What is the reason why an automatic transmission occasionally does not move when the brake is released?
3 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.
That day, I was driving my newly purchased automatic transmission car and noticed that occasionally when I released the brake, the car didn't move. It startled me, and I thought the car was broken. Later, I realized it's a normal phenomenon, especially when starting a cold engine with low idle speed—the engine power can't keep up immediately, and the transmission needs a moment to respond. If parked on a slope, the hill-start assist function becomes more noticeable. I learned from the manual that this is related to the torque converter, but I didn't delve too deeply into these technical terms. The key takeaway is to wait a few seconds and avoid rushing to press the accelerator, as it wastes fuel and harms the car. If the brakes feel sticky, pressing them a few more times after washing the car can help. I recommend noting the timing and conditions when this happens—mechanics will understand quickly and diagnose it efficiently. Also, avoid holding the brake pedal down for extended periods when parked to develop good habits and reduce issues.
