The Straight Dope

Go Back   Straight Dope Message Board > Main > General Questions

Reply
 
Thread Tools Display Modes
  #1  
Old 12-27-2002, 10:28 AM
Enola Straight Enola Straight is offline
Guest
 
Join Date: Jan 2001
Bump-starting an automatic

In the course of my duties as a Valet, I sometimes have to give jump-starts to cars with dead batteries.

When a stick-shift goes dead, all I have to do is get it rolling downhill and pop the clutch in first.

How come I can't do that with an automatic?
Reply With Quote
Advertisements  
  #2  
Old 12-27-2002, 10:42 AM
Harli Harli is offline
Guest
 
Join Date: Dec 2002
The obvious answer (to a non-driver, ie me) would be because an automatic doesn't have a clutch?

Reply With Quote
  #3  
Old 12-27-2002, 10:47 AM
starfish starfish is offline
Guest
 
Join Date: Oct 1999
With a manual transmission, you get the car rolling, the wheels get the driveshaft rotating, when you pop the clutch, it connects the rotating driveshaft to the engine crankshaft and gets it rotating, the same as the starter does.

With an automatic, there is no direct connection between the driveshaft and the crankshaft. Here you rely on the fluid couple in the torque converter. This "soft" connection makes it hard to turn the cranckshaft. You can still do it, it just requires much higher speed.

Realize that you don't need to turn the engine fast. You just need to turn it hard enough to compress the air in the engine's cylinders. But you're trying to do it by spinning fluid in the torque converter, not by spinning the cranckshaft directly.
Reply With Quote
  #4  
Old 12-27-2002, 10:53 AM
Edward The Head Edward The Head is offline
Charter Member
 
Join Date: Jun 2000
Location: Somewhere in time
Posts: 5,797
Quote:
Originally posted by starfish
You can still do it, it just requires much higher speed.

What kind of speed are we talking about and how do you do it? I'm guessing you put it into N and then drop it into Drive?
Reply With Quote
  #5  
Old 12-27-2002, 11:15 AM
SandWriter SandWriter is offline
Guest
 
Join Date: Oct 2000
IANAAM, you can't push start an automatic for the reasons starfish mentioned. Your best bet would be to jump start it with the battery from a manual car that you could push start.
Reply With Quote
  #6  
Old 12-27-2002, 11:21 AM
Ringo Ringo is offline
Charter Member
 
Join Date: Apr 1999
Location: Houston, TX, USA
Posts: 11,261
You can push start an automatic tranny, as starfish said. It requires another car to push you, as you need to get up to 25 or so mph.
Reply With Quote
  #7  
Old 12-27-2002, 11:25 AM
Sofa King Sofa King is offline
Guest
 
Join Date: Sep 1999
Quote:
Originally posted by starfish
You can still do it, it just requires much higher speed.
Woah, man! I'm not so sure about that one. I remember tearing up a friend's little Toyota pretty bad by trying to do just that. Dropping it into D from N was just like throwing the poor thing into park. We pushed in the back bumper pretty good, and I'm sure the transmission fared none too well.

I suspect that since most modern automatic transmissions are electronically controlled, some of them won't start at all without some initial charge in the battery. You'll need a real mechanic to confirm and explain that, though.
Reply With Quote
  #8  
Old 12-27-2002, 11:34 AM
Ringo Ringo is offline
Charter Member
 
Join Date: Apr 1999
Location: Houston, TX, USA
Posts: 11,261
Start with the tranny already in low gear. I've done this without problems.

Admittedly, I haven't done it for a few years, so I don't know what might prevent you from succeeding with a 2002 model.
Reply With Quote
  #9  
Old 12-27-2002, 11:39 AM
DougC DougC is offline
Guest
 
Join Date: Mar 1999
- - - The two times I've seen it tried, it didn't work at all. That could have been for any of multiple reasons, but the one that stands out in my mind is that the blades of a torque convertor are airfoil-shaped, and designed so that the blades on the centrifugal pump push fluid into the radial turbine on the transmission-side, which "windmills" in the flow of fluid. This is not a two-way connection, you don't get anywhere near the normal mechanical transmission by using it the wrong way.
-
http://www.howstuffworks.com/torque-converter.htm
~
Reply With Quote
  #10  
Old 12-27-2002, 11:44 AM
Mr. Slant Mr.  Slant is offline
Guest
 
