The mileage is over 260 thousand km, equipped with 1. 8T engine, 01V (5HP-19) automatic transmission 2004 Volkswagen Passat B5 car. It is normal when the car is reversing gear. Sometimes after hot cars hang up the reverse gear, do not take the car, or the reverse gear is weak, especially when the car is reversing on the slope. Fault diagnosis: according to the analysis of the failure phenomenon, there are clutch B, brake D and G, but the brake G is working in the 1, 2 and 3 gear of D bit, and the D positions are all normal, so that there is the possibility of the failure of the clutch B and the brake D. Asked by the owner, the car had been involved in water, and the automatic transmission was also in the water. Several transmission oils had been changed, but there was no solution to the problem. It is read by the diagnostic instrument. The fault code of the automatic transmission is not stored. The main oil pressure of the transmission is normal. The oil pressure of the transmission is 0.65 ~ 0.75 MPa and 2000r/min is 2.3 ~ 2.4 MPa when the speed is idle. Release the transmission oil, remove the oil bottom shell, check the transmission oil, no color and no butterfly taste, there is no friction flake under the oil bottom of the gearbox, it is preliminarily determined that the friction plate is not damaged, then what is the problem? In line with from outside to the inside, from easy to difficult maintenance principle to check, because the body into the water, suspect may be the valve stem card stagnation, first replace the valve body, test, failure to solve, indicating that the problem is not the body. At this time, the judgment is the internal problem of the transmission, carry the transmission from the car, disintegrate the transmission, check the reversing clutch B normal, further check the brake D shell cracking. Decomposing the brake D, it is found that the gap between the brake disc is too large because of the cracking of the shell. The piston has one side running out and the piston seal ring is squeezed out, causing the piston seal to be tight, so that the oil becomes thinner after the hot car and the pressure is not enough. Fully clean up the transmission, replace the brake D shell after the assembly of the transmission, after loading the car after the test reversing normal, other gear work is normal. After the repair is completed, the car is delivered to the owner. The owner is satisfied with the driving. Three months later, the customer will return to the customer to tell him everything is normal. Maintenance summary: because the automatic transmission oil was first checked, no oil deterioration and machine wear debris were found, so the valve body was decided to replace. The decision is a bit sloppy, so some curves are taken and some experience is obtained. When making a diagnostic conclusion, it should be fully and carefully considered in the table and in it. The occurrence of debris will invalidate the mechanism, and the cracks without debris will also invalidate the mechanism. The diagnosis process of this case is to check the fault code, to check the transmission oil, to change the body and to disassemble the internal one, to find the D shell of the brake and to replace the D shell of the brake. It is correct to check the sequence in order that it is easy to get rid of and difficult to follow first. Finally, the author concluded: "there are some detours due to the replacement of the valve test run." I think the electric control system and torque converter have been eliminated, and the failure of planetary gear mechanism and hydraulic control mechanism should be broken down and overhauled. The body can also be removed to repair the body first, pay attention to ensure that the assembly is correct and clean, there is a replacement body to try and go on, if this time the purchase of the body is an indiscreet choice. In addition, to correct the second paragraph, the brake G does not work in the 3 position of D position.