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The mileage is about 50 thousand km, equipped with 2010 Buick Buick XT cars equipped with LDE engine and manual transmission. The car owner reflected that the vehicle could not start. Know that the car has been a gap can not start the phenomenon, before 1 months encountered 1 times, now more and more frequent, sometimes 1 weeks 1 times, even 1 days 2 times can not start. Fault diagnosis: first of all, the initial inspection of the vehicle, fuel and electricity are normal, the engine wiring harness is not abnormal, the battery terminal is not loose, it is no sign of ignition when starting. Then the general purpose special diagnostic instrument GDS is connected. The result shows that it is unable to enter the vehicle system and uses a digital multimeter to measure the high and low line resistance of high speed GMLAN, the value is 121 Omega, and the normal value should be about 60 Omega, which indicates that the fault is related to the vehicle network system. The vehicle's network system uses high-speed GMLAN system, as shown in Figure 1. It mainly includes engine control module (ECM), automatic transmission control module (TCM), electronic brake control module (ABS), power steering control module (EP guide and body control module (BCM), etc.). To ensure the normal work of the subsystems of any module in the network, the necessary information provided by the residual subsystem can not be separated, and the required information can be provided for the residual subsystem, so as to realize the information sharing of the entire vehicle network system. The serial data network in the high speed GMLAN system consists of twisted pair lines, in which the "D-BU" line is the high line of high speed GMLAN, and the "WH" line is the low line of high speed GMLAN. 2 120 Omega terminal resistors are arranged in parallel in parallel to prevent the data from being disturbed during the bus transmission. As seen in Figure 1, the 2 120 - Omega terminal resistors are in the engine control module (ECM) and the body control module (BCM), and the high speed GMLAN and low line values are now 121 Omega, indicating that one of the modules is disconnected from the network system. According to the layout of the network system, the electronic brake control module (ABS) is the center for testing. Disconnect the negative pole column of the battery, pull down the ABS wire harness, use the electronic multimeter to measure the resistance between "9" and "11" at the end of the wire beam. It is 121 Omega, normal; the resistance between the "10" and "12" of the remeasured line beam end is 0 Omega, it is abnormal, and the problem is between ABS and ECM, and they pass between themselves. Dynamic transmission control module (TCM). Then the wire harness of TCM and ECM was pulled out, and the lines between TCM and ABS and the line between TCM and ECM were measured respectively. The results were all in the normal range, indicating that the wires themselves had no problem, but the measurement had a circuit breakage, and the initial judgment was that there was a virtual connection between ABS and ECM, causing the system to have no communication. The problem. In order to verify this judgment, all the wiring harnesses between ABS and ECM are connected, and the resistance of high and low lines of high speed GMLAN is measured again. It is shown as 120.5 Omega, indicating that the line connection of the network system is normal, and the previous judgment is confirmed. Although the initial problem has been found, is the fault point in the plug of the ECM module or in the plug of the TCM module? The exact point of failure has not been found, so continue to search. Test the plug of TCM, use the digital multimeter to connect the 2 lines between ABS and ECM (ABS: 10/12), and gently pull and shake the TCM module plug. It is found that the resistance of the multimeter is constantly beating, indicating that the TCM module plug has a problem; and then testing the plug of ECM is the same method, but the resistance of the multimeter is shown. It does not change, indicating that the plug of ECM is normal. The plug of the TCM module is decomposed, and the opening of the 4 GMLAN data lines is found to be large, the opening is adjusted and the conductive adhesive is adjusted. After loading, the special diagnostic instrument GDS has normal communication, and the vehicle can start smoothly. A week later, there was no more problems with the car. Fault summary: the accidental failure of the car is caused by the excessive opening of the 4 GMLAN data lines of the automatic transmission control module (TCM), which causes the sometimes virtual connection. In the actual troubleshooting, the wire or wire connector should be swayed in the vertical and horizontal direction and before and after. At the same time, the resistance of the multimeter should be observed to check whether there is any fault such as false welding, loosening and bad contact of the wire connection. In addition, in the exchange with peers, the model was found to be similar to some problems, some of which should be replaced by replacing the wiring harness and wiring harness.