What is the reason why an automatic transmission occasionally does not move when the brake is released?
2 Answers
If an automatic transmission car does not move when the brake is released while reversing, the reason may be that the auto-hold function is activated. This phenomenon will not occur if the auto-hold function is not enabled. Simply deactivating the auto-hold function will resolve the issue. Enabling this function enhances safety by preventing the car from moving forward when waiting at traffic lights or crossing pedestrian crossings, thereby avoiding potential collisions with vehicles or pedestrians ahead. Relevant information about auto-hold is as follows: 1. Introduction: Auto-hold, also known as AUTO-HOLD, is an automatic braking function. When activated, it eliminates the need to use the handbrake or electronic parking brake during stops, such as at traffic lights, simplifying operations. It also reduces the need for frequent shifting between D (Drive), N (Neutral), and P (Park) in automatic transmission vehicles. 2. Working Principle: The AUTO-HOLD function uses a slope sensor to provide precise braking force via the control unit. During startup, the parking control unit calculates information from sensors such as the clutch distance sensor, clutch engagement speed sensor, and throttle pedal sensor. When the driving force exceeds the resistance, the parking brake is automatically released, ensuring a smooth start.
After driving for so many years, I've occasionally encountered situations where an automatic transmission car doesn't move when releasing the brake, which is quite common. There could be several reasons, the most frequent being slope-related issues. When parked on an incline, the hill start assist function activates, where the computer holds the brakes a bit longer to prevent rolling back, causing a slight delay before moving after releasing the brake. If there's no slope, it might be due to low or contaminated transmission fluid affecting the creep function, or this can also happen when the engine's cold start RPM is insufficient in winter. Another factor could be the brake system, such as slightly stuck brake discs or adhesive brake pads, especially after rain when it's humid – releasing the brake doesn't immediately remove the resistance, leaving the car without enough power to move. Electronic control malfunctions, like sensor failures, can also mislead the system into incorrect operations. In short, for safety's sake, don't ignore these signs. Regular maintenance should include checking transmission fluid and brakes, and replacing worn-out parts promptly to avoid issues on the road.
