An annual output of the Mercedes-Benz 300E sedan in 1993 due to automatic transmission failure to our factory maintenance, the chassis for the W124 chassis, the engine inline 6-cylinder, equipped with 4-speed automatic transmission, model 722.409. The car's fault phenomenon is: when the shift lever is located in the P block, run the engine, slam the brake pedal, then directly linked to the R block is completely normal, but if the first hanging in the D block, and then by the D block to the R block , It will not be able to properly mount. When the shift lever is located directly in the N block when running the engine, then hanging to the R block, the same can not hang gear. Road test, in the D block state, the transmission jump and driving state are normal. Hanging upshift, as long as the P block into the R block, rather than by the D block into the reverse driving is also normal. The car has been repaired in other repair shops, many times to clean the body assembly, but the failure failed to solve.
    For this more special case, we do not have a similar fault related maintenance experience, so we do not have a good way. So we did not dismantle the fault car, but first access to relevant information, combined with the phenomenon of failure analysis, and finally determine the maintenance program.
    In general, the manual transmission due to the clutch or transmission itself failure, can not hang the gear situation is more common, but also easier to understand. However, the automatic transmission shift lever controls only the pressure regulating valve, which is controlled by the regulating valve to control the oil passage of each mechanical actuator group in order to achieve the control of the gear. The regulator itself is just a plunger valve, is the valve body assembly control valve in a very common component. And move a plunger valve does not require a lot of force, the phenomenon of hanging into the block is difficult to understand.
    If it is because the regulator valve stuck or the external shift mechanism failure can not move into the gear, it should always be unable to hang a gear, should not be in a state can be linked, and in another Kind of case can not. In order to rule out the suspicion of the shift mechanism, we will shift the lever removed, directly at the transmission shift, the results of the phenomenon is still the same. It can be judged, the fault is still inside the transmission. Then the probability of the valve body should be greater than the mechanical transmission in terms of the possibility of such a failure. But that's even more puzzling. Through the above analysis, based on the structural characteristics of the regulator itself and its working nature, it basically does not exist the possibility of causing this failure. In general, the valve body is on the shift time, shift quality, shift hysteresis and shift after the car, etc. have an impact, can not affect the hanging on the block.
    Combined with the data analysis of the valve body parts of the control of oil and related components of the working principle, also did not find a part of this failure will have an impact. The maintenance work here seems to have fallen into a deadlock. Is it the wrong direction of the previous analysis? Since the principle, the valve itself does not exist influencing factors, then the temporary should not always focus on it to consider, and then go down may be a dead end.
    We have to update the idea to think, there is no possibility in the shift control valve around the problem, because the previous maintenance errors or other factors, resulting in other parts of the regulator valve movement interference? Or the transmission there are some of the structure we are not familiar with, and precisely this part of the failure caused by it?
    After trying a variety of methods, we decided to start maintenance: remove the oil pan, the pressure regulator around the observation, combined with the relevant information for diagnosis. Let go of the transmission oil, remove the oil pan, see the valve body assembly and other components. There are two important points to discover this part: First, often speaking of the valve body assembly, we did not include the middle plate and the two on the plate on the valve cover, but this part of the transmission even part of the shell are Should be part of the oil control system. They are integral with the body assembly, and the body only concentrates most of the control valve components in the oil control system, but not all. We usually think of oil control problems, always the valve body assembly and other parts of the split to look at, only part of the valve body to be analyzed, which has almost formed a way of thinking. It is precisely because of this stereotyped thinking that led us to think erroneously, thus ignoring the analysis of a part of the relevant components.