Join Date: Nov 2001
Don't do this with a diesel. Not even if it's a stick. The compression ratio runs too high in a diesel. I had my old Mercedes 240D going downhill at 25 MPH on gravel. It didn't want to start, so when I dropped the clutch the engine turned over maybe ONCE while the car skidded to a halt and a bolt holding my best friend's passenger seat in sheared.
Reply With Quote
  #11  
Old 12-27-2002, 11:47 AM
Ethilrist Ethilrist is offline
Guest
 
Join Date: Nov 2000
If you must push-start an automatic, you can avoid the manufactured fender-bender by having the push vehicle push it up to about 30 mph, then stopping, letting the pushed vehicle travel a little ways before dropping into gear.
Reply With Quote
  #12  
Old 12-27-2002, 01:19 PM
RAWDuke RAWDuke is offline
Guest
 
Join Date: Jul 2002
The reason you can't push start an automatic is that you need oil pressure in the transmission to engage the clutches.
Before about 1964, manufacturers put rear oil pumps in trannys (driven by the rotating drive shaft). These cars could be push started, because the movement of the car turned the pump and created enough pressure to engage the clutches and start the engine.
After 1964 they stopped putting the rear oil pump in the tranny.
__________________
I will take the blame, but I won't take the responsibility
Reply With Quote
  #13  
Old 12-28-2002, 11:46 PM
hammerbach hammerbach is offline
Guest
 
Join Date: Mar 2002
Now then...

I have tried this experiment at various speeds ranging from 30 to 75 MPH in a '70 Cutlass Supreme, an '87 Volvo 240 DL, an '87 Volvo 760, and a '95 Chevy Van:

Start Vehicle (note no battery trouble).
Get up to speed.
Turn off ignition.
Turn on ignition.
Put tranny in neutral and re-start.


I'm not saying it's impossible for ANY automatic out there, but none that I have tried worked.
Reply With Quote
  #14  
Old 12-29-2002, 01:32 AM
Rebel307 Rebel307 is offline
BANNED
 
Join Date: Dec 2002
Posts: 15
Even a stick on a front wheel car can take come serious damage from push starting. The srive linkeges in a front wheel drive are just strong enough for it to operate under its own power. In my opinion a front wheel drive car is infurior to a rear wheel drive. and a car can start even without a battery on it. The alternator makes the power once the engine turns and takes the battery out of the loop.
Reply With Quote
  #15  
Old 12-29-2002, 09:23 AM
hammerbach hammerbach is offline
Guest
 
Join Date: Mar 2002
Quote:
Originally posted by Rebel307
Even a stick on a front wheel car can take come serious damage from push starting. The srive linkeges in a front wheel drive are just strong enough for it to operate under its own power. ...
The drive linkages (CV joints, etc) can take it. Otherwise you would risk breaking them every time you downshift or take your foot off the gas. Of course, it's not especially good for your clutch, but that's another question.

As to push starting a diesel, I did it just a few weeks ago. They do have a higher compression ratio, and if you don't get going fast enough are likely to stop the wheels turning and jerk you to a stop. Gasoline engines can also do this, but the momentum required to turn the engine is lower.

As to the battery being "out of the loop", well... that's another thread.

And the front/rear wheel drive issue would be an IMHO or possibly even a Great Debate.

I encourage all to try experimenting (but please do so under safe conditions). Sometimes it's the only way to really know.
Reply With Quote
  #16  
Old 12-29-2002, 09:27 AM
kanicbird kanicbird is online now
Guest
 
Join Date: May 1999
Quote:
Originally posted by starfish
You can still do it, it just requires much higher speed.
My father did this years ago. It took 50mph on the parkway but it did work. I don't think new cars can be push started though as the design has changed.
Reply With Quote
  #17  
Old 12-29-2002, 09:47 AM
Early Out Early Out is offline
Guest
 
Join Date: Sep 2002
Another hazard: when you bump-start (a.k.a. push-start) a car, it ends up putting some unburnt fuel into the exhaust system, which will damage or destroy the catalytic converter. That's a pretty pricey repair.
Reply With Quote
  #18  
Old 12-29-2002, 09:56 AM
Early Out Early Out is offline
Guest
 
Join Date: Sep 2002
...and that's true whether it's a stick or an automatic, I should hasten to add!
Reply With Quote
  #19  
Old 12-29-2002, 12:53 PM
hammerbach hammerbach is offline
Guest
 
Join Date: Mar 2002
Does that destruction happen immediately, or such that you would have to do this a lot over time?

