Automatic transmission — general description (Mercedes-Benz Sprinter 903)

Some models of the Sprinter family can be equipped with a four-speed automatic transmission type W 4 A 028. Between the gearbox and the engine there is a torque converter, which transmits torque from the engine to the impellers via the working fluid circulating in it. When the engine is turned on, the pump wheel connected to it converts the torque into hydraulic energy of the working fluid (ATF). The turbine, connected to the input shaft of the gearbox, transforms this energy back into torque. A reactor located between the pump and the turbine directs the fluid back to the pump wheel. The planetary gears of the gearbox are constantly in mesh, and when the engine is running, the working fluid tends to set them in motion, and with them the gearbox and wheels, so that the car can move slowly. Therefore, when idling, it is necessary to hold down the brake pedal or apply the parking brake. The gear ratio in an automatic transmission changes when various gears are connected through the action of friction clutches and a hydraulically driven band brake. The drive is controlled by a separate gearbox control system, which receives the corresponding signals depending on the position of the accelerator pedal and, accordingly, the engine crankshaft speed. The automatic transmission control functions are described below.

If the gear selector is in position D, the upshift is engaged as early as possible when the accelerator pedal is not fully depressed. The car accelerates slowly, which saves fuel and ensures a smooth ride. In position D all gears are engaged and this mode is used most often.



In positions 2 and 3, shifting occurs only to second or third gear, respectively. These modes should be selected when climbing hills and requiring additional engine power. When these modes are activated on a slope, for example when engine braking is required, the automatic transmission will engage a lower gear. To avoid overloading the engine, monitor the rotation speed.

In positions P (park) and R (reverse) only shift the selector when the vehicle is stationary. Otherwise, sharp jerks are possible, which will wear out the gearbox parts.

Position N·means neutral gear. The car can be rolled and towed (taking into account the recommendations below for towing and pushing starting the engine).

Original can be found on the website MercedesMAN.ru

In Kickdown mode (the accelerator pedal is pressed all the way down) switching occurs when the engine speed reaches almost maximum. If necessary, a downshift is also engaged to achieve maximum acceleration of the vehicle. To shift up in Kickdown mode, an electromagnetic valve is installed in the gearbox hydraulic drive system.

Note: When manually engaging a downshift (braking shift) it is necessary to monitor the tachometer readings. If the engine revs too high when switched on too early, the pistons may hit the valves and bend them.