What Causes the Thud Sound When Starting an Automatic Transmission Car?
2 Answers
Automatic transmission cars make a thud sound when starting because the brake pads stick to the brake discs after releasing the handbrake, and the sound is produced when the brake pads snap open under force during startup, which is a normal phenomenon. Below is relevant information about automatic transmissions: 1. Introduction to Automatic Transmissions: As the name suggests, an automatic transmission does not require the driver to manually shift gears; the vehicle automatically selects the appropriate gear based on driving speed and traffic conditions. Typically, automatic transmission cars have six gear positions, arranged from top to bottom as: P, R, N, D, S, L. 2. Working Principle: An automatic transmission uses planetary gear mechanisms to shift gears, automatically adjusting based on the degree of throttle pedal pressure and changes in vehicle speed. The driver only needs to control the speed using the accelerator pedal.
From a mechanical perspective, there are three common causes for the clunking sound when starting an automatic transmission vehicle. If the torque converter in the transmission is worn or has unstable internal oil pressure, it can cause impact and noise during acceleration. Excessive clearance at the driveshaft connection points can lead to discontinuous power transmission and noise. Aging and deformed rubber bushings in the suspension system can produce friction sounds when the vehicle starts. Worn differential half-shaft CV joints are another common factor, especially noticeable when starting during turns. The braking system, such as stuck calipers or electronic parking brake release, may also generate noise, though the latter is usually normal. Ignoring these issues and driving long-term could lead to further component damage or even loss of vehicle control, posing safety risks. Safety first—I recommend immediately visiting a professional repair shop to use a multimeter for testing related circuits and inspecting the chassis to eliminate potential hazards. Regular maintenance can prevent such abnormal noises.
