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My question is a variation on these two:

I recently bought an automatic transmission 2012 Mazda 3 after driving a manual for a number of years. As was my habit with the manual, rather than braking on a highway off-ramp, I downshift all the way to 1 to use engine braking (applying brakes right at the end).

When downshifting from 2 to 1 around 30 - 40 km/h the car has a jerk of deceleration and revs to about 4k rpm. I assume this is because a large amount of momentum is transferred through the transmission to the engine block while the clutch is engaging. Is this putting strain or wear on the transmission?

Am I saving wear-and-tear on my brakes at the cost of an eventual transmission replacement?

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    There is a reason they are called automatic, let the transmission shift on its own, yes manually downshifting an automatic causes additional wear on the planetary gears. – Moab May 4 '16 at 16:13
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    There is no clutch in an automatic. – user207421 May 4 '16 at 21:29
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    @MikeOunsworth joining a new site in the Stack Exchange network when you already have an account in any of the other sites is basically a one click operation. I routinely join new sites just to vote or comment on a question. – briantist May 5 '16 at 0:33
  • @MikeOunsworth The goal of this site is to provide people with information. EJP's comment was not made to attack you, but to correct the information in your question for future readers.I agree he could have gone into more detail, but his comment is correct and helpful, not 'nit-picky'. – MooseLucifer May 5 '16 at 15:29
  • @MooseLucifer Fair enough. I've gone on tirades on various SE meta sites about condescending comments to newbie questions - maybe I'm just over-sensitive. I'll remove my comments, thanks. – Mike Ounsworth May 5 '16 at 15:37
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Sorry for bringing up an old thread, but I'd just like to add my personal experience with this. Long story short, I recently took up the practice of engine braking on an automatic minivan, and my transmission is now dead at 100k miles. Could just be a coincidence, but here's the long story for what it's worth:

On a drive up and down the Rockies a bit, 1 year ago, I decided to try engine braking while heading down hills. We're talking about 40-50 mph in 4th gear (the gear just below my OD) and 30-40 mph in 3rd. 3k rpms max, usually closer to 2k. After about 1.5 hrs of doing this on the way back, I noticed a whirring sound. It worried me, but after getting home and checking the transmission fluid, everything seemed fine. I figured I was just being paranoid, that the sound had already been there and that I was just worried that after trying this new thing I was prone to think I did something wrong. But...

After about a year of frequently doing this, not just in the mountains, but at red lights, etc., the 3rd gear suddenly now has a terrible time shifting. And 4th seems like it's about to go as well. The transmission mechanic says the fluid looked fine and everything else is checking out. My "clutches" are going out, he says. Time for a transmission rebuild.

I'm sure it's not that rare for a 10 year old transmission with 100k miles to go out all of a sudden, but I can't help but think that it was the engine braking that made it happen so soon. Whether or not the whirring sound was already there will remain a mystery, but I'm fairly certain it wasn't. I don't want to build unfounded fear for engine braking, but the saying "better to wear out a $150 part than a $3000 part" is really hitting close for me.

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Basically, yes. Automatics don't have a clutch per se, but you're causing additional wear and tear on the internals. Basically you're reversing the power flow -- instead of the engine driving the wheels, you're driving the engine with the wheels. Obviously this occurs normally when you decelerate, but by downshifting you're increasing the amount of force that normally goes through the transmission in that way. There's no way to accurately predict how much additional wear you're putting on your drivetrain, but doesn't it make more sense to put the wear on an easily-accessible part that's designed to be replaced than on an expensive one?

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    Automatics have many clutches, they are called clutch packs. – Moab May 4 '16 at 17:13
  • AT clutches are also called "bands". – blacksmith37 Jan 21 at 16:22
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You should leave the downshifting to the transmission. Sounds like you're doing it wrong anyway. The abrupt "jerk" that you speak of means you're going Too Fast for the gear you've selected. Of course if you're towing a heavily loaded trailer, you could manually downshift to assist the cars brakes to avoid the possibility of overheating them. But the engine revving to 4k on the 2-1 downshift is really being abusive.

There are clutches in an automatic transmission, but for this discussion, you have no direct control over their operation, and should realize instead, that you're no doubt over-revving your engine.

The "1"-"2" & "L" mean that you're selecting that gear manually, and the transmission will only shift automatically between "1" and "2" if you select "2" on start-up. "L" means Lo/Low gear and there won't be an upshift at all. Same is true when you select"1". The transmission will remain in "1" or "Lo". This manual selection is useful for towing at low speed for short distances, and can help when braking on super slippery surfaces.

Additionally, there are always exceptions. A rear wheel drive vehicle will realize theoretically, an advantage when braking on ICE over a front wheel drive vehicle because the proportioning valve sends as much as 67%-70% of the braking force to the front wheels anyway. Yet, a too high Idle speed will have the rear wheels pushing while the fronts are forced to lock-up in their effort to stop the car. A.B.S. does NOT stay with you until you come to a complete stop. On top of all this, braking forces applied to each front (or rear) wheel are not guaranteed to be equal, so from a moderate speed on ICE , engine braking can conceivably help, but you MUST have a good "seat of the pants" feel for what's going on under you as conditions vary.

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    Hi, the question was: "will engine braking in an automatic cause me more repair bills?" I feel like you're answering a completely different question by explaining why a car has gears in the first place. Clearly I'm over-revving the engine at 4k, that wasn't my question, the question is: "will that cause damage, and why?" – Mike Ounsworth May 5 '16 at 1:24

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