OT: why is it that you can't push start an auto....

[ryan roopnarine]

i've been thinking about this because it doesn't appear to me that the rotational speed of the motor would be that different from a manual if you worked quickly enough. i'm thinking along the lines of: shift into drive, push the vehicle for a decent distance, or roll down a significant hill, then quickly shift into neutral and attempt to start the car. can anyone explain why this wouldn't work?

[jofusfarr]

I'm not sure but I bet it's due to the torque converter not functioning in that "backwards" manner.

[pingu]

I don't think that's the reason: when you go down a long hill (e.g. Swiss Alps), you force (by selecting 2, 3 or 4 instead of Drive) the auto into a lower gear so that you can use engine braking rather than overheating your brake pads. It therefore seems to me that the autobox (and torque convertor) is capable of sending torque from the wheels to the engine.

I suspect that the reason you can't bump start an auto is that the internal hydraulics (clutch packs etc) of the auto cannot operate without sufficient pressure from the auto's oil pump. Since the auto's oil pump is powered by the engine crankshaft, it seems to me that there's a Catch 22 - without the engine running, you can't get the autobox to engage a gear. But this is just my take on things and I'd be happy to be blown out by someone who knows better.

P.S. Ryan: good question! I hadn't really thought about it until you posted, although it always struck me as a bit odd.

[E34-520iSE]

[Quote: Pingu] Since the auto's oil pump is powered by the engine crankshaft, it seems to me that there's a Catch 22 - without the engine running, you can't get the autobox to engage a gear[/quote]

Yep, that is correct. The engine must be turning for the torque converter to work and the oil pump to spin.

Shaun M

[Bill R.]

could push start them because they had a rear oil pump in the transmission that would turn when the driveshaft turned. which would then supply oil pressure to the torque converter which would then spin the engine. Power glides were this way and the old buick hydramatics had a rear pump also.
Many times i had to have someone push me in my 65 chevy biscayne with a powerglide to get it started. The drawback was it took around 15to 20 mph to get it to go.

i've been thinking about this because it doesn't appear to me that the rotational speed of the motor would be that different from a manual if you worked quickly enough. i'm thinking along the lines of: shift into drive, push the vehicle for a decent distance, or roll down a significant hill, then quickly shift into neutral and attempt to start the car. can anyone explain why this wouldn't work?

[ryan roopnarine]

i forgot that band application depended on hydraulic pressure. thanks. i've been thinking about this since i put my transmission back into my car. i didn't screw the torque converter on enough during the first attempt, so on the second go around, after spinning the converter copiously, i put the transmission into drive and spun the output shaft of the transmission to make sure that the torque converter spun along with it. i figured that if even residual "spin" from the back wheels made its way through the diff, driveshaft, and up to the torque converter/flywheel, it would be enough to get the motor to "chug" over. i didn't think about how the spin of the driveshaft might not make it up front in a "linear" modelled fashion

[Ross]

Having a nostalgia attack here
I can still hear the sound of those old slip and slide powerglides.

[whiskychaser]

Had this file on my desktop for a while. OK its from Toyota but it makes a great job of explaining how it works:

http://www.autoshop101.com/forms/AT02.pdf

[SnakeyesTx]

Torque converter stall + slip I'd imagine. Basically you couldn't get it to spin up fast enough to lock up in time to push start.

[Zeuk in Oz]

could push start them because they had a rear oil pump in the transmission that would turn when the driveshaft turned. which would then supply oil pressure to the torque converter which would then spin the engine. Power glides were this way and the old buick hydramatics had a rear pump also.
Many times i had to have someone push me in my 65 chevy biscayne with a powerglide to get it started. The drawback was it took around 15to 20 mph to get it to go.

The Borg Warner 35 box in Volvo 164s were also able to be push started.
Guess how I know.........