What Causes the Thud When Shifting to Reverse in an Automatic Transmission?
2 Answers
There are several reasons for the thud when shifting to reverse in an automatic transmission: 1. Electronic linkage between the brake system and the automatic transmission: If the clutch is not fully depressed when shifting gears, it can cause gear damage; additionally, when not using the clutch, the foot should not rest on it, as this can lead to excessive wear of the release bearing. 2. Shifting at idle: If the gear is shifted from neutral to reverse before the engine speed has stabilized, it can cause strong shock vibrations in the transmission, potentially damaging the shift switch inside. 3. No synchronizer in reverse gear: The synchronizer is a device inside the transmission that ensures smooth gear engagement when shifting. Some cars lack a synchronizer, making it harder to shift gears smoothly. 4. Shifting before the car has come to a complete stop: If the car is shifted into P (Park) before it has fully stopped, the P gear lock hook inside the transmission can hit the gear, producing a thud. This operation can wear or even break the lock hook, and in severe cases, may lead to transmission failure.
As a long-time transmission researcher, I must tell you that the clunking sensation when shifting an automatic transmission into reverse is usually caused by internal mechanical issues. This could be due to improper synchronization of the clutch or torque converter. When transmission fluid becomes old or insufficient, inadequate lubrication increases gear friction, resulting in noticeable jerking. Another common cause is valve body sticking or abnormal oil pump pressure, leading to uneven power delivery. Sometimes faulty circuit sensors or software calibration deviations may also trigger this issue, especially after frequent hard acceleration. I recommend prioritizing inspection of transmission fluid level and quality - immediate replacement is needed if it appears black or has a burnt smell. Neglecting this can lead to more severe wear, even requiring transmission overhaul. That's why I always emphasize the importance of regular maintenance, such as fluid changes every 40,000 km, which effectively prevents problems. Also avoid rapid gear changes during normal driving to allow system buffering time.
