Problem of Kicks and Jerks in Automatic Transmissions

Nowadays, cars equipped with automatic transmissions have almost pushed out manually shifted cars off the automotive market. Automatics are popular not only among young drivers, but also among experienced drivers. It is not surprising, as modern automatic transmissions not only significantly simplify the process of driving a vehicle, but they also do not outperform manual analogs in terms of performance characteristics and fuel economy. Automatic transmissions ensure more comfortable driving and safety under the conditions of heavy traffic in urban areas. At the same time, automatic transmissions are quite complex units, which are prone to failures, especially when drivers violate rules of operation. The most common symptom of the automatic transmission malfunction lies in kicks and jerks during the process of gear shifting. In this article, we will explore the main causes of kicks and jerks in automatic transmissions and give some recommendations on how to deal with such issues.

In some cases a driver can continue operating even with jerks, but if this problem stays disregarded for a long time such symptoms can lead to the transmission damage and highly-priced repair. That is why this issue must be addressed at an early stage to avoid unwanted complications. The main task here is to determine the source of kicks and jerks in the automatic transmission.

It can become quite a challenge, as kicks and jerks can manifest themselves at different stages of gear shifting: the issue can manifest itself when moving the shifting lever, during shifting between specific gears, or when you start driving without the transmission warm-up, etc. Proper diagnostics and some indirect signs can help to determine the source of the problem.

Main causes of kicks and jerks in automatic transmissions

In many cases, kicks and jerks in automatic transmissions are caused by issues with the transmission fluid (ATF). This fluid does not only lubricate transmission hard parts, but also serves as a working fluid. The fluid transmits torque from the engine to the wheels and takes part in engagement of required gears by passing via valve body channels under the pressure.

The main reasons for jerks and kicks in the automatic transmission are the following:

• ATF is not warmed up;
• ATF level is too low or too high;
• ATF gets too contaminated and loses its operating qualities;
• ATF pressure drops;
• Transmission filter is clogged.

If the driver notices some jerks in the transmission when driving, it is necessary to check the ATF level and its condition. Normally, the fluid level must be between HOT and COLD marks on the transmission dipstick (the fluid check must be performed after the transmission warm-up). If the ATF level is too low, it will lead to increased wear of transmission components, fluid pressure shortages, slippage of frictions, and incorrect engagement of gears. If the ATF level is too high, the fluid starts to foam, and the pressure of the foamed fluid is not enough for normal operation of the automatic transmission. Fluid surpluses can be squeezed out through the breather, leading to the transmission greasing from the outside. When the transmission fluid level deviates from the norm, driver can start experiencing jolts when shifting gears.

Slight transmission jerks in many automatic cars, which start moving without the transmission warm-up, can be considered as a norm. Commonly, these jerks disappear when the transmission fluid reaches optimal operating temperatures. To avoid any inconveniences during driving, it is necessary to warm up the engine and the transmission before you take off. During the first 5-10 kilometers it is recommended to avoid rapid accelerations, high RPM rates, usage of kick-down mode or manual gear shifting.

Even if the fluid level is normal, it is also necessary to pay close attention to the general condition of ATF. The fluid must be clean, have normal color, no contaminants (wear debris) or burning smell. If the fluid condition does not comply with operating standards, it is necessary to change ATF as soon as possible.

To put it simply, automatic transmissions must be regularly maintained and ATF change serves as a key maintenance procedure, which determines the service life of the whole transmission. Some auto manufacturers may say in car manuals that ATF is filled for the entire period of the car service life and the transmission unit is maintenance-free. Such recommendations presuppose the operation in ideal conditions, but in reality it is hardly possible. In practice, ATF starts losing its properties after travelling 40-60 thousand kilometers. Therefore, without timely ATF change, the automatic transmission (with the service resource of 250-300 thousand kilometers) will get out of order before reaching 150 thousand kilometers. At the same time, the first symptoms of the upcoming failure commonly manifest themselves in kicks and jerks when reaching the mileage of 100-120 thousand kilometers.

