Mechanical parts automatic transmission

An automatic transmission contains mechanical components that transmit engine torque and engage gears. Which components perform these functions depends heavily on the transmission design. A conventional torque-converter automatic typically uses planetary gearsets and hydraulically operated multi-plate clutches and brake packs.

A fully automatic gearbox, also known as an automatic transmission, selects and shifts into the appropriate gear based on vehicle speed, engine load and accelerator pedal position. Unlike a manual gearbox, it does not require the driver to operate a clutch pedal or shift gears manually to move forward. The three most common types of fully automatic transmission are:

1. Conventional torque-converter automatic: the torque converter transmits engine torque through transmission fluid. Hydraulically operated multi-plate clutches and brake packs switch between ratios in planetary gearsets.

2. Dual-clutch transmission, DCT (Dual Clutch Transmission) or DSG (Direkt-Schalt-Getriebe, direct-shift gearbox): two clutches each operate a separate set of gears. While one gear is engaged, the next can already be selected; the transmission shifts by switching between the clutches.

3. Continuously variable transmission, CVT (Continuously Variable Transmission): variable pulleys and a belt or chain change the transmission ratio continuously. The transmission adjusts the ratio without shifting between fixed gears.

City buses such as the MAN Lion’s City and Mercedes Citaro may have a torque-converter automatic transmission. Automatic transmissions are also found in vans such as the MAN TGE and Mercedes Sprinter. The available engine torque affects the design of shafts, gears and clutches. Gear ratios, shaft diameters and revision levels can also differ within a single vehicle series.

Always identify parts using the vehicle identification number (VIN), then check the transmission type, gearbox code, serial number and part number. A visual comparison or a selection based solely on the model name is never sufficient. When replacing parts, also check the number of teeth, spline profile, bearing fit, material specification and specified endplay; small discrepancies can cause shifting problems or premature wear.

Whining noises under load may indicate damaged gear teeth or worn roller bearings. In a torque-converter automatic, slipping, delayed engagement and discoloured transmission fluid can indicate worn friction plates or insufficient hydraulic operating pressure. Jerky shifting does not in itself prove mechanical damage: a control valve or incorrect actuation may also be the cause.

Begin diagnosis by reading fault codes and carrying out a test drive, recording the gear and load at which the symptom occurs. Then check the transmission fluid level using the specified temperature procedure, and inspect a fluid sample and the magnet for metal particles and friction material. On hydraulically shifted automatic transmissions, compare the measured operating pressure with the manufacturer’s specification for the relevant operating mode. After disassembly, measure shaft endplay with a dial indicator, bearing bores with an inside micrometer and clutch pack thickness with a micrometer. Also inspect gear teeth for pitting, chipping and discolouration caused by overheating.

Do not replace only the visibly damaged gear or clutch pack. Also check the mating gear teeth, pressure plates, bearing fits, seals and lubrication passages. Set endplay using the specified shims and use only transmission fluid that meets the specification for this gearbox. After internal damage, flush the oil cooler and lines where applicable so that residual wear particles do not immediately damage new parts. If the housing or bearing seats are distorted, or the transmission has run without lubrication for an extended period, assess whether replacing the complete transmission would be more reliable than a partial repair.