A Mercedes Benz S350 car with a mileage of about 30 thousand km and a 276 engine. User reflection: the car wobble at idle speed. Fault diagnosis: this car began to be repaired in the factory because of the accident in the left front. The accident was not too serious. The accident was repaired after the repair was normal, so the car was given to the customer. A few days later, the customer went into the factory to repair again, complaining that the car was seriously vibrating at idle speed. Because everything was normal when lifting the car, so first of all, verify the fault phenomenon. Sit in the car and start the car with a clear sense of vibration. S class cars also often encounter vibration due to the decrease of engine support and shock absorption ability. The vibration of this vehicle is similar to that caused by the decrease of supporting shock absorption capability. Try to increase the throttle and find that as soon as you step on the accelerator pedal, the vibration will decrease immediately. You can feel the vibration with your hands touching the body and tires. Doubt is that when the vehicle is in use or after collision, the external force is supported, or the supporting position is not allowed to cause the damping capacity to decrease. First, release the fixed screws on the support, then tighten the torque according to the prescribed torque, and the vibration remains unchanged when it is started again. Judging from experience, it is the engine that supports problems, so orders are changed to support the engine. The car has two engine support and a transmission support. Unlike other same class cars, the car's engine support is not purely hydraulic, and its support has a solenoid type (switchable engine support) in its support, while other models are not. The engine is connected to the two support and the body is connected to the shock absorption. If the support has problems, or the damping capacity falls, the engine vibration will spread more to the body, so that there will be a strong vibration in the car. The relevant description of engine support is not found in WIS. After the replacement, it is found that there is still a sense of vibration. Is it not a support problem? In order to verify whether it is a supporting problem, the engine is disassembled, the support screw is removed, the engine is supported with Jack, and the engine is not allowed to contact the body. When the engine is started, it is clearly felt that the previous feeling of vibration disappeared. It seems to be a problem of support. But the support is new. Is it possible that the new support has problems? This possibility is very small. Connect the diagnostic device for quick testing without any fault code. The ME is upgraded first, but the failure remains. So we are going to measure the circuit, find WIS, and find out the supporting circuit diagram, as shown in Figure 1. It can be seen from the circuit diagram that the two solenoid valves pass through the Z node to the engine control module ME, and the circuit diagram shows that the two supports share the power line and from the node Z7/48. After two solenoid valves, the two supporting grounding wires are connected again and grounded through Z57/17. Then the right solenoid valve was pulled out for measuring. There were two wires on the plug. The solenoid valve was measured to be 27.5 ohm, and the left solenoid valve was 27.3 ohm. Compared with other normal vehicle models, the resistance is consistent. When measuring, turn on the ignition switch. The actual measuring power supply voltage is 11.9V, normal. After the ignition switch is closed, there is still a voltage on it (when the 87 relay is not broken), the voltage drops to 0V after about two minutes. The two support is ready to be detected, into the general list of guiding detection, and to find the "detection unit Y123" (the left variable engine support) and Y123/1 (the right variable engine support) ". After the point is put, the two supports are detected according to the hint, as shown in Figure 2. During the detection process, the engine bearing can be automatically operated by several times. The level of current consumption indicates whether the two switchable engine supports have been energized. Test premise: the engine must stop running, open the ignition switch, do not turn the steering wheel, close all the electric appliances, can not connect the battery charger, the battery voltage rating is 11 ~ 14V, the whole inspection does not allow the operating components of the vehicle. Then the diagnostic instrument will display several measurements, and then the normal interface will appear. This is strange. The support is new, and the power supply is grounded normally. When ME is tested, the control is normal. But why is the car feeling very well? And jack up the engine is normal? After careful analysis, I felt that the engine support might have gone wrong, so I did not consider the support problem for the time being. And what parts can be connected to the body? At this time, considering an important component, that is, a half shaft, is it possible for the left front part to collide when the accident happens, and whether the half shaft is also subjected to external forces? The great wheels are vibrant. The possibility of the half shaft to pass the vibration to the wheel and the body is very human. It is difficult to think of the vibration of the vehicle when it is idling in place, and the vibration of the semi axle is usually driving. Have to debug a left front half of the shaft to try, after the switch to start the car, still vibrate, after the road test, the strange is the vibration completely disappeared, the car to the customer for a period of time, the vibration did not appear again.