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Discussion Starter #1
Hey all,

One of the things I always liked about my old 5-speed Taco was I could never (usually) be stranded by a dead battery. I wanted to know if my '02 automatic tranny was able to do the same push starting, so I took it to a big hill, dropped it in 4LO, 1st gear and went down. Didn't even turn the engine once.

Can someone explain the mechanics of this?? I would have thought that having the trannys output shaft turning would make a "reverse" torque converter effect and spin the engine. But I guess not. Is the AT completely dead unless first powered by the engine?

Thanks,

Beave
 

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Long story short, yes.. it's dead unless the engine is running.. I've heard it can be bumped, but you have to be doing something like 30mph.. I have a stick so I've never tried..

From what I've heard, it's got something to do with it building up enough pressure in the system to enable the auto tranny to shift into drive from neutral.. a few dozen revs from a slow speed "bump" arn't enough, the engine got to be able pump enough times to bring it up to pressure.. with the starter the eninge kicks in and right away is running at least 600 RPMs, and builds up pressure almost immediatly.
 

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Weird. Well that hill was pretty steep and long too. I didn't get any kind of reaction from the engine, and I guess if 1st gear and 4-LO won't do it, nothing will. =(
I better buy some jumper cables!

Thanks,

Beave
 

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Those little $30 self-jumper kits that you can find at places like Target work great. Just hope you don't need it more than once without a charge in between. It will easily tuck into your toolbox too.
 

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An auto tranny is basically a large hydraulic pump that forces pressurized hydraulic fluid against vanes to convert fluid energy to mechanical energy. To bump start the engine you would need to be going at least 30 maybe faster to build up enough hydraulic pressure.
I've seen a car with an auto pull started once, it wasnt pretty, the difference between the vehicle speed and transmition speed made for a few exciting moments. The driver being pulled almost didnt get the tranny in neutral fast enough and almost rear ended the car pulling.
Who says you have to pay for a comedy show in the south.

Tim
 

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The reason you can push start a manual is because as soon as the clutch is engaged (and you are in gear), the engine and tranny are mechanically connected.

An automatic transmission transfers power by essentially "flinging" the tranny fluid in the torque converter. As was stated above, vanes catch the flinging fluid, changing its momentum. The energy is transferred from the fluid to the output shaft. There is really no directly mechanical connection in an auto tranny. At low speeds, the fluid isn't flinging fast enough, and there isn't sufficient transfer of energy.

If you try to drive an auto with little or no fluid, you won't be going anywhere very fast...
 

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Beave said:
Can someone explain the mechanics of this?? I would have thought that having the trannys output shaft turning would make a "reverse" torque converter effect and spin the engine. But I guess not. Is the AT completely dead unless first powered by the engine?


Beave

Not that I know a hole lot about this but have taken my fair share of them apart and put a few back together too. This description isn't very technical nor very specific. Probably not very acurate either. As I am not a rebuilder and do not require the technical knowledge involved in knowing all the internal parts, flows, and drives. Although if desired I could probably get a good write up on why it wouldn't work with specific reasoning. I would be willing to bet there probably is a builder in here somewhere that could explain it better.

In an automatic the torque converter drives the the pump gear and input shaft. Without this pump preasure an auto is useless. From the pump to the stator, threw the case into the valve body. Then the mechanical valves or selenoids direct the flow to the proper drum. Which has a piston at the bottom. This piston compresses a stack of friction clutchs and steel plates to transfer the rotating force from the intput shaft threw the drum to the output shaft.

The drums are stacked with a certain amout of slack so when the piston is not aplied they spin freely. when they become worn enough the piston can't play enough preasure on them or pushes up to far and the fluid just blows by the seals. Hince transmission slipping or not pulling at all. Normally in the latter ( no pull ) something else has failed the planetary gear or a sprag.

Here again the converter allows for a quite a bit of slip even threw the engine driving it until lockup. It has a clutch in it too for lockup. Lockup is controled by the computer now adays and would not come on till certain requirements are met as far as engine speed, load, tps signal and so on.

Here is a little bit of info that might be of use to why it wouldn;t work quoted from http://www.carcarecouncil.org/Auto_Transmission/torque_converter.shtml

Description: The torque converter portion has the ability to multiply torque from the engine. The impeller (sometimes called the pump) has specially curved vanes and is driven by the engine's crankshaft. The turbine also has specially curved vanes and is connected to the input shaft of the transmission. Adding a third element, the stator (also called the reactor), gives the assembly the capability it's named for. The stator has vanes and is mounted on a one-way clutch, to allow it to freewheel in only one direction. The stator assembly is located between the impeller and turbine and redirects oil that bounces back off the turbine. The force of the redirected oil assists in rotating the turbine, resulting in torque multiplication. When the impeller's speed is high and turbine's speed is low, torque can be multiplied by as much as 2:1. When the impeller's speed and the turbine's speed are about the same, torque can be transferred at almost 1:1. Carmakers took the torque converter one step further by adding a lock-up function beginning around 1980. Lock-up converters also contain a friction clutch that locks the converter impeller to the turbine, usually in higher gears. A solenoid-controlled oil passage, commanded by the car?s powertain control module (PCM), locks and unlocks the converter based on driving conditions.
 

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when i hydro locked my last tacoma that was an auto we pulled it down a straight road and got up to around 30 trying to get the engine to turn over,

(that was after we had gotten all the water out of the engine, gas tank, muffler, cab, transmission, differentials and changed the fluids many times) the radio was never dried

after working on it for a week i had to call it quits and turn it in on insurance,
they called it a loss and i was cover since it was hurricane ivans fault

we suspect that the computer was fried, it went under, whichever salavage yard got that truck got a good deal
 

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Can't you bump start a auto if you have a pump mounted in the rear, and not on the torque convertor.
 

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no new cars or trucks with slushboxes can be pulled/pused to gain enough pressure to get the tourque converter to engage properly. Not since the late 60's has this been a viable option. Buy a battery jump pack from Wally world and carry it with you.

Boheefus
 

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Like others have stated, the torque converter drives the fluid pump; if the converter ain't spinning, there is no hydraulic pressure to couple the tranny to the engine through its clutches. Some T-350's in the 1960's had rear drive pumps; accordingly, they could be push/pull started if necessary.
 
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