Is It Okay to Shift an Automatic Transmission into Gear While Rolling Backward?
You’ve probably seen older drivers do this: while backing out of a garage, driveway or parking spot, he suddenly shifts the gearbox into the '‘D' position while the car is still in backward motion, not waiting for the car to come into full stop when doing so. For sure, that dealt the transmission gears some serious damage, right? Or did it?
Actually, as long as the car is going backwards less than a couple of kilometers per hour, this practice does little damage to an automatic transmission system. So the straight answer to the question above is: Yes, it's perfectly fine, as long as the car is reversing at a very slow pace. It all relies on how the automatic transmission system essentially works.
Automatic transmissions shift gears by locking and unlocking the various orbits of its planetary gear set. When the vehicle is not in gear, these orbits turn freely, and locking them happens through clutches and bands aided by the automatic transmission fluid. These clutches and bands are designed to 'slide' a little when engaging thanks mainly to the fluid, which absorbs some of the transition, in turn allowing the system to shift smoothly and safely.
It's the same principle when you are forced to stop your car at a traffic light located on a hill or inclined road. When you remove your foot from the brake, the car will start to slightly move backward before power is transmitted through the fluid and pushes your car forward. The slowness of the speed means shifting into drive isn't likely to cause much harm.
On the other hand, if the car was previously in reverse and the car has gained a speed of around 6 to 8 kmh in reverse, that's when it's a good idea to bring the car to a full stop first before putting it onto gear. Otherwise, you're going to end up diminishing the lifespan of your automatic transmission.