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The mileage is about 40 thousand km. The model is C307, carrying the 1.8L engine and the 4 speed automatic transmission in 2010 Changan Ford Fawkes car. User response: the transmission fault light of the vehicle is bright, and there will be impact when shifting. Inspection and analysis: maintenance personnel road test, found that after the car starts, transmission failure lights go out. During the acceleration process, there is only impact during the 1-2 shift. When the vehicle speed is reduced from high, the body will shake when it reaches 30 km/h, and the transmission failure lamp will light up. Then the transmission control unit found fault, so enter the protection mode, the gear lock in the 3 gear, engine speed 3500 r/min speed can only reach 100 km/h. Detecting the control unit of the transmission, it is found that the fault code P0755 - the SSB control circuit of the solenoid valve, the failure of the P0758 - solenoid valve SSB control circuit, the P0757 - solenoid valve SSB stagnation, the circuit part and the SSB performance of the P0795 - solenoid valve are abnormal. Disconnect the solenoid valve connector and measure the resistance of the solenoid valve to 14.2 ohms, normal. When the engine is idle, measure the working pressure of the transmission. P block 400 kPa, N block 620 kPa, D block 400 kPa, normal. After removing the fault code, remove the transmission oil pan, adjust the solenoid valve SSB and the solenoid valve SSA, and check the transmission oil at the same time. The inspection found that the oil had a peculiar smell, and there was metal powder at the bottom of the oil. After changing the transmission fluid, let the user take the car out of the trial run. Soon after the car left the factory, the user called to say that the trouble was still there. Once the vehicle returns to the plant, it detects the transmission control unit again, and finds that the fault code is the same as the previous one. But this time at least SSB can be excluded from the problem hole with the IDS data recorder to see the data stream, when the 1 gear is connected by SSA, the SSB is connected; the 2 gear is closed when SSA is closed, the SSB is closed; the SSA is closed when the 3 gear is closed, the SSB is on, the SSA is on, SSB closes when the 4 gear is on. From the recorded data, the transmission control unit controls the solenoid valve accurately. The conclusion is that the solenoid valve, transmission control unit and circuit are all right. But the transmission oil has peculiar smell, and the metal powder in the oil pan is also a fact. How to explain this? In this case, the maintenance personnel decide to replace the transmission assembly (including torque converter). After the replacement of the gearbox and the test run, the transmission failure light was on the way about 70 km, and it was totally unexpected that the fault code came back. In this case, we must try our best to find the real fault. Repeated test vehicles found that when the vehicle was running at a constant speed in the 3 gear, the original solenoid valve SSB was in the state of connection, but suddenly it was broken and soon resumed, while the transmission entered the protection mode. At this point, the problem of the vehicle finally came to light: there was a fault inside the transmission control unit. Troubleshooting: replace the transmission control unit, repeat the trial run to confirm that the fault is completely eliminated.