Sometimes, car owners are facing the problem of an automatic transmission that jerks. Unlike manual transmissions where maintenance is quite rare, automatic transmissions need more care and attention from the owner.
There are some causes why automatic transmission could jerk. A common cause can be an insufficient transmission (ATF) fluid level. Other reasons are a clogged oil filter, a clogged oil cooler, worn friction discs, a failing oil pump, problems with the valve body and/or solenoid.
All these causes are due to improper maintenance performed by the car owner or by mechanics. In most cases, well-done maintenance is all is needed for the automatic transmission to work properly.
Also, there are “experts” that deliberately mislead their customers by telling them that the automatic transmission on their cars is maintenance-free and there is no need to perform one.
What causes an automatic transmission to jerk?
After buying a new car and running about 100 thousand kilometers (62.000 miles), of course, this value is approximate and conditional, you may experience to see these symptoms in your car’s automatic transmission. Below we will talk about all the possible reasons why your transmission could jerk.
Using an incorrect automatic transmission fluid
Every car manufacturer has certain transmission fluid requirements, it is necessary to use a fluid of a certain viscosity, and with the performance characteristics specified by the car’s manual. Using a fluid with incorrect parameters will cause the car’s automatic transmission to jerk and kick.
Related to the AT fluid, you will often see the transmission jerking in winter especially as soon as you start the car after the car sat at low temperatures for a while. However, after warming up, the problem disappears. This means you initially chose the wrong oil, which thickens in the cold, and does not ensure the normal operation of the automatic transmission. Thus, you need to choose a more liquid oil.
A bad automatic transmission oil filter
Because of wear in the transmission parts that are formed naturally depending on the car’s mileage and operating conditions, metal chips are formed over time, and more wear and tear result in more chips formed.
Because of this metal debris, the AT oil filter will clog over time and won’t be able to supply the right amount of fluid. This will lead to the operating pressure in the system to decrease which will affect the friction discs. These discs won’t stop at the right moment and won’t have enough force causing them to begin to slip. That is the reason why you will feel shocks and shakes when changing gears during acceleration and braking.
In worst cases, when the fluid level is very low, this will cause the load on the discs to be even higher and can cause them to burn. This situation is especially true when you see jerks in the automatic transmission when hot.
The conclusion is simple. It is necessary to monitor the condition of the transmission fluid filter, use high-quality ATF fluid with the correct parameters specified by the manufacturer. And of course, change the filter and the fluid itself in time.
Failing friction plates
The friction plates in the automatic transmission act as a clutch when changing gears. They are driven by the transmission fluid that circulates through the system under pressure, and they contract and expand.
If the pressure is not enough, then the plates are not able to compress and decompress with the necessary force. They will simply slide and rub against each other, and if there is a lack of oil, they will heat up and burn.
When these plates are worn-out, you will feel shocks when changing gears. Also, the transmission fluid will wear-out much faster and loses its properties. In worst cases, when the discs are significantly worn or simply burned, the jerks, kicks, or twitching will be felt more intense, and you will hear, in most cases, unpleasant crunching noises coming from the transmission.
Also, you might get to the point where gear shifting is impossible. Wear of friction discs is a serious problem, and without dismantling the automatic transmission and opening it, repairs are impossible.
A failing automatic transmission oil cooler
Logically, the role of this part is to cool the AT fluid. When its performance decreases, the fluid becomes very hot and burns. As a result, the operation of the automatic transmission as a whole will suffer for the reasons described earlier.
The main reason why the cooler fails is related to replacing the old fluid under pressure. When performing this procedure on so-called high-pressure devices, the system is installed in the gap between the cooling radiator and the automatic transmission itself. Then, under pressure, the old fluid is extracted from the system and a new one is added.
However, the process does not involve replacing the oil filter or flushing the box tray, including the radiator. Therefore, the dirt from the bottom rises to the top and clogs individual elements of the transmission, especially the radiator, oil filter, hydraulic plate, and solenoids.
The radiator consists of many small pipes and honeycombs, which are simply clogged with dirt. And when the temperature rises, it will significantly reduce the efficiency of the radiator because of the burns.
So, in order to get rid of the jerks when shifting gears, one of the options is to clean the radiator cooling the ATF. In the future, it is advisable not to change the AT fluid under pressure, change the oil filter, and clean the radiator as well. This will cost more, but in the long run, it will extend the life of the automatic transmission as a whole.
A failing hydraulic plate
This hydraulic plate is also known as the valve body. This part consists of many small channels through which the AT fluid moves under pressure. It can be noted that each gear has its own channel with fluid, which, under pressure, makes the clutches compress or expand.
If these channels will be clogged with dirty liquid, then the pressure will not be enough for clutches normal operation. They will not be able to contract and compress normally, and you will feel jerks in the automatic transmission behavior.
