can't speed up) is very similar to the emergency protection operation mode of the engine control unit when the electronic throttle system fails, but there is no fault code storage in the engine control unit of the car, and the throttle body is assembled and accelerated. Both the pedal position sensor and the engine control unit have been replaced. Why do the faults still exist? After careful examination of the engine control system circuit, it is found that the terminal 28 and the terminal 30 of the vehicle engine control unit receive the brake signal 1 and the brake signal 2 respectively. What is the use of the brake signal from the engine control unit? According to the maintenance data, the combined electronic 1788 engine control system has the brake priority control strategy, that is, when the engine control unit receives the brake signal, the driver has the braking intention, and the engine control unit will limit the engine torque at this time. According to this control strategy, when the vehicle is able to speed up normally, a man can disconnect the wire on the terminal 30 of the engine control unit, and the fault is reproduced. Thus, it is inferred that the fault of the car is related to the wrong brake signal received by the engine control unit. Check the brake signal line, no abnormalities, doubt that the brake switch is not properly adjusted or the brake switch is damaged. Troubleshooting: re adjust the brake switch after trial run, speed up the normal return of the vehicle, troubleshooting. Fault analysis: throttle is an important part of the gasoline engine to control the intake volume and adjust the engine load and power. The electronic throttle is a throttle device driven directly by an electric motor. Like the cable throttle, once the electronic throttle opening increases, the throttle position sensor (TPS), the air flow sensor (MAF) or the intake manifold absolute pressure sensor (MAP) will have the corresponding signal input engine control unit. According to the main control program, the design rules and the engine control list will be fixed. Yuan will increase the fuel injection pulse width, so that the engine power and torque will greatly improve. When the electronic throttle system fails, such as the acceleration pedal is jammed or accelerates the pedal position sensor signal abnormal, and the electronic throttle card is stagnant at a large opening, the engine power and torque will be immediately improved according to the main control program. In the condition of no forewarning and prevention, the failure of the electronic throttle system is very easy. It is easy to cause the vehicle to get out of control and cause an accident. To this end, a set of protection procedures has been written in the engine control unit for the vehicle equipped with electronic throttle system to prevent such phenomena. The protection program can be divided into 2 types: one is triggered when the electronic throttle system has related faults (such as the engine control unit detected the throttle position sensor, the acceleration pedal position sensor, etc.); the other is triggered when the brake switch signal is received, that is, the brake priority protection program. There are several conditions for the brake priority protection program to be enabled: one is the accelerator pedal is in a larger stroke, the two is to step on the brake pedal (no need to step on, only need to step lightly); and the three is the speed greater than a certain value (such as 60 km/h). Due to the improper adjustment of the brake switch of the car, the engine control unit occasionally receives the abnormal brake switch signal at the speed of acceleration, thus the brake priority control program is enabled and the signal of the accelerator pedal position sensor can not be responsive to the speed of the accelerator pedal position and the injection pulse width is limited to the idle speed, the engine power and speed. Limited, so that vehicles can't speed up occasionally. Through this case, the author reminds the majority of the maintenance personnel not to restrict the detection objects to the electronic throttle assembly, the accelerant pedal position sensor and the engine control unit when diagnosing the failure of the electronic throttle system. As shown in Figure 1, there are many factors affecting the control of the throttle actuator of the engine control unit, such as the brake switch signal, the clutch switch signal (manual gear type), the air conditioning compressor working request signal and so on. In addition, in the diagnosis of electronic control system failure, in addition to the diagnosis of the hardware part of the electronic control system "components of the electronic control system", we should also understand the control strategy of the software part of the electronic control system (control program within the control unit).