Automatic Transmissions from Mercedes and their Typical Repair Issues

During most of its history, the German automaker Mercedes has been independently developing and manufacturing transmission solutions (manual as well as automatic transmissions) for its cars. As a rule, Mercedes transmissions are considered by many as a benchmark of quality and reliability. At the same time, Mercedes engineers always try to implement the process of gear shifting in a special way, which fully reflects the philosophy of the company.

However, like any other piece of machinery, Mercedes automatic transmissions should be operated in accordance with specific technical rules. Mercedes automatics require regular diagnostics, maintenance services, and professional technicians recommend car owners to avoid aggressive driving, slippages, etc. Otherwise, even the most reliable Mercedes transmissions will get out of order pretty fast. In this article, we will consider the development history of Mercedes transmissions and their typical repair issues.

Bits of History

It is believed that the 1-st Mercedes-Benz equipped with an automatic transmission was the Mercedes-Benz 770 (also known as Grand Mercedes), which was produced during 1930-43. Initially, this model was equipped with a 3 speed manual transmission, but since 1938 it was produced with a 4 speed overdrive manual transmission.

Mercedes-Benz 770

First automatic Mercedes models hit the automotive market in the 1950s. Here we should mention the Mercedes-Benz 300C, which after some restyling in 1955 received a 3-speed hydro-mechanical automatic transmission from BorgWarner (marked as DG 150M). In 1957, the Hydrak transmission was optionally available on 220S and 219 models. It was a 4-speed manual transmission with a gear shift gate located on the steering column. The clutch of this transmission was complemented with an electro-vacuum servo mechanism produced by Fichtel & Sachs and a hydraulic coupling provided by Daimler. Cars equipped with the Hydrak unit didn’t have a clutch pedal, and shifts were performed with the help of a lever mounted on the steering column.

Check out Mercedes Ponton equipped with Hydrak transmission in action

Mercedes presented its own-produced hydro-mechanical automatic transmission 722.2 in 1961. This transmission found application on W111 220 SEB/300 SE models. Instead of torque converters applied in modern automatics, this transmission was equipped with a hydraulic clutch/coupling. This transmission had been produced until 1983. In 1967, Mercedes released an upgraded version of this transmission with a torque converter (722.1). This transmission found application in the W116, W123, and other models. In 1964, Mercedes engineers developed an enhanced 4-speed hydro-mechanical transmission specifically intended for the 6-seater Mercedes-Benz 600. In this transmission solution the number of planetary gears was increased from 3 to 6. In 1970, the German car manufacturer provided a special 3-speed transmission unit (722.0) that was able to operate in pair with V8 engines. A special improved transmission version (722.003) was developed in 1975 to be coupled with the 6.8-liter engine installed in the 450 SEL 6.9 model. This transmission unit still impresses transmission repair experts with its durability when compared to other hydro-mechanical automatics.

In 1981, automatics of the 722.0 – 722.2 Series started to be gradually replaced by the more advanced 722.3. This transmission was broadly used in many passenger cars produced by Mercedes. For instance, this unit was installed in all versions of the W124 model, except for a version equipped with a 3.2 gasoline engine. The 722.3 unit also found application in the W126 series of S-Class automobiles (including 560SEC and 560SEL) as well as in its successors from the W140 Series. High reliability of the 722.3 transmission was proved by the fact that it had been installed in the 1-st Mercedes SUV.

In 1983, Mercedes presented the 722.4 transmission, which, in terms of design, was a smaller copy of the 722.3 transmission. This automatic transmission was specially designed for the Baby-Benz model. The first 5-speed Mercedes automatic transmission (722.5) was built in-house in 1989. This transmission was installed in E-class and S-class models (W124, W140).

The release of the next Mercedes transmission (722.6) became a real breakthrough in evolution of Mercedes transmission. This transmission (also known as Steptronic) was the first solution, where Mercedes engineers managed to practically implement electronically controlled gear shifting and the torque converter lock-up system. The 722.6 transmission became the main option for passenger car models produced by Mercedes from 1996 to 2003. Before the release of the 7-speed 7G-Tronic (722.9) transmission in 2003, the 722.6 unit was successfully utilized in W211, W203, and W220 models.

722.3, 722.4, 722.5 – design simplicity is a key to reliability

It is hard to find any modern transmission unit that can compete with the 722.3 transmission developed in the 80s in terms of the service lifetime and reliability. Perhaps, on the one hand, it can be explained by the fact that the 722.3 design is quite simple. On the other hand, it can be assumed that these old transmissions were designed and assembled with higher quality compared to modern analogs. In general, 722.3 automatics were extremely reliable were able to operate without problems for decades, sometimes covering up to 1 mln km before the first overhaul.

722.3

Four-speed Mercedes automatics (722.3/722.4) with purely hydraulic control frequently get out of order because of improper operating conditions and non-compliance with service intervals (ATF change and other maintenance services). According to the factory operating rules, ATF should be changed together with the filter each 60 thousand miles. Nevertheless, the resource of these robust transmissions has its limits – old automatics start having issues with friction discs (linings come off) and the brake band plates get damaged.

In terms of design, the main drawback of the 722.3 transmission is its vacuum control system. Structurally, this transmission mechanism is integrated into a single vacuum system of the vehicle. Thus, even the slightest leak in vacuum lines of old Mercedes cars leads to ATF pressure losses, resulting in possible hard shifts and slippages during upshifting.