I'd have to think it would take a lot, but catalytic converters do not fall into my area of expertise...
Reply With Quote
  #20  
Old 12-29-2002, 01:51 PM
kanicbird kanicbird is online now
Guest
 
Join Date: May 1999
Wait a sec. How does turning the engine via pushing dammage the CC by unburnt fuel passing through but turning the engine via a starter motor doesn't.

Iv'e heard that push starting an automatic, or for that matter towing on the drive wheels could damage the trany because the tranny fluid pump is driven on the engine side, not on the axil side. So turning the axile side will not allow fluld to circulate.
Reply With Quote
  #21  
Old 12-29-2002, 02:26 PM
sailor sailor is offline
Guest
 
Join Date: Mar 2000
Quote:
Originally posted by Jonathan Woodall
Don't do this with a diesel. Not even if it's a stick. The compression ratio runs too high in a diesel. I had my old Mercedes 240D going downhill at 25 MPH on gravel. It didn't want to start, so when I dropped the clutch the engine turned over maybe ONCE while the car skidded to a halt and a bolt holding my best friend's passenger seat in sheared.
You can start a diesel just like you can start a gasoline motor. In any case, if you let go the clutch and the motor jerks the car to a stop then that means you are in too low a gear and need to use a higher gear.
Reply With Quote
  #22  
Old 12-29-2002, 02:35 PM
sailor sailor is offline
Guest
 
Join Date: Mar 2000
BTW, if you are going to push a car with another car the technique is to push until the car has enough speed, then let it go ahead and only then release the clutch. never release the clucth while the car is still being pushed.
Reply With Quote
  #23  
Old 12-29-2002, 03:10 PM
racer72 racer72 is offline
Charter Member
 
Join Date: Mar 2002
Location: Covington, WA
Posts: 5,388
There are some modern automatics that can be pushed started but it is not recommended. Automatics rely on the pump to push fluid throughout the transmission. Beside providing hydraulic pressure for the torque converter and the transmission clutches, the fluid also acts as a lubricant. When attempting to push start an automatic, the clutch pack will spin with the drive line but is not getting any lubrication. That is also the reason cars with automatic transmissions should not be towed.

When push starting a manual, have the transmission in 3rd or 4th gear. Push starting a car in first gear can spin the engine over too fast and cause some serious damage.
Reply With Quote
  #24  
Old 12-29-2002, 04:33 PM
Early Out Early Out is offline
Guest
 
Join Date: Sep 2002
Quote:
Originally posted by k2dave
Wait a sec. How does turning the engine via pushing dammage the CC by unburnt fuel passing through but turning the engine via a starter motor doesn't.
According to this site and others, the problem occurs because, when you're push-starting a car, the engine is turning over, and pumping fuel, but not firing. This is why unburnt fuel gets into the exhaust. Apparently, according to another site I visited (but can't seem to find again!) doing one quick push-start probably won't get you in trouble, but repeated attempts is a prescription for trouble. Other sites, however, tell you never to do it. You pays your money, you takes your choice.

Quote:
Originally posted by hammerbach
Does that destruction happen immediately, or such that you would have to do this a lot over time?
Can't lay my hands on the cite at the moment, but what happens is that the fuel in the cat converter results in overheating that will, over time, destroy it. In extreme cases, however, it can melt the core of the converter completely, blocking the exhaust - at that point, the car won't run at all.
Reply With Quote
  #25  
Old 12-29-2002, 04:43 PM
kanicbird kanicbird is online now
Guest
 
