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Car Forum / Toyota / Camry / February 2008What happens when I take the foot off the gas pedal?
Assume automatic transmission, we are at the cruising speed of let's say 40 mph and I take the foot off the gas... Start loosing speed, aproaching read light, brake - stop. What will transmission controller be doing at these moments? Is it shifting to neutral, disconnecting wheels from engine? Is it reducing to a lower gear? But I do not feel it in my car... Something else is happening? Where can I read more technical details of such scenarios? My car is 1995 toyota camry, 2.2 liter 4 cyl. My friend with 2004 vw passat GTX V6 tiptronic transmission has the "currently engaged gear" indicator on the dashboard and when he takes his foot of the gas he sees the transmission to downshift from 5 down to 1 when he finally break to stop. Camry seems to behave differently since I do not feel engine breaking which would happen if the engine got engaged to a lower gear succesively. Many transmission manuals only briefly touch on the theory of operation. So I don't see a lot of detailed info on the subject. However, wikipedia's automatic transmission (and the torque converter link) plus the 5 pages on Performance Oil's site should help: http://en.wikipedia.org/wiki/Automatic_transmission#Parts_and_operation http://www.performanceoiltechnology.com/transmission_operation_1.htm Tiptronic: http://en.wikipedia.org/wiki/Tiptronic From what I can tell, the answer has to do with the "fluid coupling" nature of the torque converter and the one-way clutches used in the tranny. It allows the engine to run without stalling even when the car is stopped at a red light. When you first released the accelerator and the speed hasn't changed much, the tranny was still in "coupling" mode. As the car slows down 10% or so, the lockup clutch in the converter disengages and the engine decouples from the tranny. And as the speed further decreases, the ECU commands many "apply devices" (clutches and brake bands) to change to lower gears. No, you were never in "neutral". You can probably build your own "shift indicator" by tapping the signals to various shift solenoids as long as the signal integrity isn't affected. Then you can have LEDs lighting up indicating different gears, etc. Should be fun. :) As far as why engine braking is more pronounced in VW maybe it has to do with performance programming? Because I drove an Audi Q7 some while ago and the excellent ZF Friedrichshafen 6 speed Tiptronic coasts more like a good 'ol GM Hydramatic than a VW. Audi is a VW company and both get technology from Porsche. The Toyota Aisin tranny designs started when Borg Warner began a joint venture with them in 1969, so your A140E should feel like a typical American transmission. Aisins aren't the best transmissions around, but they work reasonably well and coast comfortably. FYI only: http://www.pbase.com/kocho/camry > Assume automatic transmission, we are at the cruising speed > of let's say 40 mph and I take the foot off the gas... [quoted text clipped - 16 lines] > breaking which would happen if the engine got engaged to > a lower gear succesively. > Many transmission manuals only briefly touch on the theory of > operation. So I don't see a lot of detailed info on the subject. [quoted text clipped - 23 lines] > isn't affected. Then you can have LEDs lighting up indicating > different gears, etc. Should be fun. :) I was thinking about it :-)) > As far as why engine braking is more pronounced in VW maybe it has to > do with performance programming? Because I drove an Audi Q7 some while > ago and the excellent ZF Friedrichshafen 6 speed Tiptronic coasts more > like a good 'ol GM Hydramatic than a VW. Audi is a VW company and both > get technology from Porsche. So you felt this difference in your Q7 as well? > The Toyota Aisin tranny designs started when Borg Warner began a joint > venture with them in 1969, so your A140E should feel like a typical > American transmission. Aisins aren't the best transmissions around, > but they work reasonably well and coast comfortably. I have nothing against the well working transmission in my old good camry. I was just curious about the differences making described symptoms. > FYI only: http://www.pbase.com/kocho/camry What this link has to do with the discussed subject? Confused... 1. Looks like the "coast brake" is on for 2nd and 1st gears during manual downshift. But in D, the coast brake is NOT on. This probably explains the VW experience as their equivalent of the coast brake. Then it would be more like a safety feature. (imagine going downhill without it). The coast brake is "programmable". But don't disable it I'll say. So if you shift into 2 (while the D mode is on the 2nd gear obviously), you should feel the coast brake kick in giving you engine braking. 2. No just test driven one, don't have an Audi Q7, Jaguar XJ-10, Range Rover or even an ultimate driving machine etc etc. If I do I won't be driving a Toyota ;) ;) ;) 3. Just some pictures of the tranny in discussion. Not directly related otherwise. > > You can probably build your own "shift indicator" by tapping the > > signals to various shift solenoids as long as the signal integrity [quoted text clipped - 22 lines] > > What this link has to do with the discussed subject? Confused... > Assume automatic transmission, we are at the cruising speed > of let's say 40 mph and I take the foot off the gas... [quoted text clipped - 16 lines] > breaking which would happen if the engine got engaged to > a lower gear succesively. We have two '95 LE fours. I think that one of the reasons these cars get such amazing fuel economy for their size and weight is that the transmission goes into a 'coast' mode when you take your foot off the gas. If you look at the tach, you will see it drop to about 500 rpm above idle, even at freeway speeds. I vaguely remember someone mentioning an additonal clutch in this unit that disengages to accomplish this. At any rate, careful driving and use of this 'coast mode' will get you overall MPG in the mid 30's in Summer, and about 30 in Winter. The D mode doesn't engage the coast brake, so there is no engine braking. However, the manual downshift modes (2nd and 1st) do. Engine braking would kick in if you switch from D to 2 (while you are in that speed range of course). On Feb 4, 9:15 pm, "mjc13<REMOVETHIS>" > We have two '95 LE fours. I think that one of the reasons these cars > get such amazing fuel economy for their size and weight is that the [quoted text clipped - 5 lines] > mode' will get you overall MPG in the mid 30's in Summer, and about 30 > in Winter. > The D mode doesn't engage the coast brake, so there is no engine > braking. However, the manual downshift modes (2nd and 1st) do. Engine > braking would kick in if you switch from D to 2 (while you are in that > speed range of course). Engine braking also kicks in when you disengage the overdrive. I use that feature all the time in the Winter, to go down steep hills. > On Feb 4, 9:15 pm, "mjc13<REMOVETHIS>" > [quoted text clipped - 7 lines] >>mode' will get you overall MPG in the mid 30's in Summer, and about 30 >>in Winter. > What will transmission controller be doing at these moments? > > Where can I read more technical details of such scenarios? ======== See: Toyota technical articles: http://www.autoshop101.com/techarticles/technicalarticles.html#AT |
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