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1. Introduction

Theory versus reality

Many owners of cars equipped with automatic transmissions hesitate and delay changing the oil in the gearbox. Opinions on this subject are very divergent. In addition, many car manufacturers do not provide a schedule for oil changes in automatic transmissions when performing standard periodic inspections. However, attention should be paid to the fact that car manufacturers do not prohibit or advise against changing the oil, but only do not provide for it during the standard service procedure. By design, the automatic transmission must maintain efficiency and enable trouble-free operation during the warranty period of the vehicle. In order to ensure trouble-free operation over a longer period of time, well after the warranty period, the automatic transmission should be properly taken care of. According to the principle "prevention is better than cure", preventative measures are recommended. The service practice clearly shows that automatic gearboxes, without maintenance operating at distances over 200,000 km is when the first signs of wear, tearing or stalling of gears will appear, and the unit usually requires an overhaul, as an oil change alone is usually not able to return the gearbox to trouble-free operation. In order to unambiguously deal with the myths about this topic, you should start from the beginning and present the oils used in terms of the principle of operation of automatic transmissions.

2. Transmission oil functions in individual automatic transmission systems
The oil used in automatic transmissions is a very complex mixture, so the term "automatic transmission fluid" (ATF) is often used as a synonym. It is connected with many functions that this medium must fulfill. In addition to the basic base, the oil has many additives fulfilling specific functions, including: corrosion inhibitors, oxidation inhibitors, depressants (temperature), friction modifiers, viscosity index modifiers (polymers), detergents, dispersants, anti-foam, swelling and anti-wear additives.
2.1. Torque converter
The converter is the first component of the automatic gearbox to test the properties of the gear oil (Fig. 1). The element responsible for transmitting the torque from the crankshaft to the input shaft of the gearbox. Often referred to interchangeably as a torque converter or torque transformer. During driving, especially when accelerating, the transmission fluid is the medium that ensures the transmission of torque from the engine to the gearbox. There is no mechanical connection here, as is the case with a classic clutch in a manual transmission. It is the oil set in motion by the pump blades (Fig. 1 - B) that sets the turbine in motion (Fig. 1 - A) in mesh with the gearbox input shaft. The parameters of the oil, appropriate viscosity and friction index, its anti-foam properties, resistance to temperature and compression are extremely important here. Additionally, there is a friction clutch in the converter (Fig. 1 - C). It is activated under specific operating conditions to increase the efficiency of the converter, thus reducing fuel combustion. When the clutch is engaged, the temperature increases in a spot, which leads to oil degradation. The appropriate coefficient of friction must be maintained. The lubricating properties are also extremely important so that the friction lining does not burn. Very important components of the oil are dispersants and decarbonizers, whose task is, among others, dissolving of soot particles and preventing the clumping of particles formed during friction between the friction linings.

The converter also has a Reactor (Fig. 1 - D) with a bearing with a one-way clutch. Temperature transfer properties, proper coefficient of friction and lubricating properties are crucial for its proper operation. In service practice, the clutch is often broken, as a result of which the car accelerates very slowly, not responding dynamically to pressing the accelerator pedal.