Join Date: May 1999
OK I can buy the unburnt fuel argument if the battery is dead as any power the alternator produces will be sucked up by the battery and not have enough power to fire the plugs. But if you are push starting due to a broken starter I can't see this damage happening.
Reply With Quote
  #26  
Old 12-29-2002, 04:59 PM
Early Out Early Out is offline
Guest
 
Join Date: Sep 2002
Quote:
Originally posted by k2dave
But if you are push starting due to a broken starter I can't see this damage happening.
Yeah, that's basically what one site said on the subject: if the problem is, indeed, just a bad starter, you can get away with push-starting the car, since it will, in fact, start firing pretty quickly. But if the car still doesn't start, your problem probably isn't just a bad starter, so trying it again and again might lead to heartache!
Reply With Quote
  #27  
Old 12-29-2002, 05:25 PM
Doc Nickel Doc Nickel is offline
Guest
 
Join Date: Feb 2001
No offense guys, but I seriously disbelieve that you managed to push-start an automatic-equipped car.

The clutch packs in an auto are not only lubed by, but engaged via the hydraulic fluid. The "at rest" position for all automatic transmissions is disenaged- meaning that the pressurized fluid engages the clutches, not disengages them.

I cannot imagine being able to push the car fast enough that the driveshaft will turn the clutchpacks fast enough that the sheer viscousness of the tranny fluid will turn the rest of the tranny fast enough that it will in turn drive the torque converter, which is itself an indirect coupling that must have hydraulic pressure and fluid flow in order to even move.

There is no mechancial coupling at all between the input and output of the torque converter. It relies on turbine-like vanes on one half driving fluid against the vanes on the other half (internally) in order to transfer power. Most converters will simply "freewheel" at anything under about 700 rpm, and have varying degrees of "slippage" (less than direct coupling) on up to anywhere from 1,800 rpm to 4,000 rpm depending on application.

So the driveshaft will have to spin fast enough to turn the clutches by mere viscous drag, and those in turn will have to spin fast enough to get the converter to the point it begins to transfer power itself... and then these two indirect couplings will have to transfer enough power to turn the engine over.

And, since there's no active lubrication of the tailshaft, this all has to happen before the driveshaft destroys the unlubed bronze sleeve bushing that supports the output shaft.

Sorry- I want to see reliable cites about push-starting an auto equipped car, and not anecdotes. I'm all but certain it can't be done.
Reply With Quote
  #28  
Old 12-29-2002, 08:36 PM
AHunter3 AHunter3 is offline
Charter Member
 
Join Date: Mar 1999
Location: NY (Manhattan) NY USA
Posts: 16,310
My experience meshes with that of people who say you can't do it in anything remotely modern that has an automatic tranny.

Here's how you start an automatic transmission-equipped car in the absence of a battery charger (a method approximately as safe as pushing one car with another):
  • Remove battery from functioning car (e.g., the same hypothetical car you were gonna use to push-start the dead car with).

    Remove battery from dead car and replace with battery from functioning car. (If car A has top terminals and car B has side terminals, see Appendix A, "Choice Pit-worthy Cuss Words")

    Start car, which is presumably no longer dead.

    Without turning off engine, remove battery from car. (Yes, engine keeps running because alternator makes electricity. If battery is dead due to alternator being dead, see Appendix A again). Hold live end of positive battery cable CAREFULLY in such a way that it does not contact any part of automobile body. If you have something akin to a rubber boot to stick it in, so much the better.

    Place dead battery in battery cradle. Rehook battery cables, making sure to hook them to the correct posts. Positive cable first. At the precise moment of making contact between negative cable and battery, turn head away from battery as eyeball insurance against rare but not quite dismissable possibility of bad things happening. Tighten clamps or cable bolts.

