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Discussion Starter #1 (Edited)
I have an automatic sGT. When I'm coasting down a gentle slope in heavy traffic, I like to manually downshift to 2nd gear to control my speed and ease my braking. When doing so I've noticed that my RPM will steadily rise to 2000, then suddenly drop down to 1000, then steadily rise back up to 2000 only to drop back down to 1000. This also happens in 1st gear at slower speeds.

What is going on here? Is the car automatically working the clutch at 2000 RPM, which would explain the drop in RPM? This is my first automatic so I have no idea what the tranny is doing on the backend, and that scares me.

EDIT: corrected the RPMs, they are changing at 2000 and 1000.
 

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I have an automatic sGT. When I'm coasting down a gentle slope in heavy traffic, I like to manually downshift to 2nd gear to control my speed and ease my braking. When doing so I've noticed that my RPM will steadily rise to ~2400, then suddenly drop down to ~1300, then steadily rise back up to ~2400 only to drop back down to ~1300. This also happens in 1st gear at slower speeds.

What is going on here? Is the car automatically working the clutch at 2000 RPM, which would explain the drop in RPM? This is my first automatic so I have no idea what the tranny is doing on the backend, and that scares me.
I'm pretty sure what you're seeing is the fuel being reintroduced into the engine. When coasting at high to moderate speeds, the engine is powered by the wheels only and the fuel is cut off. At a certain speed, the engine reintroduces fuel to maintain the engine speed above idle (and prevent stalling of course).

It's around that point that the 'current MPG' display will turn from 99.9 to -- as well, which should give you a clue as to what is happening.
 

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I have an automatic sGT. When I'm coasting down a gentle slope in heavy traffic, I like to manually downshift to 2nd gear to control my speed and ease my braking. When doing so I've noticed that my RPM will steadily rise to ~2400, then suddenly drop down to ~1300, then steadily rise back up to ~2400 only to drop back down to ~1300. This also happens in 1st gear at slower speeds.

What is going on here? Is the car automatically working the clutch at 2000 RPM, which would explain the drop in RPM? This is my first automatic so I have no idea what the tranny is doing on the backend, and that scares me.
Sounds like after a while it's going back to D from 2nd, so it's continuing to upshift. Is this a steep hill?

I do the same thing with my automatic S Touring and I've only seen it do this after I slow down from 2nd to very slow speeds, then the car automatically drops it to 1st at ~1K RPM if I don't do it manually. In that case the RPM just blips up as it does the shift, if I do it manually, the RPM will soar up to the 4.5K range.
 

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Discussion Starter #5 (Edited)
The display on my dash says I'm still in the gear I manually shifted into (2nd). I am also not stepping on either the brake or the gas pedal, and just letting it coast downhill with light engine braking to control speed. I can test it again tonight on my drive home but I don't believe it's upshifting, if it was two things would happen, 1) the rpm would stay low and not bounce back up to ~2400 (since I'm not gaining speed), and 2) the display on the dash would display 3 or D.

I'm pretty sure what you're seeing is the fuel being reintroduced into the engine. When coasting at high to moderate speeds, the engine is powered by the wheels only and the fuel is cut off. At a certain speed, the engine reintroduces fuel to maintain the engine speed above idle (and prevent stalling of course).
If this was the case, wouldn't the engine be reintroducing fuel when my rpm drops too low, causing it to spike up, and not vice versa?
 

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You don't feel any sort of stutter cause by a shift. Most automatics will override the manual shifting you told it to do if it thinks it's safer. The car automaticly realizes it is inefficient and causing excess wear on the engine (a new engine or transmission is more expensive than new brakepads) so it upshifts for you, but since you've forced it into 2nd which it doesn't like it fights itself and forces itself back down, then up then down. That's my guess. Especially if you aren't touching the gas. If you aren't touching the gas the car thinks you aren't paying attention and thus it will shift for you.

Same thing happens if you shift down and have your foot on the same position of the accelerator without pressing the accelerator more, it will give it a second or two then upshift thinking that you don't actually want to downshift and thus have more power from the higher RPMS but that it was an accident.
 

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Because automatics are called automatics for a reason.

There is no benefit to engine braking with an automatic. It doesn't work like that at all.
All you're doing is confusing the transmission processor.
 