Converter elements: A - Turbine, B - Pump, C - Lock-up clutch, D - Reactor
2.2 Clutch and brake baskets. Influence of the arising contaminants on the correct operation of the hydraulic module
The elements that significantly affect the degradation and contamination of the oil are multi-plate clutches and brakes (Fig. 2). When the clutches are engaged, the oil temperature increases pointwise (approx. 400 ° C) and its carbonization occurs. This causes rapid wear of additives necessary for the proper operation of the gearbox mechanisms and protecting it against excessive wear. If the dispersants and decarbonizers wear out, they do not prevent the formation of soot and dust particle separated from the friction material, which then end up in the mechatronic module.
Set of friction plates and clutch spacers
Set of friction plates and clutch spacers
The resulting debris clogs the internal channels and filters of the hydraulic module. They block the movement of the pistons in the module and solenoid valves (fig.3) which leads to a reduction in operational comfort, i.e. jerking when shifting gears. In the event of longer use of the vehicle, the accumulated dirt and filings causes the channels and pistons to wear, which eventually lose their tightness.
Piston for the hydraulic control module
Piston for the hydraulic control module
2.3 Planetary gear unit
Another key element of any classic automatic transmission is the planetary gear unit (Fig. 4) consisting of gears and a ring wheel permanently in mesh. The transmission fluid must have adequate properties to prevent wear of the gears, as rubbing them causes too much inter-tooth play. Excessive play first manifests itself in the louder operation of the automatic transmission. For this planetary system to perform its function, its satellites must be able to rotate around their axis. Therefore, these elements are mounted on yokes. If, as a result of a long period of use of the car without changing the oil, the friction modifiers contained in it are worn out, and the oil itself is oxidized, the satellite bearings wear out quickly. Additionally, in many automatic transmissions, to reduce their weight, the yokes are made of aluminium, which is very susceptible to abrasion. Improper satellite bearings and their axial clearance caused by abrasion of the bearing seat is very dangerous and often leads to serious and costly repairs. The planetary assembly is a compact piece. If the satellite gets stuck, it is pushed outside the assembly, destroying it and nearby elements: clutch baskets with sets of friction plates and spacers. 
Automatic gearbox planetary gear unit
Automatic gearbox planetary gear unit
2.4 The seals
Damaged rubber seals in the gearbox are often responsible for a significant proportion of faults and malfunctions in automatic transmissions. Therefore, their protection and maintenance is crucial for long and reliable operation of the automatic transmission. The clutches and brakes mentioned earlier are piston actuated. They have rubber seals (Fig. 6). If the oil is not changed, the additives that protect and soften the rubber seals lose their properties, leading to hardening and cracking of the rubber parts. In such a situation, the electrohydraulic controller is not able to build up sufficient pressure in a sufficiently short timeframe, which causes incorrect engagement of a particular clutch or brake. This results in the gearbox jerking, and with high pressure losses, the emergency operation mode is started. The shaft seals are also exposed to increased wear. Their destruction causes leakage and loss of oil. The oil that stands out in the context of compatibility with seals is Castrol Transmax ATF DEXRON®-VI MERCON® LV Multivehicle which is not only dedicated to applications requiring Dexron VI but also to earlier standards, starting from Dexron II C, D, E ending with Dexron III G and H. Additionally, many types of automatic transmissions have several different oil circuits. In addition to the classic parts with clutch baskets, brakes and planetary gear units, they have integrated differentials that are lubricated with other gear oils. Damage to the double-sided seals (Fig. 5) between the oil chambers leads to the mixing of different types of oils. Too much unsuitable transmission oil in the automatic section causes greatly accelerated wear of the friction linings, resulting in jerks when shifting gears.
Double-sided output shaft seal
Piston with seal
Double-sided output shaft seal
Piston with seal
3. Effect of gear oil change on the service life of an automatic transmission
One more thing to pay attention to: when changing the oil, filings and other impurities are additionally removed, which have a negative impact on the operation of the gearbox. All kinds of solid particles, filings and contaminants in the oil intensify the degradation of mechanisms, faster wear and destruction of transmission elements, among others. gears, bearings, shafts, seals and hydraulic modules. Therefore, this seemingly not very important thing is crucial and ensures trouble-free operation of the gearbox for a long period of time. This is extremely important, especially in continuously variable CVT transmissions, where service practice shows that regular oil change in this type of gearbox increases their service life by at least two times. To choose the right oil, we can use the oil selection tools, such as the "Find the right oil" tool at www.castrol.comIn summary, the key to a long, comfortable and trouble-free operation of an automatic gearbox is regular service and gear oil change. The right decision, however, is always in the hands of the user and owner of the vehicle.