Alright, I've wondered about this for years, but I thought it was a stupid question so I never asked, but I really want an answer.

One of the reasons I have always purchased manuals, one of the many reasons, is that every time I have had a dead battery I've been able to push start. Even with my heavy F-150, if I have a slight incline somewhere, I can get going. My question is, what is the limiting factor as far as push starting an automatic transmission (i.e. why won't it work)? Does it have something to do with the planetary gears that won't let it work, or is it the fact that you just can't get a "hard" grip immediately to spin the flywheel (like you can when you pop the clutch) between the impeller and turbine?

Also, isn't it a misnomer to say you can push-start a manual when you have a "dead" battery? There has to at least be some electricity or else the primary windings in the alternator wouldn't be able to collapse into the secondary windings to produce the spark for the plugs....Correct?

 

Dunno about the auto question, but I always thought you needed 'enough' power in the battery to supply the low voltage side of the coil, it was just that there wasn't enough power to spin the starter....Open to suggestions...

 

The reason you can't push start an auto trans is that the oil pump in the auto trans is driven by the input shaft. If that isn't turning, no pressure will be built up to engage the bands and gears. That is also the reason that towing is strictly limited to speed and distance. If there is no oil pressure, nothing gets lubricated.

Back in the dark ages, some manufactures put oil pumps on both the input and output shafts, so those could be push started. Last vehicle I remember that on was my folks '49 Buick.

 

I had a 71 Mercedes 280SEL w/ an auto tranny and the manual said it could be push started if you could get up to 35MPH !
I dont think I could push that fast but if a hill were nearby.......

 

that mercedes has a rear driven pump, the pump is driven off the output shaft, like a ford C3.

 

But won't pushing the auto turn the impeller and turbine, thus turning the input shaft so the oil pump would work?

 

nope

 

No, it won't.

If the input shaft isn't turning, the pump isn't turning, so there is no pressure to engage any of the clutchs. If the clutches are not engaged you can't turn the torque converter, so you can never get get input shaft to turn.

Mark

 

Mark

Gotcha...I think. But I thought that the clutch hub was attached to the turbine and was also splined to the big sun gear, so I figured movement would transfer to the input shaft. What am I missing? Sorry, don't know too much about automatics, that's why I stick

 

In order for the output shaft (which is the only thing mechanically attached to the driveshaft) to have any connection to the input shaft, there first has to be hydraulic pressure and a valve body selection of where to put the pressure in order to engage any clutches. So the output shaft and whatever drums, etc. that may be attached wont turn anything else without prior fluid pressure.

 

rj998:

Actually if it wasn't for the computer you wouldn't need the battery for a truck/car to run. Again with the exception for the computer the only reason you need a battery is for energizing the starter to turn the motor. This causes the alternator (generator) to spin. The alternator provides low AC voltage to the primary side of the coil which is steped up to any where between 10,000 to 20,000 volts on the secondary side depending on the coil. If I still remember correctly it takes approx. 9,000 volts to arch over on the plug, or to create the spark. The coil can only step-up/step-down AC voltage. The coil is actually a transformer and only AC voltage can be "transformed" because it is Alternating Current.

On older cars it was one way to tell if the alternator was working properly by disconnecting the battery while the truck/car was running. If the engine seemed to slow down you knew you had a problem with the alternator. Another way to tell was when driving at night if your lights started to dim it was because the alternator was not able to put out enough energy so it (lights) would draw current from the battery.

By all rights if you didnt run any accesories you could run your truck/car without a battery once started. Now days with the computers you need the battery to power the computer. If the computer is not functioning you won't be able to run your car/truck because the computer relies on inputs from sensors etc. and if something is not correct some times your car/truck won't run. You may have heard from people with Superchips having a problem with their trucks quitting etc. It is usally because the chip comes loose or dirty connectors etc. Anyway hope some of this helps.

 

The input side of one of the clutches is attached to the torque converter turbine, but the output side of the clutch isn't attached to the turbine. If the whole clutch was attached, how would you get the clutch released? If both sides are connected, replace the clutch with a shaft.

You need fluid pressure to connect the input and output sides of the clutch. The fluid pressure does the same job as the springs in a manual transmission clutch's pressure plate.

On the next post, older cars didn't have alternators. They had generators that only produced DC current.

I have a '30 Model "A" Ford. That generator does not have any AC ouput, it's all DC. Later model cars switched to alternators, which do have AC output. There is a rectifier circuit that converts the AC to DC current.

The coil works by charging and collapsing an electric field, not by stepping up AC voltage. A similar, but different device called a transformer steps AC voltage up or down.

Mark

 

01 XLT Sport

Thanks for the input. Aside from being part of a circuit, my understanding was that the alternator produced enough electricity for all the electrical demands of the vehicle except under extreme circumstances, and at higher vehicle speeds actually creates too much electricity, whcih is why voltage regulators are needed. So I'm confused as to why the computer would need the battery to run.

I guess what I'm saying is that barring any use of accessories, the alternator should be enough to power the computer, if under normal circumstances it is enough to power the computer and recharge the battery. There really is no difference between a dead battery and a regular battery when both are being charged, except that with the dead battery the chemical process has gone kaput. The alternator output still leaves the positive terminal and travels to the battery, with the return circuit running through the block and frame. You made it sound as if you need a fully charged battery in to run the F-150. Maybe I misunderstood you.

 

Wrong.



The output of alternators is DC. Even on old old stuff, the after the regulator, it was DC. An old points system took DC current and switched it on and off to the coil. The collapse of the magnetic feild when the DC current is switched off is what causes a secondary votage in the coil.

The ONLY thing AC output of a alternator has ever been used for is old choke heaters.

 

Dont worry, you are correct in your thinking.