As you may have already guessed, the main reason why this is happening is not changing the AT fluid at the right time or replacing it under pressure. Cleaning these channels is possible only when removing and disassembling the transmission, and this can be very costly.
Another important reason is the plate wear. This part consists of two plates and between them, there is another special plate. The rod in the form of a ball from the solenoid rests against it. This ensures that the pressure is closed (locked). There is a hole in the specified plate, which is exactly covered by the specified ball.
Over time, the hole cracks, leading to minor bumps when switching gears. In order to resolve this problem, it is necessary to replace the mentioned plate. However, it will be necessary to remove the AT for detailed diagnostics and repairs.
Solenoids are special valves that open and close at the right time, and they regulate the supply of transmission fluid to the hydraulic plate channels. Over time, the solenoid may fail due to mechanical damage or simply due to aging. Thus, they either do not close or, more often, do not open the channel, causing the solenoids to not work smoothly and affect the operation of the entire automatic transmission, jerking when shifting gears.
The situation is worsened because of the material from which they are made. In modern cars, solenoids are made of plastic. So, at high temperatures, they can melt, lose their geometry and stop working correctly. Metal solenoids don’t have these problems. To repair the solenoids, it is necessary to dismantle the hydraulic plate, clean it, and adjust it, including the solenoids.
Other reasons why automatic transmission jerks
There are also rare cases that causes the car’s AT to jerk.
A bad Electronic Control Unit (ECU)
A failing ECU can be responsible for jerks in the automatic transmission. Its software can cause errors. In this case, the car not only jerks when shifting gears, but also other malfunctions can occur. For example, the engine speed will be unstable, and the AT will not react when changing gears. In this case, it is necessary to perform computer diagnostics of the ECU.
Moreover, you should not drive the car in this state, but instead bringing the laptop with the necessary software program and connectors to the car. Repairing the ECU requires special hardware and software skills, this is a serious thing, and it is better to entrust this to specialists who give a warranty for their work.
A bad automatic transmission oil pump
The AT fluid pump fails rarely. The design of the pump is very reliable, however, in case of mechanical damage or a factory defect, it can also break. Under these conditions, the transmission fluid will cease to circulate at operating pressure or it will simply stop circulating. The pressure will drop, leading to jerky gear changes and, in extreme cases, complete transmission failure.
Bad ECU wiring
The automatic transmission is controlled by the ECU by electrical signals through the corresponding wires attached. Especially in older cars, and in rare cases, the wiring is oxidized or completely torn.
Causes that are not related to transmission problems
Sometimes, the jerks felt when changing gears have other causes than a bad automatic transmission. The throttle valve can be the culprit, you should clean it to ensure proper air passage. The same applies to the air filter. In general, cleaning the throttle valve and changing the engine air filter will lead to a reduction in shifting shocks and relieves prolonged gear changes on the automatic transmission.
The causes of shocks and jerks when changing gears can be worn mounts on which the engine and transmission are fixed. These mounts are two metal plates with a rubber insert located between them. Over time, including under the influence of vibrations, the rubber loses its properties and wears-out. This leads to the appearance of jerks when switching gears. You can get rid of this by replacing those mounts with new ones.
Role of additives for automatic transmission against jerks and shocks
Special additives for automatic transmissions are intended for the following purposes:
- increasing the smoothness of the transmission, especially when switching gears;
- increased service life of both individual transmission elements and the automatic transmission as a whole;
- reducing the noise in the AT;
- Protection of individual elements of the car’s transmission;
- Protection of plastic and rubber parts;
- cleaning the At fluid from metal debris and dirt, which will increase the service life of the oil filter;
- prevention and elimination of transmission fluid leaks from the system;
- eliminating jerks when operating the automatic transmission.
The additive from Liqui Moly proved it’s usefulness. It’s one of the best additives for the car’s automatic transmission. It prevents oil loss, has cleaning and protecting properties, and regenerate seals.
You should use these additives in the AT fluid every 20.000 km or 12.400 miles, but no more than three times on the same transmission oil.
There are some causes why your car’s automatic transmission can jerk, but there are also other causes that are not related to your transmission. When jerks or kicks are coming from the automatic transmission when shifting gears, the first thing you should do is computer diagnostics. The errors, if any will show how serious the problem is.
To avoid problems with the automatic transmission in the future, try to monitor it, and, periodically change the oil and filter. Also, try to not use the automatic transmission in rough modes.
If you don’t want to remove the whole transmission, you should first replace the transmission fluid and the oil filter. Remember, don’t do the replacement under pressure. If the problem was the fluid, this should be enough to eliminate the jerkings. If the oil change did not help, you will have to dismantle the gearbox and carry out diagnostics in a car workshop. This procedure is quite expensive but necessary.