The internal circuit of automobile computer can be divided into two parts, namely, the conventional circuit including input, output and conversion circuits and the microprocessor. Conventional circuits mostly use common electronic components, which can be repaired if damaged. In the actual use process, most of the faults of automobile computers occur in conventional circuits. If you want to repair the car computer, you must first make sure that it is a computer fault, so as to avoid blind maintenance, causing unnecessary waste of time and other circuit failures.
(1) The usual method to judge whether the computer is damaged is that the computer can't output the control signal correctly to drive the actuator when the relevant sensor signals can be normally input into the computer. Although this sentence is simple, it needs a lot of detailed basic inspection work. For example, if the engine cannot be started, make sure that there is no frequency voltage on the fuel injector plug when it is started. Check that the relevant circuits are normal, and the starting signal can be input to the engine computer normally, but the computer does not output the driving signal to the injector, so it can be concluded that there is a fault inside the engine computer.
(2) Find the damaged component according to the circuit. According to the circuit diagram or the actual circuit trend, find the corresponding computer terminal connected with the fuel injector, and then use the on-off block of the digital multimeter to start from the determined computer terminal and search along the printed circuit of the computer until a transistor is found. This is because computers usually use high-power transistors to amplify execution signals to drive actuators, so most of these failures are caused by short circuits of transistors that act as switches.
(3) Measure the triode to determine the triode. The pin corresponding to the printed line is the collector of the triode, and the thinner printed line next to it is the base. The confirmation method is to plug in the porous plug of the engine computer, start the engine, connect the voltage block of the multimeter to the printing line to be confirmed, and display 5V as the base. Test triode with multimeter. If it is found that the forward and reverse resistance between the collector (C) and the base (B) is infinite, it means that the triode has been disconnected. If the resistance between the collector (C) and the emitter (E) is found to be zero, the triode has been broken down. In addition, other triodes and diodes connected near the triodes also need to be measured.
(4) There are roughly the following methods to determine the model of the triode: ① Model. Check the model of triode, and determine the matching domestic triode through the triode correspondence table. ② Resistance. The base of a triode is generally connected in series with a resistor, and the resistance value of the base is close to that of the original triode, and the resistance values of different colors are different. Because the base of the transistor is controlled by the current and the computer voltage is fixed, it is necessary to control the current with a resistor. Too much current will burn out the triode, and too little current will not trigger it. ③ measurement. Measure the characteristics of triode with diode measuring block of multimeter. According to the characteristics of triode, only the 1 pin should conduct in one direction relative to the other two pins. With this property, it can be determined that it is a triode. Only one pair of pins is conductive in one direction, and the conductive pin of the other two pins is the base of the triode.
(5) When welding the replacement transistor to the circuit board, pay attention to the minimum amount of solder to avoid overheating. After welding, use a multimeter to measure whether the pins are not connected.
(6) Test the maintenance effect. Connect the computer board to the body harness, start the engine to check whether the corresponding function is normal, and touch the triode with your hand at the same time. Some heat is normal, and when it is hot, there is something wrong. Observe whether the fault light is on, and carry out a road test for a certain mileage.