The current automatic transmissions are an important system element of every vehicle. Most manufacturers have decided on the electrical control of transmission since the beginning of the 1990s. Since electronic fuel injection systems qualify as “mandatory equipment” for gasoline and diesel engines, it is difficult to imagine that the transmission can be controlled through intricate hydraulic systems. The centrifugal advance mechanisms, vacuum cans, or rope or wire coatings which determined the switch point positions are already history.

A modern transmission is a mechanical-hydraulic element. It completes its tasks exactly according to electrical signals given by the EGS module (electro-hydraulic transmission control), which also calculates the best transmission ratio using the automotive speed, the accelerator pedal position, mechanical motor load, oil temperature, ambient conditions, and even driver’s habits.

Hardly anyone thinks about how many factors and parts of the whole vehicle have an impact on the proper activity of the automatic transmission. The more interrelations which exist for the functioning of the automatic transmission, the higher the possibility of failure causes there are. The discovery of the original cause of failure is a solid basis for effective repairs. Through using some examples, I will try to explain to you why it is so important to entrust professionals with the repair of an automatic rather than relying upon laypersons who are trying get money for doing learning exercises with your vehicle.

One can imagine how a current automatic transmission functions by using the example of the 5-speed automatic transmission, which has been fitted as standard in the cars of Mercedes since 1996. Its compact construction is based upon on the use of the planetary gear, which is operated by means of three multi-plate clutches, three multiplate brakes (no more brake bands!!) and two overrunning (or freewheeling) clutches. The torque converter with the lockup clutch is responsible for the transfer of the drive from the engine. Everything is powered and lubricated by a gear oil pump. This pump is operated directly by the engine. The closing of the individual clutches and brakes is carried out through means of an electro-hydraulic control unit which supplies the supply ducts of the execution elements with pressurized oil. There are three sequence solenoid valves (switching solenoid valves), two impulse electro-valves (solenoid modulating valves) and a solenoid valve which controls the lockup clutch on the electrohydraulic control unit. Several spool valves of oxidized aluminum, which are shifted using the oil pressure, are responsible for the direct distribution and regulation of the oil pressure. The transmission’s activity is constantly monitored by two revolution speed sensors and one oil temperature sensor.