Does going into drive while still reversing damage an automatic transmission?

I was the designated driver the other day and was driving some of my overly sensitive friend’s home in one of their cars. I pulled out of the driveway into the street and then went along on our way. But, apparently I had made a grave sin in the eyes of my non-technically inclined, non-car enthusiast friend. That sin was that I didn’t come to a complete stop before putting the transmission into drive… I haven’t heard the end of it since.

Now, it’s not like I was racing in reverse and then slammed it into drive like some sort of stunt driver. I was backing out normally, braked to less than a mile per hour, and then put it into drive. It was a very smooth transition and there were no clunks, bumps or any drama at all. If I was going any slower, I would have been stopped.

The problem is that most people don’t understand how an automatic transmission works. I believe they think that gears are actually moving into one another and that if you’re not completely stopped, you would grind the teeth off the gears. As you probably have guessed, this is not the case.

Automatic transmissions use a planetary gear set which have gears that are always meshed. Shifting gears happens by locking and unlocking the various orbits; and when not in gear, those orbits are left to freewheel. Locking these orbits is done by clutches and bands (basically brake shoes with friction material on them) which are activated by hydraulics. These clutches and bands are made to slide a little when engaging, and that’s that gives smooth shifting.

Furthermore, automatic transmissions need a method to be in gear and be able to have the car stopped without stalling the engine. This is achieved by using a torque converter which connects the engine to the transmission though a fluid coupling. The torque converter works like an egg beater mixing thick pudding in a bowl: if you beat the pudding without holding onto the bowl, the bowl will spin. But if you hold onto the bowl (like applying the brakes in your car) the egg beater will continue to turn the pudding, but the bowl (your wheels/transmission) will be stopped.

The transmission doesn’t get damaged when shifting into drive while the car is still (slowly) moving backwards because there’s no hard mechanical links between anything in the transmission. The slow engagement of the clutches and the slop of the torque converter absorb any speed differentials and allow you to do this. This ease of use of automatic transmissions is what they were designed for, and if this caused serious problems, you would hear about it all the time.

 

 

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7 Comments to "Does going into drive while still reversing damage an automatic transmission?"

  1. daniel's Gravatar daniel
    November 17, 2011 - 8:23 am | Permalink

    I’ve always wondered how a tranny works. Well here is something that possibly broke on my 99 expedition when shifted into drive and gas pedal pressed while rolling in reverse at about 3-5 mph. Since that incident at a tire shop by their speedy car mover @ speeds over 45mph there is a vibration/groaning sound around transfer case or front axle while slightly accelerating. Also won’t come out of 4hi or 4lo unless engine is turned off and cranked up again. I’m guessing its my transfer case shift motor.

  2. Glenn Ballard's Gravatar Glenn Ballard
    December 11, 2011 - 10:24 pm | Permalink

    Going from reverse to drive while moving slowly will not DAMAGE an automatic transmission, but it is a bad habit. The foreward clutches in your transmission are NOTa substitute for the foot brake, and wear and possible failure will result from chronic reverse/drive or drive/reverse shifts while car is in motion.

    • Lorr's Gravatar Lorr
      May 22, 2014 - 10:18 pm | Permalink

      I’m thinking you nailed this one right on the forehead, sir. Just like many other bad driving habits, they might not break the car right away, but if done continually, will cause the affected part’s life span to diminish substantially, resulting in its defective operation, if not its catastrophic failure. Another similar driving habit is applying the brakes while hitting bumps or potholes. I’ve read it in a book somewhere that this is bad on the suspension, particularly the ball joints and the tire rods. Since then, I always make it a point to lift off the brakes when hitting sharp road irregularities.

  3. Carlos's Gravatar Carlos
    August 21, 2013 - 4:22 am | Permalink

    Hello I have a 01 corolla 3spd automatic
    Accidentally I shift from neutal to reverse
    At 100mph the tires brake like stepping in the brakes
    For lest then a second then I shift back neutral and then drive
    Nothing happens to the tranny.

  4. Eric P's Gravatar Eric P
    March 11, 2015 - 1:01 pm | Permalink

    Good explanation, a visual would help though.

  5. Thomas Luggiero's Gravatar Thomas Luggiero
    August 14, 2016 - 6:53 am | Permalink

    Automatics have things called one way clutches. They are roller sprag type. You can grenade these or simply wear them out. When an automatic is on a hill and it doesn’t roll back when you take your foot off the brake, that is the owc holding. They are all in a timing when shiftng from reverse to drive to stop shafts/planetaries. You damage these everytime you drive your car foward frm rolling backwards in reverse. The transmission needs time to catch up sorta speak.

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