Why does an automatic transmission car's engine speed increase but the speed doesn't when accelerating?
4 Answers
When an automatic transmission car's engine speed increases but the speed doesn't when accelerating, it is typically an issue with the automatic transmission, with the most common cause being the torque converter not locking up. Symptoms of a torque converter not locking up: Under normal circumstances, when the engine speed, vehicle speed, and throttle depth reach a certain level, the torque converter should lock up. If the torque converter fails to lock up, it often results in the car revving loudly without accelerating. This phenomenon mainly occurs in vehicles equipped with torque converters, such as all AT (automatic transmission) cars and some CVT (continuously variable transmission) models. Solution: For smooth and powerful acceleration in an automatic transmission car, it is necessary to press the accelerator pedal more firmly. Whether the torque converter locks up depends not only on the vehicle speed but also on the engine speed and throttle depth. If the driver consistently applies light throttle input and maintains low speeds, the engine speed may not rise sufficiently, preventing the torque converter from locking up.
I've been driving for over a decade and encountered this automatic transmission issue of high RPM without corresponding speed increase several times. The most common cause is torque converter slippage in the transmission - when it fails to lock during power transfer, the engine revs but power doesn't reach the wheels, likely due to severely worn friction plates inside the converter. Another possibility is aged or insufficient automatic transmission fluid (ATF), where inadequate lubrication leads to abnormal hydraulic system operation and acceleration failure. Additionally, faults in the transmission control module, such as sensor signal misjudgment, can also cause speed-RPM mismatch. I recommend starting with basic checks: pop the hood to inspect transmission fluid - if it appears black or has a burnt smell, replacement is needed. Neglecting repairs may lead to overheating and component damage from prolonged slippage, potentially doubling repair costs. Regular maintenance every 20,000-30,000 km with fluid changes can prevent this issue.
I was quite panicked when I first encountered this situation. The engine roared when I stepped on the gas, but the car just wouldn't move, which felt really strange. A friend told me that with an automatic transmission, it could be a loose drive belt or an issue with the drive shaft, like severely worn universal joints in older cars, preventing power from reaching the wheels. Another possibility is wheel spin, especially on wet roads during rain, where the tires spin in place without gaining speed. I checked the transmission fluid level myself, and adding some new fluid when it's low can help alleviate the issue. If the problem persists, the transmission internals need to be inspected, such as a failed clutch pack or stuck gears. If you notice poor acceleration while driving, slow down and stop immediately to avoid risking an accident. During maintenance, have the technician check the transmission system pressure to prevent problems before they occur.
As a car enthusiast, I've researched this issue. When the engine speed increases but the vehicle speed remains unchanged, it indicates the engine is functioning normally but the power isn't being transmitted. Automatic transmissions rely on torque converters to transfer torque - if the converter is worn or has design flaws, it can slip, resulting in high RPMs without effect. Transmission fluid overheating causing viscosity reduction can also cause this. Control system issues like faulty computer modules may interrupt signals. When I dealt with this, changing to new fluid resolved the oil quality problem.
