Why can't a manual transmission car shift gears when running but can when the engine is off?

[VanAngelo]

When a manual transmission car cannot shift gears while running but can do so when the engine is off, there are two possible causes: 1. Clutch not disengaging: The clutch fails to fully disengage, while the transmission's gear-shifting mechanism functions normally. Solution: This issue can be resolved by adjusting the clutch or replacing certain clutch components to ensure proper free play and disengagement clearance. 2. Detent ball failure: The positioning components of the gear lever's detent ball lose functionality, allowing the gear lever to rotate freely around its own axis and making accurate gear control impossible. Alternatively, the spherical working surface at the lower end of the gear lever may be worn out, preventing the lever from controlling the shift fork or shift head's groove, thus making both gear engagement and disengagement impossible. Solution: For the former issue, repair is necessary to resolve the fault; for the latter, the gear lever must be replaced.