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A Buick GL8 business car with a mileage of about 183 thousand km in 2004. The left front window glass elevator does not work, and the window lifting main switch on the left front door can not control the right front window glass lift, but the right front window lift switch can normally control the right front window glass lifting. Fault diagnosis: first, confirm the fault after receiving the vehicle. After repeated tests, it was found that when the failure phenomenon started, the window and lifting main switch could not control the left front window glass lifting or the right front window glass lifting, and the right front window lifting switch could control the right front window glass lifting. The driver was told that the car had changed the main window switch in other repair plants, but it failed to recur after a period of time. Remove the main window switch and check the voltage of each terminal in turn. Using the test lamp to measure the yellow power line, it is found that the lamp filament of the test lamp is red and can not reach the normal brightness. The main switch of the window lifting and lifting, the light intensity of the test lamp is darker; the voltage of the yellow power line is measured by the multimeter, only about 3V. According to the above test results, the reason that the main switch of the window lift does not work is due to the poor contact of the yellow power line, resulting in its low actual voltage. It is found that there are 3 wire connectors between the fuse box and the main switch of the car window, that is, C200, C303 and P500. First, it is doubted that there is a bad contact in the 3 wire connectors. Disassemble the rubber sleeve at the hinge joint of the left front door, find the connector P500, be measured normally, and then find the connector C303 from the right A column, and be measured normally; follow the wire harness to continue the inspection, and find the harness under the driver's side into the floor leather; the driver side seat and the front passenger side seat are dismantled and checked and found in At the front passenger foot board of the right A column, the yellow wire that supplies the power to the main switch of the car window has a hinge point, which has been seriously rusted, and it is doubted that the fault is caused. Troubleshooting: the rusty hinge point is reprocessed and welded firmly after commissioning. The main window switch is back to normal, and the trouble is completely eliminated.