BMW, which is less than 3 years old, why is gearbox so badly damaged?

Why is gearbox so badly damaged for a BMW under 3 years old?

The three main parts of a car are engine, gearbox and chassis. If something breaks, repair of car will cost a lot of money. A BMW gearbox less than three years old is badly damaged, mainly for following reasons:

BMW, which is less than 3 years old, why is gearbox so badly damaged?

First, bad driving manners. Many drivers think that their cars are of good quality and do not value them while driving, they even develop some bad habits while driving, which can easily damage car parts, some habits have a great influence on gearbox. big. For example: when shifting gears, brake or clutch is not fully depressed. When many people drive an automatic, they do not press brakes enough at first. It is a little difficult to change gears. time, but do not want to press brakes hard when working., but pull gear lever hard, at this time damage to car's gearbox is very large, for cars with manual transmission it is even more serious, when clutch is not fully depressed when shifting, then it even heard The sound of gears colliding with each other is caused by bad driving habits, another bad habit is slapping accelerator pedal. accelerator, transmission computer automatically adjusts speed according to vehicle's engine speed. Automatically shifts gears. If you hit accelerator hard and brake hard, frequency of this automatic gear change will be very high and damage to gearbox will be relatively large.

Secondly, habit of parking is bad. Many times when parking, gear is released first and then handbrake is applied. If handbrake is not applied properly at this time, especially when parking on a rough road, transmission will apply parking force. If you drive again Sometimes, it will be very difficult for you enable transmission. In general, first pull handbrake, and then remove gear to improve situation.

In short, in addition to improper driving and parking, damage to gearbox can also be caused by your own bad luck, just a collision with a poor-quality gearbox.

2014 BMW 525Li sedan, powered by a naturally aspirated 2.5L engine and mated to an 8-speed electronically controlled automatic transmission (AT) with 8 hp. and a power of 45 hp. manufactured by German company ZF.

BMW, which is less than 3 years old, why is gearbox so badly damaged?


At first, gearbox showed signs of slipping and blocked gears when changing gears, but then it simply could not go. At same time, a transmission fault message appears on instrument and fault indicator lights up.

Overhaul process:

Tow faulty vehicle to a repair shop for inspection using a rescue vehicle.First connect a diagnostic tool for troubleshooting.As a result, two fault codes are generated in electronic transmission control system, namely:< /p>

During a basic inspection of gearbox, it was found that color of ATF was particularly black, accompanied by smell of paste, so it is more likely to judge an internal mechanical failure of gearbox (experience judges that gearbox friction plate has burned out), thus, do not analyze fault codes and do not decide to disassemble for inspection.

BMW, which is less than 3 years old, why is gearbox so badly damaged?

Figure 3. Condition of ATF lubricating oil flowing out of two oil line holes

Check basic information before disassembly (see fig. 3).

BMW, which is less than 3 years old, why is gearbox so badly damaged?

Figure 4. Transmission information identification position

When removing gearbox oil pan, it was found that there were many metal particles inside (see fig. 5 below), indicating that mechanical parts of gearbox must be damaged inside gearbox.

BMW, which is less than 3 years old, why is gearbox so badly damaged?

Picture 5 - large amount of metal particles in oil pan

When removing electro-hydraulic control unit, it was found that there were more metal particles in gearbox housing (see Fig. 6). When checking flange on shaft or output shaft, a scraping sound is heard inside. gearbox, and a stagnation phenomenon is observed, indicating that it is very likely that large particles that have fallen off after damage to mechanical parts will block rotating parts, so continue to disassemble gearbox assembly.

BMW, which is less than 3 years old, why is gearbox so badly damaged?

Figure 6. A large amount of metal particles in gearbox

Finally, it was confirmed that last set of planetary gears in gearbox was severely damaged. At same time, during dismantling process, it was found that each group of friction discs also has a different degree of entrainment.

Error Analysis

Carefully analyze cause of failure of planetary gear mechanism (see Fig. 7) and find that planetary gear in last row of planetary carrier has fallen off and is stuck in transmission system. cause of planetary gear? How severe is damage?

BMW, which is less than 3 years old, why is gearbox so badly damaged?

Figure 7-BMW 8HP45 transmission power transmission diagram

There are several possible reasons:

Firstly: gear is poorly lubricated, which can be judged by signs of wear on gear and ablative wear on bearings;

Second: in process of transmitting torque, planetary gear is damaged due to limited load capacity of instantaneous change in torque;

Third: deterioration of ATF oil leads to friction disc slippage and too high gearbox temperature.

In addition, let's analyze original two fault codes: - at 4, it should be caused by a power interruption or an instantaneous slip of transmission when shifting gears, which is recorded by computer as an incorrect gear ratio when shifting gears; and - position Regulation: A damaged planetary gear may be stuck in transmission system and computer determines that transmission is still faulty when transmission is in neutral.


1. The manufacturer of damaged planetary gear does not provide it separately, buyer is advised to replace entire planetary gear and use a small repair kit.

2. Clean torque converter and electromechanical hydraulic control unit.

3. After damage to planetary gear, a large number of small particles will contaminate cooling system when oil circulates, so replacing cooler is best solution.

After loading car, test run problems were completely eliminated.

Error summary:

The gearbox of a car that has just passed less than three years is seriously damaged, which is likely to be directly related to vehicle's operating conditions and user's driving style, otherwise planetary gear will not be damaged so much. .

4S stores are mostly outsourced to companies like Xinfumei, but sometimes they can't find a suitable outsourcer or parts can't be stocked and they repair them themselves.

BMW, which is less than 3 years old, why is gearbox so badly damaged?

Because manufacturer does not recommend dismantling 4S gearbox, accessories in system are not only expensive but also seriously defective. BMW and Buick are more likely to make their own refurbished units and sell them in system. You can only first try to change input shaft of valve body, replacement price is more expensive than an outsourced overhaul. In addition to not necessarily being good, warranty cannot fully guarantee, meaning this time repair slipped through and next hit is not covered by warranty. I just want a warranty and there are no matching accessories in system.