Can an automatic transmission be bump start an engine - Fuel Economy, Hypermiling, EcoModding News and Forum

saand · 10-03-2010, 11:16 AM

I have been thinking about the concept of bump starting and its annoying that i cant do it in my automatic because i dont have control of the clutch. I have searched through some threads and not seen anyone actually bump starting an auto. I have recently converted my automatic to a manumatic which gives me control of the torque converter, and gear selection.

Now i am wondering am i now able to bump start. My idea is as follows
- if i am driving along and i go into neutral, my car is going say 60kph (which is top gear).
- i turn my engine off while moving at 60 kph in neutral, i then turn it back to the on position so that the engine is ready to fire but isn't actually turning.
- the car slows a bit and i want to speed it back up but i don't want to use my starter because i don't want to wear it out and i really don't want to get into a situation where my car wont start when im on the road.
- So i unlock the torque converter and shift the car into drive again at an appropriate gear. Hoping that the engine will get energy from the wheels and start back up.

I haven't tried this yet but before i potentially kill my transmission i thought id ask the group.
So is this a crazy idea, will the torque converter work in reverse, will this kill my transmission?

What does everyone think.

Angmaar · 10-03-2010, 11:39 AM

I think it will kill your transmission.

user removed · 10-03-2010, 12:34 PM

+1

regards
Mech

Christ · 10-03-2010, 01:13 PM

Ok, it won't kill your transmission, because you have no way to introduce torque from the wheels back into the transmission. Unless you have a fluid pump at the rear of your transmission, which you probably don't, it won't start your engine back up, either.

Essentially, unless you're pumping fluid, the transmission won't go back into gear at all, so you'll remain in neutral, until you start the engine back up and introduce fluid sufficient to fill the passages under pressure and engage the Drive gearset.

While it's not going to do anything for you, it's also not going to harm your transmission any more than coasting engine off in neutral actually would.

IOW - don't do it.

Sularus · 10-03-2010, 01:50 PM

actually it could harm the transmission. The transmission needs constant fluid to maintain temperature and lubrication. If the engine is not spinning the torque converter and then the pump, there is no fresh fluid being pumped through the tranny. Over time, this could result in the tranny being overtemped in certain areas, reducing the life of clutch packs and other parts.

It is advisable to either coast in nuetral or coast in gear. Do not shut the engine off while coasting in an automatic transmission car.

Christ · 10-03-2010, 02:03 PM

Quote:

Originally Posted by Sularus

actually it could harm the transmission. The transmission needs constant fluid to maintain temperature and lubrication. If the engine is not spinning the torque converter and then the pump, there is no fresh fluid being pumped through the tranny. Over time, this could result in the tranny being overtemped in certain areas, reducing the life of clutch packs and other parts.

It is advisable to either coast in nuetral or coast in gear. Do not shut the engine off while coasting in an automatic transmission car.

You understand, of course, that some automatic transmissions are actually designed to use in neutral with the engine off?

Several of them also have a "recommended" towing distance that the vehicle can be used in neutral without the engine running before damage becomes an issue.

Most people aren't going to coast for miles in or out of gear with the engine off, and the chances of the transmission overheating in the few hundred feet (up to a mile) that the average driver would coast is almost nil. I've been EOC with automatic transmissions for awhile now, and have yet to damage one, including the Dodge Caravan in which you can actually set the TCM to display clutch wear and recalibrate the transmission controller based on clutch pack use.

Frequent clutch displays showed no abnormal wear from EOC in that particular vehicle, which isn't rated for flat towing by the MFR.

Sularus · 10-03-2010, 02:11 PM

a few hundred feet at a time adds up. I wouldn't do it, Automatic transmissions are too costly to replace and refurbish to risk it. There is no way to save that much money in P&G or engine off coasting to risk any damage to the tranny. Remember, in most areas a automatic transmission replacement will run over 3500 USD.

It is all done based on someone's personal desire for risk vs. reward. I know that a transmission requires the pump to help cool and lubricate a transmission, especially after it is warmed up. I have a automatic and I will throw it into N for my coasts.

ShadeTreeMech · 10-03-2010, 02:35 PM

Look into having a hydraulic pump that runs off the output shaft and it will work. The only thing you're lacking is a pump running ogg the output shaft.

Years ago, older American autos were designed to be bump started; I read some older shop manuals for recommended speeds required to do just that.

Sadly for most of us, automatics are very enigmatic; I'm just now starting to learn about how the internals work. One time I attempted a bump start with a transmission that had a lock up torque convertor. That was a very expensive sounding grinding sound! It didn't hurt the transmission, but the torque convertor wouldn't correctly lock up afterwards. I had to manually engage it to make it work.

Rolling in neutral engine off? Not likely to hurt anything. But attempting to shift into gear may cause residual pressure to attempt to engage the clutches, but there not be enough pressure to completely do the job.

As creative as you are, you may be able to find an electric pump able to give you the start up pressure required. Otherwise a pump running off the output shaft would be the answer. I believe this to be possible, just not easy. And having the torque convertor unlocked would be advisable.

How'd you make the manumatic work? I can see the possiblity of making an auto shift manually, especially when it is electronically controlled, just not sure how to pull it off.

Christ · 10-03-2010, 03:58 PM

Quote:

Originally Posted by Sularus

a few hundred feet at a time adds up. I wouldn't do it, Automatic transmissions are too costly to replace and refurbish to risk it. There is no way to save that much money in P&G or engine off coasting to risk any damage to the tranny. Remember, in most areas a automatic transmission replacement will run over 3500 USD.

It is all done based on someone's personal desire for risk vs. reward. I know that a transmission requires the pump to help cool and lubricate a transmission, especially after it is warmed up. I have a automatic and I will throw it into N for my coasts.

No, the "few hundred feet at a time" do not add up.

The entire argument pleading destruction of the transmission is based entirely on the fluid overheating, losing lubricity, and causing friction damage inside the transmission. The problem is that every time you start the engine, regardless of what gear the transmission is in, even neutral, the fluid gets exchanged at the clutches and all wear parts, which means that the heated fluid has a chance to cool and be circulated, mixing with the other fluid.

Another thing that most people don't realize is that once your torque converter locks up, the transmission is no longer pumping fluid (in most cases) to the cooler at all. It's circulating strictly within the transmission, which means that the parts in the transmission don't even seem to be capable of overheating a large volume of fluid, provided it's exchanged regularly. The overheating of transmission fluid normally occurs when the torque converter slips for an extended period, such as abnormally long take-offs, brake launches, and uphill where alot of torque is needed and the converter is operating at higher speeds.

The worst thing that can happen "a few hundred feet at a time", as you put it, is premature fluid change requirement from the fluid being heated excessively, which still isn't likely to happen in a few seconds of coasting with the engine off.

This argument reminds me of people who honestly believe that if you start an engine after the oil has been drained, that it's going to grenade instantly. In both cases, there comes a point where I just roll my eyes and stop debating the issue.

You can consider this that point.

saand · 10-03-2010, 06:52 PM

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.