The next transmission component that can cause jerks in the automatic transmission is its filter. The main purpose of this element is to keep dirt and wear debris from ATF. Sooner or later, the filter gets clogged, and its filtering capacity deteriorates. As a result, the oil pressure in the transmission becomes reduced. It leads to operating irregularities of friction discs. Friction discs are special transmission components that are compressed or expanded, respectively, stopping/releasing the required speed gear inside the automatic transmission. If the fluid pressure decreases, frictions do not provide proper compression and start to slip and wear rapidly. Slippage raises the temperature and the fluid starts burning.

Problems with friction components mean that gears are not stopped/released at the right time. The driver experiences friction slippages in the form of jerks during gear shifting.

In some cases, gears cannot be actuated at all or the car can continue moving even when N gear is engaged. In the first case friction discs are heavily worn, while in the 2-nd case it can be assumed that friction elements get stuck. If the issue reaches such proportions, it won’t be solved only by the transmission filter replacement. It will be necessary to remove the transmission unit, perform diagnostics, and replace worn friction elements and other transmission components.

That’s how an automatic transmission oil pump looks like

The problem with fluid pressure losses in the automatic transmission can be caused not only by contamination, but also by issues with the automatic transmission oil pump. The oil pump can be positioned behind the valve body or in the transmission pan. The main purpose of this unit is to provide necessary pressure for the working fluid. If the pump performance drops, the transmission starts kicking and the driver can experience some jolts.

Automatic transmission problems are also frequently caused by issues with the valve body and solenoids. In fact, the valve body is a control element of the transmission. It consists of a wide range of channels, which transfer the working fluid to a separate gear. Solenoids, in turn, open or close the channels of the valve body. Solenoids are electric coils with a rod. Voltage comes to the coil, extending the rod, and blocks the channel. When voltage disappears, the spring pushes the rod back and opens the channel. Frequently, solenoids get out of order and stop opening or closing channels in the valve body. If solenoids do operate, but not correctly enough, the automatic transmission starts kicking and jerking during the process of gear shifting.

Automatic transmission valve body with solenoids

To solve the problem, it will be necessary to remove the valve body, disassemble it, and clean the channels. Then, it is required to check the operability of solenoids, and fix or replace the faulty element. It should be noted that jolts and kicks do not always occur due to issues with the transmission unit. Problems with the ignition system or the feed system, faults in the throttle area, or incorrect operation of ECM sensors can too have a negative impact on the automatic transmission operation.

That is why apart from automatic transmission maintenance, it is necessary to pay attention to the air and fuel filters, check the state of the spark plug and plug wires, flush the fuel injectors, clean and, if necessary, adjust the throttle unit, etc. For example, the automatic transmission can start jerking because of weak or strong tension of the gas pedal cable (if the gas pedal is electronic, the voltage in such pedal may also deviate from the norm). As a result, it may lead to jerks in the automatic transmission.

Summary

As you can see, there is a lot of potential causes for kicks and jerks in the automatic transmission. As a rule, if the automatic transmission starts working incorrectly, “A/T” or “Check Engine” warnings signals appear on the panel. In this case, it is necessary to visit the nearest repair shop for computer diagnostics to find out the source of the problem. However, diagnostics can be efficient if the problem lies in electrical components, but if the problem is caused by hard parts it will be necessary to remove the transmission unit and disassemble it to find a faulty element. It should be noted that in classic automatic transmissions (in contrast to CVTs) shifts from one gear to another can be accompanied by slight jolts (especially when driving without a warm-up) and they are considered as peculiarity of the transmission operation rather than a malfunction.

It should not be forgotten that so-called adaptive automatic transmissions adapt its operation to the driving style of the car owner, remembering certain parameters. Thus, when another person gets behind the wheel of such car, the process of gear shifting may seem hard and can be accompanied by jerks. It is caused not by any transmission malfunctions, but simply by the fact that the transmission requires some time to adapt to the style of a new driver.