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Dual clutch (DCT) automatic does it automatically, I don't know yet how does it work on Mazda's transmission. DCT work in this way, take you foot off the gas pedal, a car slows down by itself, not completely but good enough for a heavy traffic. There is no need to press the brakes or downshift, which is even on manual transmission you do in critical road condition, mostly.
 

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Because automatics are called automatics for a reason.

There is no benefit to engine braking with an automatic. It doesn't work like that at all.
All you're doing is confusing the transmission processor.
Is this still applicable when used conjunctively with brakes before rolling down a steep grade? I typically do this and combine with brakes to avoid excess engine load.
 

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Discussion Starter #13
Just verified again on my drive home tonight. On 2nd gear at between 15-20mph while rolling downhill, no brakes no gas, the RPM will slowly creep up to 2000, then suddenly dip down to 1000, then slowly creep back up, over and over again. Stick is set on manual override. Dash displays 2nd gear the whole time.

Really boggles my mind because 2000 RPM is by no means excessive...
 

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I mean if you're going down a hill, yes, it's fine to see if you can force your car to hold a gear.
If you're doing it around town.... chances are it won't accept it, and go back to whatever gear it was in.

I personally will not recommend going from N to D while rolling.
It's really bad for insurance reasons too; if you get in a wreck and your transmission was found to have been put in neutral and you wrecked... they're going to likely blame a lot of it on that, and less of a failure of the vehicle etc.


Strange that the revs jump all over the place.
Does it do it when you're going down a hill as well?
 

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Hold on!
What is the difference, if I have manual tranny, driving in heavy traffic switched from say 2nd gear to neutral, stopped my car and released a break pedal on a straight road, then proceed again.
Or, driving automatic tranny going through the same process but from D-N-D, while car is rolling fwd most likely cannt be switched to R, not the one I know of, so how this can cause a wreck?
It just cannot fit in my brain how you can be blamed from switching from D-N-D?
I suppose nonsense is a word unless info is verifiable
 

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Discussion Starter #16
Strange that the revs jump all over the place.
Does it do it when you're going down a hill as well?
lol yes, I've only experienced this RPM fluctuation while going downhill. That's the main reason I downshift in the first place (control speed going downhill).
 

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D-N-D, while car is rolling fwd most likely cannt be switched to R
This is why I don't recommend it.

Yes.... it can lol.
Ask me how I know....... :pinch:


This is probably a dumb question, but the revs fluctuate.. and your speed continues to rise correct?

Do you think you can get a video of it?
 

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Discussion Starter #18
No the speed stays within the same range. As the revs rise to 2000 the engine brakes just enough to slow me down a little. When it drops to 1000 it speeds me up a bit. Speed variance is within 5mph.

Here's a quick topo of the area where I experience this (405 freeway from Skirball Center to the 101). You can see I'm driving down a "mountain" so it's fairly steep, and the thing about the 405 is there's always traffic so I'm always going about 10-20mph on this section.

https://goo.gl/maps/cf8iP
 

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Although I'm not sure whether you are confusing the matter by attempting the engine braking manually, there is an element of overthinking in here. This modern car uses various inputs to control the transmission and does in fact understand that you're going downhill, and thus will AUTOMATICALLY have the transmission downshift if the downhill is long enough and you brake a little. This sort of logic has been in automatic transmissions and their associated controls for a while now.

Really, I'm not even sure you have to brake, just let off the gas, because personally I find it cuts in with downshifting a bit TOO MUCH! I'm used to the hills I drive on daily, and when driving my old manual Integra I never would have downshifted on these hills, whereas my 3i auto will drop down a gear fairly regularly. Probably need to use the manual mode to keep it in a higher gear instead. ;-)
 

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Discussion Starter #20
During this stretch of downhill my Mazda3 always gears up to 3rd even when I'm under 20mph. This causes me to pick up speed pretty quickly and, due to the heavy traffic, I would have to constantly apply the brakes. It seems super inefficient to me. If I manually shift into 2nd gear then I can maintain a slow speed going downhill that matches the flow of heavy traffic pretty closely. Less headache, less wear on brakes, happier driver.

But yes I do experience what you mention, when I'm driving at a moderate speed and slow down I do feel the car downshifting automatically. However if I'm already at a slow pace while going downhill, it won't stay in 2nd and seems too eager to upshift into 3rd all the time.
 
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