Analysis of common Transmission faults

I. Oil Leakage

This is one of the more common faults. Usually, oil seepage can be observed at the transmission housing or the oil pan. The main reasons for this are the aging of the seals and poor sealing. With long-term use, the seals lose their elasticity and are unable to effectively prevent the leakage of oil. It may also be due to poor manufacturing processes, resulting in small holes in the housing, which in turn causes oil seepage. Oil leakage not only leads to the consumption of transmission oil, affecting the normal lubrication and heat dissipation of the transmission, but in severe cases, it may also cause excessive wear of the internal components of the transmission, leading to more serious faults.
Once oil leakage is detected, it should be checked in a timely manner. If it is just the aging of the seals, new seals can be replaced. If there are small holes in the housing, slight holes can be repaired, and in severe cases, the transmission housing needs to be replaced.

II. Abnormal Gear Engagement

(I) Gear Engagement Delay or No Response

Under normal circumstances, after starting the engine, idling, stepping on the brake, and engaging the gear, the vehicle should have a gear engagement feeling within 1 to 1.2 seconds, and it can start slowly after releasing the brake pedal. If the gear engagement time exceeds 2 seconds, it is defined as gear engagement delay. If the vehicle shows no response after gear engagement, it may be due to the damage of the lock-up solenoid valve, which cannot control the oil circuit properly, resulting in the inability to transmit power. It may also be that the lock-up control valve is stuck in a certain position and cannot work properly, affecting the gear engagement operation.
In such a situation, professional diagnostic equipment should be used to read the fault codes to determine which specific component has a problem, and then replace the damaged lock-up solenoid valve or repair the stuck lock-up control valve.

(II) Gear Engagement Impact

Any automatic transmission will have a certain degree of impact feeling when engaging the gear. However, if the impact is too large, it is abnormal. This may be due to the transmission oil being too dirty. The dirty oil will cause the valve body to stick, making the oil pressure regulation unstable. It may also be due to the engine idle speed being too high. A high idle speed will cause the engine to output excessive power, resulting in a large impact when engaging the gear. Or the throttle cable or throttle position sensor is adjusted improperly, causing the transmission to receive inaccurate signals, thus affecting the power matching during gear engagement.
To solve the problem of gear engagement impact, first, check the condition of the transmission oil. If the oil is too dirty, it should be replaced in a timely manner. At the same time, check whether the engine idle speed is normal. If necessary, adjust the engine idle speed. For the throttle cable or throttle position sensor, it is also necessary to check and adjust to ensure its normal operation.

III. Shifting Problems

(I) Shifting Impact

There is an obvious jerking feeling during the shifting process of the vehicle, as if the vehicle shakes violently. This may be due to the wear of the rubber pads between the transmission and the engine. After the rubber pads are worn, they cannot effectively buffer the vibration and impact between the engine and the transmission. The loose connection bolts make the connection of the transmission system less tight, causing displacement and impact during shifting. The excessive clearance or looseness of the transmission system leads to instability in power transmission. It may also be that the accumulator fails and cannot effectively regulate the oil pressure, resulting in excessive oil pressure fluctuations during shifting. Or the working clearance of the friction elements of the shifting actuators such as the brakes and clutches is abnormal, making the frictional force during shifting uneven.
For shifting impact, it is necessary to check and replace the worn rubber pads, tighten the loose connection bolts, and adjust the clearance of the transmission system. If the accumulator fails, a new one needs to be replaced. For the friction elements of the shifting actuators, check and adjust their working clearance to restore their normal working state.

(II) Excessively Long or Short Automatic Shifting Time

A normal automatic transmission has a specific shifting speed and vehicle speed. If it is found that the shifting time is suddenly too long or too short, it means that the gear ratio of the transmission is not in proportion. This may be due to sensor failures, such as the vehicle speed sensor, throttle position sensor, etc. The sensors cannot accurately transmit signals, causing the transmission control unit to be unable to correctly determine the shifting timing. It may also be that the valve body is stuck, affecting the regulation of oil pressure and the switching of the oil circuit. Or the transmission oil is too dirty, affecting the fluidity of the oil and the stability of the oil pressure.
When the automatic shifting time is abnormal, first check whether the sensors are working properly. Professional equipment can be used to detect the signal output of the sensors. Clean or replace the stuck valve body. If the transmission oil is too dirty, replace the new transmission oil in a timely manner.

