A modern Beijing Sonata limousine with a mileage of about 20 thousand km and V equipped with manual transmission. The car broke down on the road and stopped for a while before it stalled. The engine was silent. Fault diagnosis: 4S store personnel arrived at the scene after the arrival of the ignition switch can not hear the start of the start of the start of the start of the battery, check battery to find electricity, the dashboard is normal, determined as the starter does not work. At the scene, we used the original starting method - the trolley gear to release the clutch, and the vehicle was able to start. We drove the car back to 4S shop in time for further inspection. Check the maintenance data, check the circuit diagram carefully, check the circuit diagram carefully, check the circuit diagram in detail, the fuse is normal, the relay has no "click" sound when starting the starter, the relay coil is good, and the positive terminal of the relay coil has starting electrical pressure when the test lamp is started, which indicates that the loop of the coil is problem. One end of the test lamp is connected to the battery positive electrode, and one end is connected with the relay coil to connect the terminal. As a result, the test lamp is not bright, indicating that the terminal of the connection is not connected with the negative circuit of the storage battery. It is known from the circuit diagram that the relay coil is G15 and is located near the left rear air filter shell of the engine cabin. As shown in Figure 1, the iron lap is fastened with a bolt and the body iron. Using multimeter to measure the resistance between the iron plate and the bolt, the result is infinite, that is to say, the circuit is blocked. Using the test lamp, one end of the test lamp is connected to the positive electrode of the battery. One end measures the iron plate, and the test lamp is not bright, as shown in Figure 2. It is found that when viewed alone, it is found to be well fixed, and the touch bolts are not loose, but they are not conductive. The analysis suggests that there should be insulation between the bolts and the iron plate. After the problem is found, it will be easy to screw down the bolts of the fixed iron plate, and use the sandpaper to polish the bolts and the iron plate to re fasten. When the test lamp is re tested, the test lamp is lit, the starter turns normally, and the engine starts and troubleshooting. Maintenance summary: no matter how difficult it is to solve the problem, there should be a clear way of thinking. Fault review: This is a car with a mileage of 20 thousand km, suddenly cannot start, the starter does not turn, the battery power is not rotated, and the battery is normal. When the starter relay is observed, no action is found. When the relay coil is measured, it is found that the tie end is not guided. Except. The fault is not complicated. The author is simple and simple, and the way of fault diagnosis is correct. Each automobile manufacturer has four branches: stamping, welding, painting and assembly. The painting plant is a dust-free workshop, and it is not allowed to visit because it prevents dust from entering. The body coating is divided into skimming, cleaning, phosphating, electrophoretic primer, coating, topcoat and so on. All the surfaces of the body have paint to avoid rust and corrosion, but it also blocks the current path of the steel wire and steel plate. Although the assemblers try to remove the coating on the iron point, they also need to be conductive, not to expose the steel plate, so the coating acts as an insulator. 4S shop to pre - Inspection and new car sales before the electric, electronic control problems, must consider the undesirable problems of the iron lap, the maintenance personnel should be familiar with all the location of the whole car, so that the fast handling of the malfunction of the malfunction.