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?
The obvious answer (to a non-driver, ie me) would be because an automatic doesn’t have a clutch?
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.
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?
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.
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.
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.
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.
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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.
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.
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.
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.
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.
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.
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.
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.
…and that’s true whether it’s a stick or an automatic, I should hasten to add!
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…
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.