    Return other battery to car from whence it came.
__________________
Disable Similes in this Post
Reply With Quote
  #29  
Old 12-29-2002, 08:48 PM
Rebel307 Rebel307 is offline
BANNED
 
Join Date: Dec 2002
Posts: 15
If you cant git a fast enough speed to successfully turn the engine, try useing a higher gear it puts the wheels at a bigger advantage. And as far as screwing up your catalytic converter(s) go on I will give you 20$ each for them as long as the pellets are still in them,,,(heh heh heh a guy I once knew took a truck load of them to the right place and made 17,000$ something about platinum I think). Your altenator,, puts out 12.5V DC sometimes more and the battery isnt gonna suck enough to hamper starting the car. Push starting an auto,, I drove dirt cars for 4 years and I would git mad the car would die and then the little old man on the tractor would come out and push you very fast to git you off the tiny 1/4 mile track and I tried several times to break away from him by enguaging the car in drive hoping it would start. The tractors they used would probally do 40 and they would run them wide open. Besides if your into racing you know that the alternator it riged with a switch so you can cut it from kicking in while racing it robs power. I have wrecked and had the battery pulled out of its box I just killed the power from the battery to the electrical and ran with only an alternator and vice versa. after a night of racing many times the battery would be toasted completly and not have anything left but a good push when I ran a 4-speed always did the trick.
Reply With Quote
  #30  
Old 12-29-2002, 08:54 PM
kanicbird kanicbird is online now
Guest
 
Join Date: May 1999
In a modern car you very well could fry the electronics by removing the battery and ru solely off the alternator.

The last car I ran off the altnator soley was a '80 dodge omni 024
Reply With Quote
  #31  
Old 12-29-2002, 09:06 PM
sailor sailor is offline
Guest
 
Join Date: Mar 2000
AHunter3, in a car equipped with an alternator if you remove the battery with the alternator running, you'll most likely fry the alternator regulator diodes. Don't do it.
Reply With Quote
  #32  
Old 12-29-2002, 09:33 PM
stockton stockton is offline
Charter Member
 
Join Date: Mar 2002
Location: Atlanta, GA USA
Posts: 930
Quote:
Originally posted by k2dave
In a modern car you very well could fry the electronics by removing the battery and ru solely off the alternator.

The last car I ran off the altnator soley was a '80 dodge omni 024

Flashback! Wow, when had one it was a Plymouth Horizon, but you still have my sympathies.

Rebel307, GIT?
Reply With Quote
  #33  
Old 12-29-2002, 10:25 PM
Rebel307 Rebel307 is offline
BANNED
 
Join Date: Dec 2002
Posts: 15
git= get=got dont blame me my ex g/f was from texas
Reply With Quote
  #34  
Old 12-29-2002, 10:38 PM
Rebel307 Rebel307 is offline
BANNED
 
Join Date: Dec 2002
Posts: 15
oh and I am sorry to mention that I had forgot that they stopped making cars in 1984. I was reading on howstuffworks.com that some of todays cars have large 5.0 (302-305) engines ,large?? Man if a 5 litre is the best I can do I shouldnt even leave home. 350 (5.8) is the absolute smallist engine I will put in any of my cars and yes I build them myself,, (want pictures?) My truck has a 400 modified (6.6) which has been punched out .040 so it may be a few CI bigger but it push starts with no trouble at all. got a .625 cam with 11-1 compression (gitting close to the diesel ratio) but its not exactly the compression ratio that counts here its the cylinder pressure in that case its close very close to a diesel. Cause of the long duration cam with its high lift it gits a big gulp of feul/air mix through the 850 double pumper holly. and its low geared at that so the engine is at an advantage. I am sure you dont want me to explain the mechanics that gear ratios play when push starting. Basically the higher the gear the less of an advantage the engine has while under its own power ( this is why we have trannys). With the low 3.37-1 gears I run the engine is almost always at an advantage under its own power. So if I push start I use 3rd gear or even 4th but the engin turns it will lock the wheels in 2nd or 1st which isnt used for anything other then pulling 3,000 pounds or better. And to git a better picture the truck weighs about 5,000 pounds at any time ( weighed it) unless its loaded. So wheel adheasion would seem to have a better chance as well. But still in 2nd and 1st they just lock down.
Reply With Quote
Reply



Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 07:14 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.

Send questions for Cecil Adams to: cecil@chicagoreader.com

Send comments about this website to: webmaster@straightdope.com

Terms of Use / Privacy Policy

Advertise on the Straight Dope!
(Your direct line to thousands of the smartest, hippest people on the planet, plus a few total dipsticks.)

Publishers - interested in subscribing to the Straight Dope?
Write to: sdsubscriptions@chicagoreader.com.

Copyright 2013 Sun-Times Media, LLC.