Possible causes
Old fluid or wrong level
Transmission fluid loses its properties over time and collects wear particles. A level that is too low or too high also affects how it works. The type of fluid and the change method depend on the gearbox model.
Adaptations and control
The electronic control unit adapts the shifts to wear and driving style. After repairs, disconnecting the battery or a fluid change, some gearboxes need to relearn their adaptations.
Wear of internal parts
If shifts have been getting harsher for a long time and the problem is growing, the cause may be wear of the internal clutches or the valve body. Servicing the fluid on time slows down wear.
What to tell the mechanic
- In which gears and conditions the thumps appear: cold or warm gearbox, accelerating or braking.
- When the gearbox fluid was last changed, and whether it has ever been changed.
- Whether warnings appear on the dashboard and whether the gearbox goes into limp mode.
What you can do
If the gearbox no longer engages gears, slips badly or has gone into limp mode, do not keep driving hard. Tell the mechanic the car and gearbox model; the right fluid and procedure depend on them.
Where the check starts
The mechanic finds the exact cause during the inspection.
Frequently asked questions
Does an automatic gearbox oil change help?
If the problem is related to the condition of the fluid, a change can improve how it works. Wear of mechanical parts is not removed by changing the fluid. The fluid approved for your gearbox is important.
Is it true that automatic gearbox oil never needs changing?
Some manufacturers say the fluid is for life. In practice, with heavy use its condition is worth checking and maintaining.