why can't you push start an automatic?

Amongsy other things, there is not direct connection between the engine and the transmission. The torque converter uses fluid.

In a manual transmission, the clutch provides a direct connection between the engine and the transmission.

The last car with an AT I successfully started by 'jump starting' was a

1964 Chevy Biscayne, and it needed to be going >25 MPH to start. We had to park the car on hills in case the starter didn't work!

The principal reason is the lack of hydraulic pressure. Automatic transmissions engage gears via clutches and/or bands. These clutches and bands are engaged by hydraulic pressure. The pressure is provided by a hydraulic pump. In most modern automatics the hydraulic pump is driven by the input shaft from the engine. If the engine isn't running, the pump isn't either. Without hydraulic pressure to engage the internal clutches and bands there is no way to properly reverse drive the input shaft by driving the output shaft. It is a vicious circle - without pressure you can't engage the gears and unless you engage the gears you can't reverse drive the hydraulic pump. Years ago (50's and early 60's) there were automatics that had two pumps one driven by the input shaft and another driven by the output shaft. These automatics could be push started.

The reason you might be able to push start a car with an automatic by pushing it at high speed is "windage." Although the clutches and bands are not properly engaged, there is a tendency for rotating internal components connected to the output shaft to drag along the other rotating components. If you go fast enough you can turn the forward rotating elements fast enough to turn the main hydraulic pump. If you develop enough pressure you can engage a gear and then you have a solid connection through the transmission. Even after you have the hydraulic pump engaged, you still have to push the car at a pretty good rate for a significant period of time. The hydraulic pump will need to fill the torque converter, and because of slippage in the converter, it will require pushing at a significant rate to spin the engine fast enough to start the car.

Push starting an automatic transmission in this manner is a very bad idea. You have to push the car at a pretty high speed. Without the pump running, you are not providing proper lubrication to the internal components. Because you are slowly building pressure it is hard to say exactly what gears will engage when. Because you are slowly building pressure, there will be a lot of partially applied clutches/bands and a lot of slippage. I've seen a car started this way (not mine), but it involved a lot of towing at relatively high speeds. I'd never do it. Get a portable jump start unit.

Ed

Bucky wrote in news:1184139123.844754.109860 @d30g2000prg.googlegroups.com:

You CANNOT push start any modern automatic at ANY speed. In fact, it is dangerous to even try to do what you suggest.

The pump that generates fluid pressure is in the torque converter. It only operates when the engine is running. If the engine is not running, there is no pressure and no lubrication.

In the old days (before about 1960) there was a second pump, at the driveshaft end of the transmission. This generated pressure when the wheels were turned, so you could push-start the car.

The second pump was eliminated from transmissions with the advent of

12-volt starting systems, which tended to be far more reliable than the old 6-volt ones. So for this reason you can no longer push-start an automatic.

I saw several RVs goint 55 mph dragging cars behind them during the holiday, I'm not sure if they were AT or MT but the front wasn't hinched up as it should be.

Some cars are made so that they can be safely towed that way.

Jeff

EdV wrote in news:1184160435.991196.65840 @q75g2000hsh.googlegroups.com:

If the car is a manual there is no need to have the drive wheels off the ground.

I suppose there may be conversion kits available to retrofit some kind of output shaft pump on automatic transmissions, I don't know.

The Owner's Guides for some vehicles with manual transmission specifically say not to flat tow the vehicle. The owner's guide for many, but not all, vehicles with automatic transmissions say not to flat tow the vehicles. Saturn used to be one of the manufacturers that specifically allowed for flat towing. Some Honda can be flat towed as well. These guys ->

make devices that allow a wider range of vehicles to be flat towed. You can find a list of vehicles that can be flat towed (aka "dinghy towed") at . Ed

"C. E. White" wrote in news:4694f096$1 @kcnews01:

That may be for safety reasons. The links you provide below say that it can be difficult to hook up the tow rig to some cars.

There is no reason at all that any manual transmission cannot run indefinitely driven by the output shaft rather than the engine. Manual transmissions are spash-lubed by the gears, not by a pump.

Huh. Very interesting.

A better link here:

It looks like, out of all the *cars*, GM makes seem to be the only ones that can be flat-towed. Of the *SUVs* just about everybody but Nissan appears to make them flat-towable.

I wonder if the necessary hardware for 4WD includes an output shaft pump.

And there is in fact a kit available to convert non-compliant vehicles. Cool.

"C. E. White" wrote in news:4694f096$1 @kcnews01:

.

But none of that has anything to with with push-starting the car, just with lubrication issues when the engine is not running. Even the towable ones cannot be push-started.

BTW, I know how to push start a M/T but not an AT, how would one push start an AT? do you have to push the car while in N, with key in ON and then shift to D when you pickup speed?

EdV wrote in news:1184177427.925267.150810 @o61g2000hsh.googlegroups.com:

Good question!

I'd think you'd have to do it just like you say.

If the transmission was is Drive, I'd guess you wouldn't be able to move the car without great effort once pressure built and the bands engaged.

Yup, that's how it's done! Have to be going >25MPH, at least with a '64 Biscayne...

That was the era, the last one I push started was a '61 Chevy, at about 30 mph. Ah, to be young and foolish. The deal, if I remember correctly, was that the tranny had to have a hydraulic pump on each end of the tranny.

Come to think about, it was a '57 chevy. Came back to me because the fins chipped paint off the headlight cowls on the '55 ford I was driving.

Must have still been there in '64!

I suppose everyone knows the old joke about the guy who needed a push to start, and since he had an automatic trans., he told his wife with the other family car that she'd have to go 30 mph before his car would start. He sat in his car waiting for her, and she'd been taking quite a long time to help him, but suddenly he looked in his rear view mirror, and there she was, bearing down on his car and going 30 miles an hour, just as he'd asked her to!

the early fordomatics (54-55) had a pump connected to the rear shaft. this was deleted in all later trannys. you must have hydraulic pressure to apply the clutches, etc. else there's no solid connection thru the tranny. the newer trannys, you can't push start, period. they have only the pump connected to the crank, which isn't turning.