IV. Slipping and Weak Acceleration

During the driving process of the car, when stepping on the accelerator to increase the speed, the engine idles, and the vehicle speed does not increase. Or the vehicle can drive normally on a flat road, but it has weak power when going uphill, and the engine speed is abnormally high. Most of the time, it is due to the burning of the clutch plates. After the clutch plates are excessively worn or affected by high temperature, the friction material on the surface is damaged, and it cannot effectively transmit power. It may also be that the automatic transmission oil level is too low, resulting in insufficient lubrication and oil pressure. Or the oil level is too high, and a large number of bubbles are generated after being violently stirred by the planetary gear mechanism during operation, affecting power transmission. The wear of the oil pump or even the leakage of the main oil circuit will cause the oil supply pressure to be too low, unable to meet the requirements of power transmission.
If the clutch plates are burned, the transmission needs to be disassembled, and new clutch plates need to be replaced. Check the automatic transmission oil level and adjust it to the normal range. For the wear of the oil pump or the leakage of the main oil circuit, repair or replace the relevant components to ensure the normal oil supply pressure.

V. Abnormal Noise

(I) Bearing Noise

Generally, the noise of the bearing is closely related to the rotational speed. The higher the rotational speed, the more obvious the squealing sound. This is because after long-term use of the bearing, the balls or rollers are worn, and pits or scratches appear on the surface, generating vibration and noise during high-speed rotation.
When bearing noise occurs, the damaged bearing needs to be replaced in a timely manner to avoid further damage to other components.

(II) Differential or Main Drive Gear Noise

This kind of noise is generally more obvious at a vehicle speed of 50 to 70 kilometers per hour. Due to the different meshing surfaces of the gears, the noise is usually obvious when accelerating and significantly weakens when releasing the accelerator. It may be caused by gear wear, tooth surface spalling, or excessive gear clearance.
For the noise of the differential or main drive gear, check the wear condition of the gears. If the wear is severe, the gears need to be replaced. Adjust the gear clearance to restore it to the normal range.

(III) Noise in the Hydraulic System such as the Oil Pump

This kind of noise is closely related to the rotational speed but has nothing to do with the vehicle speed. When the rotational speed is high, a squealing sound is caused because the oil pump has difficulty in sucking oil. It may be due to the wear of the internal parts of the oil pump, such as gear wear and vane damage. It may also be that there are impurities in the oil, blocking the oil suction port or the oil circuit of the oil pump.
When noise occurs in the hydraulic system such as the oil pump, check the internal parts of the oil pump and replace the worn parts. At the same time, filter or replace the transmission oil to remove the impurities in the oil.

VI. Unpleasant Odor in the Transmission Housing

When the temperature of the hydraulic oil in the transmission housing is too high, a burning odor will be generated, which indicates that some of the actuators in the transmission housing are not working properly. It may be due to the transmission operating under overload for a long time, such as frequent rapid acceleration and braking, or driving on a steep slope for a long time, which makes the transmission work under high intensity and unable to effectively control the temperature. It may also be that the transmission oil is too dirty or deteriorated, losing its good lubrication and heat dissipation performance.
If an unpleasant odor is detected in the housing, first check the working status of the transmission and avoid operating it under overload for a long time. At the same time, check the quality of the transmission oil. If the oil is too dirty or deteriorated, replace the new transmission oil in a timely manner.

VII. High Engine Speed during Driving

(I) High Engine Speed during Smooth Driving

When the vehicle is driving smoothly, if the engine speed remains at a relatively high level continuously. For example, for a car with a 2.0-liter displacement, under normal circumstances, the vehicle speed can reach 100 kilometers per hour when the engine speed reaches 2600 revolutions per minute. However, for a certain period of time, it often requires an engine speed of 3000 revolutions per minute or even higher to reach the same vehicle speed. This is very likely due to a fault in the high gear of the transmission, which cannot shift into the high gear normally, forcing the engine to maintain the vehicle speed at a higher speed. There is also a possibility that the lock-up clutch inside the torque converter fails and cannot effectively transmit power, causing the engine’s energy to be wasted in the hydraulic transmission of the torque converter. In this case, it is necessary to check the shifting mechanism and control unit of the transmission, repair or replace the damaged components to restore the normal operation of the high gear. Check the lock-up clutch inside the torque converter and repair or replace it in case of a fault.

(II) Excessively High Engine Speed during Acceleration

During the acceleration process, the rise of the engine speed is closely related to the accelerator pedal. However, if the engine speed is too high and exceeds the normal range, it may also be a problem with the electronic control system of the transmission. For example, sensor failures may cause inaccurate signals to be transmitted to the control unit, resulting in the control unit wrongly controlling the shifting timing. Or the control unit itself malfunctions and cannot correctly process the sensor signals and control the shifting actuators.
When the engine speed is too high during acceleration, use professional diagnostic equipment to detect the electronic control system, check whether the sensors and the control unit are working properly, and repair or replace them in case of a fault.