Tester, no I have not noticed that you can’t push start an automatic, I haven’t tried to push start an automatic since I had my 55 Chevy. But I have noticed that engine braking still works with an automatic transmission, not as well as with a manual transmission, but it still works and it increases if I downshift. RPMs rise even though I am not touching the gas so something is turning the engine at those higher RPMs.
Maybe we are confusing a transmission that is in gear and running and the engine shuts down compared to a car that the engine has not been running in and has been parked for awhile. Possibly if the vehicle has been shut down and parked for awhile and you start pushing it, there isn’t enough fluid in the torque converter for it to work effectively where if it has been driving, the fluid pressure is already built up and then the torque converter will transfer power in both directions.
If that is the case, and I admit that I don’t know, then I still stand by my post as that is the condition the poster asked about.
The engine will be driven by the car’s momentum until the engine’s RPM drops too low to develop line pressure in the transmission. Once the line pressure drops the clutch(es) will release and it will coast to a stop.
The vibration you felt was the engine turning over slowly as it reached the disengagement point. When you shifted to Neutral, the transmission disengaged; the engine stopped rotating; and everthing smoothed out.
Since little power is being transmitted at the time of disengagement there would be little damage to the transmission. As others have stated you would not want to coast a long time in Neutral with the engine ‘off’ as there is no lube pressure to oil bushings, needle bearings, and gears.
On most cars, when you lift your foot off the accelerator you will get engine braking. In the case of idle fuel cut, the engine is not producing any power and is just acting as an air pump trying to suck air through the closed throttle plate. Lets say you are coming to a stop from freeway speed. As you slow the turbine of the torque converter is driving the TC pump sending power to the engine. The stator is uncoupled from its support so there is no torque multiplication and it just goes with the oil flow. As the engine RPM drop below ~ 1000 RPM idle fuel is reindtroduced and the engine keeps the transmission from disengaging as you brake to a stop. On a lot of transmissions, automatic 1st is a freewheel engagement so the engine recouples to the transmission while the car is still rolling. This is why you don’t get a jerk as you come to a stop as if you shifted into manual 1st.
BTW there are some manual transmissions that do not have gears turning in the oil when the engine is ‘off’ and the output shaft is turning i.e. towing with the rear wheels on the ground drive shaft not disconnected. with the engine ‘off’ the counter gears are not turning so there is no splash from them The output shaft bearing, the yoke bushing, the output shaft nose bearing, and all the gear bushing/bearings will not be getting lube. That is a reason that some manual transmission cars cannot be towed long distances.
Researcher, so if he is cruising at speed and the engine dies and he leaves the transmission in gear, then the torque converter will continue to turn the engine over, thus maintaining vacuum for braking and power for the power steering as he works his way to the side of the road to stop. That would be safer would it not?
Exactly. If you wanted to make a distant off ramp, you could shift to Neutral to get extra coasting distance. Just expect to muscle the steering wheel and use the brakes once i.e. don’t move the pedal up and down – down once and hold…
I once had a Pontiac Grand Prix come in with blown intake manifold gaskets. When I told the owner what the problem was and what it would cost to repair, the owner decided to hold off on the repair until they could save the money for the repair. They then said they would come and pick up the vehicle to tow it away.
They then showed up with a pickup truck and a towing chain. I told them that they couldn’t tow the vehicle with the drive wheels on the ground as this would damage the transmission. They didn’t believe me. So while they were here I looked up the towing recommendations for the vehicle. It stated that the vehicle should be towed with the drive wheels off the ground. And if this couldn’t be avoided that the vehicle shouldn’t be towed at no faster than 5 MPH and not for a distance longer than ten miles.
The guy driving the pickup said he knew a guy who owned a tranny shop. So he gets on his cell phone and calls the guy and explains to him what I just told them. He gets off the cell phone and shakes his head no. And tells the owner that they can’t tow the vehicle with the drive wheels on the ground as his pal at the tranny shop said it would wreck the transmission.
I always thought the xmission pump that lubricates the xmission was driven by something turning in the xmission, not the engine. So as long as the transmission is turning – i.e. in gear – you’d be ok to coast without damaging the transmission. For reasonably short distances anyway. But it is the engine itself that drives the xmission pump? Good to know. Yes, it seems coasting with an automatic transmission could indeed damage it.
George, not exactly. The transmission has an input shaft and an output shaft. If the transmission is in neutral and the engine is not turning, then the input shaft is not turning. The pump is on the input shaft so if the input shaft is not turning, then the pump is not pumping. If the transmission is in gear, then the output shaft should turn the input shaft and that in turn causes the engine to turn.
I am not that familiar with the inner workings of an automatic transmission so I am not sure that if the engine is off and the car has been in neutral, that the ATF may return to the pan so if you start moving the vehicle in gear, it may not have enough hydraulics to turn the input shaft, in other words, it may still be in neutral as far as the transmission is concerned. I have not tested that and do not plan to so I don’t know if that is true or not.
My assertion was that if the transmission was left in gear when the engine quit, the transmission would continue to turn the engine at least until the car rolled to a stop. I don’t know what happens after that.
I believe that Tester is correct in everything he has said, but I also think that in this specific case, that I am also correct.
If I remember correctly, I felt the most vibrations around 35 MPH, so that must have been the engine-transmission disengagement point, according to Researcher. So, I am guessing no transmission lube below 35 MPH.
I hope there was not excessive clutch slippage right around that disengagement point that would lead to clutch wear. Of course the vibration itself might not be good for seals, etc.
George, not exactly. The transmission has an input shaft and an output shaft. If the transmission is in neutral and the engine is not turning, then the input shaft is not turning. The pump is on the input shaft so if the input shaft is not turning, then the pump is not pumping.
Keith, not exactly. The transmission’s pump is not on the input shaft, as that would require the vehicle to be moving before the pump would turn.
The pump is driven by tangs on the torque converter’s housing itself. As long as the engine is turning, the convertor is turning, therefore the pump is turning. Note that the input shaft is driven separately, by the stator INSIDE the torque convertor, and independently of the convertor housing.
The input shaft is driven by the torque converter’s turbine. The stator is a third member in the middle that increases efficiency in high-slip conditions, like starting from a stop.