An automatic transmission will never “reverse drive” an engine. When you shut your engine off in an automatic transmission, thats it, the engine stops spining which in effect takes away all hydraulic pressure to the transmission releasing friction elements which are engaged. This is because the trans pump is no longer turning. The internals of the transmission are still spinning due to the driveshaft/s still spinning. These parts are being damaged because there is no atf lubricating them.
Then there would be a nice BIG backfire…Oh wait that was back in the days of the carburetor…We used to do that going downhill in my Buds 1970 Chevy Wagon…BOOM! I want to say Impala Wagon but…did that exist?
Mine does, as do many others. My specific case is a Lincoln LS if you want to look it up. The computer turns off all fuel when coasting. Unfortunately I also verified that the transmission will keep turning the engine when the fuel system failed one day out on the interstate.
You’re driving along and the engine is going and the torque converter is in full lockup. If the engine shuts down, the transmission keeps turning, front and back, so the transmission pump keeps going and all the hydraulics continue to function. Of course, when your speed gets pretty low, the front part of the transmission is no longer turning fast enough for the hydraulics to keep going.
Not true. There are even patents to look up on this. Nearly all modern cars shut off all fuel to the engine while decelerating. They can do this because automatics now can and do in fact “reverse drive the engine.” Also note that if they couldn’t do this, using a low gear to do engine braking going down a hill wouldn’t work.
Now, turning the key off, which would remove the electronic controls that keep the transmission in the correct gear and keep the torque converter locked up would probably stop all of this, but I am responding to the false statement that " An automatic transmission will never ‘reverse drive’ an engine."
Seems to me, as long as the transmission stays in gear and there is no “free wheeling” feature, the front pump is still being driven providing pressure so it STAYS in gear spinning the inert engine. Since the engine is turning over, there will be power to the accessories like brakes and steering…The car will simply coast to a stop, a non-event…
The old automatic transmissions had a front AND rear pump so you could push-start them…
I have done this on many cars over the years. It’s rather uneventful.
Pretty much what happens with an automatic is the engine will stay coupled with the drive train for a short while, depending on how fast you’re going. The RPMs will slowly drop as the car slows a bit, then the transmission will lose enough pressure with no input torque, or the engine will lose enough RPMs that the engine will stop turning, you’ll lose your power steering, and you will coast like you’re in neutral until you run out of momentum, restart the car, or stop the car with the brakes. (which will be good for one or two pumps until the vacuum reservoir is used up)
Then the car will be harder to steer and much harder to brake, but you will never lose your steering or brakes. And the column won’t lock unless you manage to jam the shifter into park and turn the key all the way back to where you could pull it out.
Pretty much what happens with an automatic is the engine will stay
coupled with the drive train for a short while, depending on how fast you're going
A VERY short time, (seconds) until the clutches bleed down, unless you have one of those old iron Torqueflites with the rear pump. (If you can find one anymore) I havnt seen one in years.
And this was exactly the case for my 91. However, my 2006 will go until the speed drops to about 25 MPH. Starting from 80, that was well over a few seconds. Vehicle speed is the factor now, not time. The clutches don’t bleed down because the pump is still turning and pumping.
Transman, you may be an expert at overhauling transmissions, but it is obvious you don’t understand how they work at 55 or so. When you turn off the engine and the fuel shuts off, the engine continues to turn over because the drive wheels are pushing it. Now the engine WILL stop when the RPM falls to a low speed.
The safety issue of no power steering or brakes could be a big issue depending on the type of car. A lesser issue could be transmission damage if this habit was done often enough and long enough. Automatics for the most part use bushings and bushings do not hold up well with no lubrication.
My vote is with Transman on this. Since the converter is a fluid coupling I’d like to hear an explanation from anyone as to how fluid pressure is playing a part in the converter and forcibly turning over a non-running motor. After all, this little connection does involve multiple sets of splines and turbine blades for a reason.
The front pump is driven by…
" As described above, impelling losses within the torque converter reduce efficiency and generate waste heat. In modern automotive applications, this problem is commonly avoided by use of a lock-up clutch that physically links the impeller and turbine, effectively changing the converter into a purely mechanical coupling. The result is no slippage, and virtually no power loss."
So, the front pump is driven by the transmission itself which is driven by the wheels of the car still turning due to the momentum of the car. This also keeps the engine turning.
A friend had a 53 Chevy with the 216 in it and we would turn the key off in tunnels (we lived in Seattle) and the backfire was huge,it always got a good laugh.
Yes, that works too. Note that PS is not needed when the car is in motion, and there is still enough vacuum in the brake booster to give you several stops even when the engine is off. But I hope the driver has enough sense to perform this maneuver only on a clear road; steering and braking should not be a factor at all.
When we we kids (back in '68) the oldest one of us had a 50’s era Cadillac,not newer than '55 I am sure, and of course an automatic transmission). We used to get this car started with a push start (would not work until over 35 mph). Sure we were completely without power braking,steering,etc but push starting this Cadillac was not even close to the most dangerous thing that my older friends were about to be told to do. All came back though.
" As described above, impelling losses within the torque converter reduce efficiency and generate waste heat. In modern automotive applications, this problem is commonly avoided by use of a lock-up clutch that physically links the impeller and turbine, effectively changing the converter into a purely mechanical coupling. The result is no slippage, and virtually no power loss."
What you say here is true but how about this.
Converter lockup is not only hydraulic, its ELECTRONIC. Fluid for lockup is directed through the valve body. How is it directed?? By valves and SOLENOIDS. SO, ignition off= solenoid/s off= no converter lockup.
Ellis, you have already proven the fact that you know nothing of what you talk about so I wont waste any more precious time responding to your posts. You have a blessed day.