722.4

When we are talking about the 722.4 unit, this transmission is a smaller version of the 722.3 solution, as mentioned above, thus these transmissions have similar strengths and soft spots.

The Mercedes-Benz 5G-Tronic (also known as 722.5) is known to be quite reliable, but transmission repair specialists frequently have to deal with the failure of 5-th gear in this transmission. The problem is caused by transmission fluid shortages or pressure issues. This problem can be detected in the following way: when the transmission shifts from 4-th to 5-th gear, driver can notice unusual increase in the RPM rate. If the problem is not addressed timely, 5-th gear will stop working with time, as friction discs get burnt.

722.5

Then, wear debris from burnt friction discs contaminate the whole transmission system, clog the transmission filter. The pressure in the system drops, increasing the overall wear of the transmission unit and eventually resulting in the transmission malfunction. To preserve the Mercedes 722.6 transmission from premature failures, it is necessary to replace the planetary gear and the package of friction discs of 5-th gear. Moreover, “old” 722.5 transmissions are quite sensitive to overheating. ATF leaks lead to the oil starvation and consequent damage of friction mechanisms.

722.6 worth a special mention

As all conceptually new solutions, the 5-speed automatic transmission 722.6 (also known as Steptronic) gave birth to numerous arguments and myths on the internet. In terms of design, this transmission turned out to be much more complex than its predecessors, as it is the 1-st Mercedes transmissions equipped with an electro-hydraulic control system. It allowed Mercedes engineers to significantly increase the speed of gear shifting and improve the quality of this process.

722.6

Despite its technical complexity, this transmission can be hardly called structurally unsuccessful, but it has its own specific design features that require experience and some special skills from technicians when it comes to diagnostics and repair services.

Most of the problems in this transmission occur because of ATF leaks. If the fluid penetrates in the valve body, the electronic control unit may get out of order. The transmission can also be damaged in cases where antifreeze substance gets mixed up with ATF. There are repair recommendations depending on the glycol concentration in ATF. Under the influence of glycol, the material of friction discs may swell. In this case, it is necessary to eliminate the radiator leak, causing the mixture of antifreeze and ATF. Another problem that can lead to the control system failure (and its subsequent replacement) is the failure of the RPM sensor integrated in the control unit.

Common problems for 722.6 units (its early versions) lie in wear of thrust bearings in planetary mechanisms of the transmission and wear of slide (sleeve) bearings in the turbine shaft of the torque converter (in 1999 this problematic component was replaced).

With time, all 722.6 transmissions with high mileages are affected by wear of the torque converter lock-up mechanism. Wear debris of friction linings of the lock-up clutch contaminate ATF. The transmission fluid mixed with frictional dust leads to wear of solenoids (as well as valve body), and friction mechanisms of the transmission. The result is always the same – a slow “death” of the transmission. The torque converter overrunning clutch also can be a source of trouble in “old” 722.6 transmissions.

722.6 cut-away

It is interesting to point out that initially the 722.6 transmission model was supposed to become a maintenance-free transmission solution, i.e. it was supposed to be unnecessary to change ATF during the whole service life of the transmission unit. Thus, all Mercedes models equipped with this transmission initially didn’t have a dipstick. But with time, it turned out to be just a marketing trick.

Currently, even official Mercedes service centers no longer say about the lack of maintenance intervals related to ATF change and recommend car owners not to disregard this important maintenance operation (change fluid each 60.000 km). When complying with rules of operation and ensuring timely replacement of spare parts, this popular automatic transmission will easily reach a standard for Mercedes mileage (500.000 km).

722.8

When it comes to Mercedes CVTs, namely the 722.8 transmission (also known as Autotronic), it should be noted that this transmission didn’t gain popularity among Mercedes admirers, thus remaining the only Mercedes CVT transmission. Specialists often encounter the following issues in 722.8: wear of cones, belt, oil pump, and valve body in the CVT. It should be remembered that the repair of Mercedes transmissions frequently turns out to be quite expensive, and when we are talking about the 722.8 transmission, the repair process becomes more complicated because of more sophisticated design of this unit.

Recommendations

In reality, Mercedes automatics gained recognition as reliable and maintenance-friendly solutions. In most cases, Mercedes cars come to repair shops with transmission issues because of non-compliance with the rules of operation, ill-timed maintenance, or natural wear of its spare parts. Owners of Mercedes cars should not disregard standard maintenance services (ATF change, replacement of spare parts), relying on “invincibility” of their transmissions. It is important to remember that service intervals specified by the manufacturer are usually based on laboratory studies (operation under standard conditions). In reality, cars are frequently exposed to severe operating conditions (temperature fluctuations, rough roads). Therefore, in most cases, it is necessary to reduce intervals between ATF changes and replacement of spare parts.

Learn how to change ATF in 722.6

Responsible drivers have to monitor the transmission condition, perform diagnostics, and change ATF, even if it is not specified in the manual. Even if the transmission unit operates properly, regular checks can help to detect some hidden malfunctions and prevent more serious issues in the future. Monitoring of ATF level and regular cleaning of the transmission system will help to prevent overheating of the transmission unit. Transmission fluid should be changed each 60,000 km or at most 80,000 km. If the mileage in the car is over 100,000 km and it frequently operates under severe operating conditions (aggressive driving, poor roads), then it is recommended to reduce service intervals to 40-50,000 km.