Automatic trannys

[Tom]

Lately I've been manually shifting the foward gears in my mom's van.  Her car has a tach, so I thought I would give it a try for a while.  The first thing I noticed is that coming off the gas in a particular gear makes the car "buck" kind of like with a manual.  However, in D this does not happen.  It was also nice not having to brake going down hills.  I got back into my neighborhood today and tried to stall it by selecting 2nd gear and going really slow, then coming to a stop.  It wouldn't stall. 2 questions:

1.  Do auto trannys in D shift into neutral or disengage the current gear when the driver is not applying the gas pedal?  If so, how does it reengage into gear so quickly and smoothly when the gas is reapplied?

2.  Why can't I stall it?  If I am really in 2nd gear I can't come to a complete stop, right?

Thanks is advance :)

[giant_mtb]

I'm not so sure about the answer to question 1, but for number 2, I'm pretty sure you should be able to stop even if you have "2" selected...it shouldn't stall, either.

Possibility for 1...The torque converter probably has something to do with it, but since auto trannys are less connected with the engine than a manual, it might make lifting off the gas smoother...

[93JC]

1. No, they don't shift into neutral. No they don't disengage the current gear. They shift to first. The torque converter will slip so much that the hydraulic "connection" between the engine and transmission is not great enough to overpower the brakes. When you step off the brake pedal the "connection" re-establishes itself and you begin moving forward.

2. You can't stall it because the torque converter will, under normal operation, slip enough so as to never stall. You're not really in second gear all the time by selecting "2": you're limiting the transmission to first and second gear. When you come to a complete stop the transmission shifts down to first and the torque converter slips, as described above.

[Tom]

1. No, they don't shift into neutral. No they don't disengage the current gear. They shift to first. The torque converter will slip so much that the hydraulic "connection" between the engine and transmission is not great enough to overpower the brakes. When you step off the brake pedal the "connection" re-establishes itself and you begin moving forward.

2. You can't stall it because the torque converter will, under normal operation, slip enough so as to never stall. You're not really in second gear all the time by selecting "2": you're limiting the transmission to first and second gear. When you come to a complete stop the transmission shifts down to first and the torque converter slips, as described above.

Thanks.  That clarifies things.

[Raza]

The car's bucking in 1st gear, right?  

See, what happens when you rev it higher in manual mode and let off the gas, the car jerks, and the trans copes with the movements and the changes in input.  Some manual cars do this as well.  What happens in "D" is that the transmission reads that you're off the gas, and then automatically upshifts, therefore you don't buck and you drop your revs.  

You can't stall an automatic.  The torque converter won't let you.  That's why you don't stall when you let off the gas at a stoplight.  What do you think that 432(1) at the end of some cars is?  Hell, in Mercedes cars, there's a button to start in second gear, which essentially locks you out of first gear (even in manual mode, you need to move the shift lever again to bring it back to first).

[footoflead]

Believe me, it is possible to stall an automatic, did it when i was about 8 or so backing slowly out of a ditch  

[Tom]

Believe me, it is possible to stall an automatic, did it when i was about 8 or so backing slowly out of a ditch

Probably caused by something else.

You drove at 8 years old?  My brother's 8, and he doesn't want to drive despite me offering instruction  

[Tom]

The car's bucking in 1st gear, right?  
It will buck if I let off the gas after accelerating hard, but gradually slowing down, there's no problem
See, what happens when you rev it higher in manual mode and let off the gas, the car jerks, and the trans copes with the movements and the changes in input.  Some manual cars do this as well.  What happens in "D" is that the transmission reads that you're off the gas, and then automatically upshifts, therefore you don't buck and you drop your revs.  
Are you sure about the upshifting?  I've watched the tach and Mark said it downshifts to first when there is no throttle input...
You can't stall an automatic.  The torque converter won't let you.  That's why you don't stall when you let off the gas at a stoplight.  What do you think that 432(1) at the end of some cars is?  Hell, in Mercedes cars, there's a button to start in second gear, which essentially locks you out of first gear (even in manual mode, you need to move the shift lever again to bring it back to first).

..

[Run Away]

Yeah it's possible to stall an auto.
Find a snowy parking lot and find a way to get it whipping around in circles nice and fast, and when you come to a stop the thing will normally have stalled.
Done it many times in my van, and also in a few friend's cars.

