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Fault diagnosis: connect TECH2, test engine electronic control system (ECM) have fault code P0700, transmission control module (TCM) MIL start request, this is the TCM request bright engine fault indicator light (MIL). The automatic transmission control module (TCM) is detected with fault code P1781 and engine torque signal failure. The description of the fault code P0700 in the maintenance data is that the fault code indicates that the transmission related transmission fault code is set in the transmission control module (TCM), the engine control module (ECM) receives the TCM information through the serial data circuit, and when the TCM sends the message to the fault indicator when the TCM is sent through the serial data circuit, ECM lighting fault indicator. P0700 belongs to A fault code. The fault indicator light is not turned on. The description of fault code P1781 in maintenance information is that the transmission control module (TCM) is at 2. The engine speed signal fault is detected and the trouble free code U2105 is detected. The action after the fault code setting: (1) the transmission control module (TCM) will turn on the fault indicator and store the fault code when the fault is detected in the first ignition cycle. (2) no engine torque control. (3) no joint pressure control. (4) No 4 gear. (5) no self-learning control. From the above analysis, it is known that the engine fault indicator light is because the transmission control module (TCM) stores the fault memory that affects the emission, sends out a fault light request to the engine control module (ECM), and produces a series of explanations for the 6 miscellaneous annotation of the starboard frog, which is not in the TECH2 and the maintenance data. Same, but this does not affect our judgement of faults. From the theoretical analysis, the possible reasons for the failure include: (1) the wiring harness or connector between the transmission control module (TCM) and the engine control module (ECM). (2) engine control module (ECM). (3) the transmission control module (TCM). The communication between Excelle car ECM and TCM is CAN BUS bus. The circuit is shown in Figure 1, and the signal content transmitted by bus is shown in Figure 2. Information sent from ECM to TC M includes engine speed, throttle position, engine temperature, intake temperature, intake pressure, ignition timing, engine torque and so on; information sent from TCM to ECM includes speed, gear, transmission temperature and so on. In the case of a bus failure, the affected signal may not be more than one of the signals. According to this point, the possible cause of the failure is judged within the TCM and ECM modules. Considering that if the engine speed sensor fails, the engine will not run, so it is suspected that it is an internal fault of TC M. But at that time no goods, no replacement of TCM. According to the order from simple to complex, from outside to inside, first check the communication lines between ECM and TCM, and the result is good. Then consult with the client to clear the trouble code first, let the customer drive the car back. Nearly a month later, customers never came to repair, call, ask customers, said the fault never appeared again. After another month, the customer was repaired by the same trouble. At that time, the TC M was loaded, because the failure did not happen frequently, so that customers could drive back to trial run. After more than a month, the car was repaired with the same fault. This time, the ECM was replaced, and it had been tested for more than 5 months. The failure never appeared again. Later, we encountered two similar failures, all of which are to replace ECM to solve the problem. It can be seen that this fault is a fault in the internal processing circuit of ECM, and it does not send all the information sent to the CAN BUS line.