Can an automatic transmission be bump start an engine

[Christ]

Originally Posted by saand

Thanks all for your replies, looks like a resounding "no that wont work" however it seems like from the responses it wont kill my transmission if i try it out once.

I had also expected the drive wheels to not be able to back power the engine therefore making bump starting impossible however some time ago i once tried turning off the engine while traveling at a good speed and the engine kept rotating, i turned the key back into the on position and it fired up again. So it makes me wonder.
I think ill have to try this out just once to see if its possible now that i know it wont kill my transmission straight off.
I also like the idea of a pump powered off the output shaft, may be a bit too much effort for little gain though, will have to think about this if my bump starting attempt doesn't work

Shadetree regarding the manumatic conversion, i have written up the method in the wiki have a look here (even has some pretty pictures)
Manumatic - EcoModder

regarding the age old auto transmission lubrication debate, im not getting into this one but i will say I am happy to risk my transmission to go up in smoke over time as long as i have fun doing it. My car is on the way out anyway so im really at the point of seeing what crazy stuff i can do even if there is a bit of a risk of damage.

A couple things could have happened, regarding the engine starting back up, but I can't say for sure without more information.

1. Vehicle speed = engine speed. If you're going fast enough, and only turned the key off for a second, it may not have stopped rotating completely before you turned the key back on, and thus would fire back up.

2. Locked torque converter. On some cars, if the converter is locked, and you shut the engine off, it doesn't default to unlocked, it just stays that way. This would cause the engine to continue to spin, because w/ the torque converter locked, the transmission can drive the engine. The clutch in the torque converter, however, is not strong enough to instantly transfer transmission load into the engine to start it, so using that method is strongly advised against.

For #2 to happen, a series of events would have to be fulfilled:

1. You can't let off the gas, as this signals the torque converter to unlock (or touch brakes, in some cases).

2. You turned the key off, but not back on, as this would reset the TCM and cause an unlock command to be sent.

3. You were traveling in top gear, or any gear in which the torque converter could lock.

[Frank Lee]

Engine off coasting will damage the trans as there are some spinning parts in there w/o lube even though it's in N. It needs to have the oil pump pumping and for that the input shaft has to turn.

Of course the manual boxes are designed to get splash lube no matter what.

As a flatlander, the opportunities for coasting with engine off aren't there and P&G sux. I can see wanting to do extended coasting engine off in the hills like in California. I did do an engine off coast down a mountain in the Sable then I remembered D'Oh! DON'T DO THAT AGAIN!

[saand]

christ think you might be right there, the car was going fast enough to have the TC locked up although when i tried it i had only recently started up the car (which might suggest the TC wouldn't have had time to lock up yet as the engine wasn't warm). So im not sure. Everything and everyone is telling me this wont work but now that there is something that doesn't make sense to me i shall have to try it out and see what happens.
Ill let you all know how it goes.

[ShadeTreeMech]

when you shut the engine off with the car in gear, the driveshaft kept the fluid pumping which kept the clutch packs engaged which drove power to the TC so whne you turned the key back to on, the engine "restarted" --it never stopped spinning though. I've done this before on a 90 tempo which had a 3 speed auto and I don't think it had lockup. Same thing with a Plymouth Reliant I had from circa 1983.

[Don76]

The automatic trans needs fluid pressure to make the torque connection; without it you're always in 'neutral'. It is true that older cars often had 'secondary' pumps at the driveshaft (my 91 Mercedes had one).

An AT can go a little while without lubrication - just as an engine can run for a little while without any oil - but it doesn't do it any good. Once you pass a certain point, it gets ruin-your-week bad. So, if the car is moving, the engine should be on.

Manual transmissions are lubricated differently - it's OK to have the engine off.

[saand]

I tried it out today and yes as everyone expected it didn't restart when i put the car into neutral, turned engine turned off, turned key back to on then shifted back into drive.

Shadetree your explanation would explain why my car would still keep the engine rotating when i turn it off and the TC (torque converter) isn't locked. Thanks for explaining that, would have been wondering about that for a while.

Looks like my crazy idea dies here, (unless i get a pump).

Thanks all that posted for your help

[Sularus]

Thanks for posting the results. I was wondering last night if I should give this a shot. Good luck with your future projects.

[Christ]

Originally Posted by saand

I tried it out today and yes as everyone expected it didn't restart when i put the car into neutral, turned engine turned off, turned key back to on then shifted back into drive.

Shadetree your explanation would explain why my car would still keep the engine rotating when i turn it off and the TC (torque converter) isn't locked. Thanks for explaining that, would have been wondering about that for a while.

Looks like my crazy idea dies here, (unless i get a pump).

Thanks all that posted for your help

If you're considering flat towing, or extended coasting periods with the engine off in neutral, you can alleviate worry of transmission damage by using what amounts to a fish tank pump to keep the fluid flowing in the transmission while the engine is off.

You plumb it either inline w/ the transmission cooler, or tee off the lines and make a loop from input to output.