Auto trannies are essentially connect the wheels to the engine with fluid. That's why they're not as effecient as manuals, and also smoother.
AUto trannies only shift into 1st when you come to a stop.

[93JC]

Yeah it's possible to stall an auto.
Find a snowy parking lot and find a way to get it whipping around in circles nice and fast, and when you come to a stop the thing will normally have stalled.
Done it many times in my van, and also in a few friend's cars.

Yes it's quite possible to stall an automatic. In the case of what Ben here's describing, the entire drivetrain is getting FUBARed by the sudden changes in direction from doing donuts fast.

Auto trannies only shift into 1st when you come to a stop.

Some. Many (the good ones) will shift down as you slow (with your foot of the accelerator). My Dodge's Ultradrive will shift down to 2nd around 45km/h, and into first between 5-10 km/h. In city (i.e. regular 50km/h roads) traffic I can often coast down to 25km/h before applying the brakes.

[Raza]

The car's bucking in 1st gear, right? 
It will buck if I let off the gas after accelerating hard, but gradually slowing down, there's no problem
See, what happens when you rev it higher in manual mode and let off the gas, the car jerks, and the trans copes with the movements and the changes in input.  Some manual cars do this as well.  What happens in "D" is that the transmission reads that you're off the gas, and then automatically upshifts, therefore you don't buck and you drop your revs. 
Are you sure about the upshifting?  I've watched the tach and Mark said it downshifts to first when there is no throttle input...
You can't stall an automatic.  The torque converter won't let you.  That's why you don't stall when you let off the gas at a stoplight.  What do you think that 432(1) at the end of some cars is?  Hell, in Mercedes cars, there's a button to start in second gear, which essentially locks you out of first gear (even in manual mode, you need to move the shift lever again to bring it back to first).

..

Mark was talking about coming to a stop.  Automatics are programmed to upshift ASAP, so when accelerating and then pulling off the gas completely or mostly, the trans will move up as far as it can, only coming back down if you floor it.  I hate that.  If you've a car without a flexible powerband, you constantly have to slam the gas to even gain a little speed.  That's why I manually control my goddamned automatic.  I can proactively downshift instead of reactively, like a regular automatic does.  

[Tom]

The car's bucking in 1st gear, right??
It will buck if I let off the gas after accelerating hard, but gradually slowing down, there's no problem
See, what happens when you rev it higher in manual mode and let off the gas, the car jerks, and the trans copes with the movements and the changes in input.? Some manual cars do this as well.? What happens in "D" is that the transmission reads that you're off the gas, and then automatically upshifts, therefore you don't buck and you drop your revs.?
Are you sure about the upshifting?? I've watched the tach and Mark said it downshifts to first when there is no throttle input...
You can't stall an automatic.? The torque converter won't let you.? That's why you don't stall when you let off the gas at a stoplight.? What do you think that 432(1) at the end of some cars is?? Hell, in Mercedes cars, there's a button to start in second gear, which essentially locks you out of first gear (even in manual mode, you need to move the shift lever again to bring it back to first).

..

Mark was talking about coming to a stop.  Automatics are programmed to upshift ASAP, so when accelerating and then pulling off the gas completely or mostly, the trans will move up as far as it can, only coming back down if you floor it.  I hate that.  If you've a car without a flexible powerband, you constantly have to slam the gas to even gain a little speed.  That's why I manually control my goddamned automatic.  I can proactively downshift instead of reactively, like a regular automatic does.

Oh, ok.  And I agree;  low powered economy cars are a bitch to drive with an auto.  

[Laconian]

If you've just accelerated, you're probably outside the AT's threshold for that gear (too high! mpg suffering!!) and so what you are feeling might just be the upshift.

My car bucks during powered (>3000rpm) upshifts, so...

[footoflead]

Believe me, it is possible to stall an automatic, did it when i was about 8 or so backing slowly out of a ditch

Probably caused by something else.

You drove at 8 years old?  My brother's 8, and he doesn't want to drive despite me offering instruction

Well, yes, living in the middle of nowhere with ten acres of land all around and barely anything to hit, and i learned stick at 11 on our 97 jeep wrangler         , and i'm not even joking about that