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Does input shaft/turbine turn when in Neutral?

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New User

Apr 16, 2019, 8:31 PM

Post #1 of 5 (788 views)
Does input shaft/turbine turn when in Neutral? Sign In

Does anyone out there know if an automatic transmission's input shaft/turbine is spun up by the TC impeller when the transmission is in Neutral? other words, is the input shaft/turbine free to be turned by the impeller, or is there some mechanism that I am not aware of that prevents the input shaft/turbine from turning while the transmission is in Neutral?

My online research about automatic transmissions in general would seem to indicate that input shaft/turbine IS spun up by the TC impeller when the transmission is in Neutral, but a professional experienced mechanic is telling me that the input shaft/turbine is to NOT supposed spin up when the transmission is in Neutral.

My symptom is that when I shift (at a standstill) from Reverse into Drive quickly and directly, the shift is very smooth, but if I pause in Neutral at all then the shift is comparatively violent (clunk/lurch). One explanation for this would be that if the input shaft/turbine is spinning up in Neutral, then that would allow the input shaft/turbine to build up rotational momentum, which would then cause a more violent shift into Drive.

Tom Greenleaf
Ultimate Carjunky / Moderator
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Apr 17, 2019, 1:57 AM

Post #2 of 5 (775 views)
Re: Does input shaft/turbine turn when in Neutral? Sign In

? Just from ancient hand on training and trust me you probably were not born yet. In neutral the trans isn't turning or you'd be moving if wheels were on the ground so the pro is right for most of the question.
The TC as we know them is a coupler that locks up such that you get one to one engine RPM and same to transmission vs loss of about 10% varies how much slip/loss.

This is calculated for each application. The engine will turn I'll call a front pump that allows fluid/hydraulic pressure to be available in most, at the ready to engage bands for transfer of torque available at the ration of which ones.
How much do you want to learn about "automatic transmissions" and what for? One you own now to work on or learn how they work so you can make it a trade of your own? It will totally matter as there are near endless ways they operate TMK most on the first concepts of "braking" a drum mechanically to vacuum actuated to electrical actuators most limited by governors at least once just centrifugal forces against weights on springs at a set RPM forced the next move.

It did not become my specialty part of a whole trade for one simple reason. I've never had the room to spread out all the components, clean area with the unique tools, parts cleaners and more. It has to be clean always important more imperative for this.

Concepts began with the Model T now well over 100 years ago, some totally different idea came and went that worked well lost due to size, weight, ease of control plus of course not wasting the power available however something turning is powered.

Not my trade obviously did learn enough to know I'd never have the room to work on many types AND other service items as well.
Good luck if you can grasp the concepts of how many work that alone is quite the course of study before you touch a thing totally understand what it's supposed to do when it's right as intended,


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Apr 17, 2019, 3:31 AM

Post #3 of 5 (769 views)
Re: Does input shaft/turbine turn when in Neutral? Sign In

To answer your question, yes the input shaft will be turning all the time when the engine is running in neutral. The output shaft will only turn when the wheels are moving. That is how the module interprets what is or isn't happening in the trans. If the input shaft was not turning, there would be no response when the trans is shifted into gear. I believe once you shift into gear, the clutches will hold it still against the torque converter until the vehicle moves.

I'm sure our resident trans expert (gsferraro) will be along to clarify.


We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.


Apr 19, 2019, 4:14 PM

Post #4 of 5 (737 views)
Re: Does input shaft/turbine turn when in Neutral? Sign In

Yes input shaft is turning. What type of car do you have? what is the year make and model? Gary

New User

Jun 11, 2019, 9:49 AM

Post #5 of 5 (620 views)
Re: Does input shaft/turbine turn when in Neutral? Sign In

In Reply To
Yes input shaft is turning. What type of car do you have? what is the year make and model? Gary

1999 Jeep Cherokee XJ with 4.0L and Aisin Warner AW4 AT.

I think the other previous responder misunderstood my question...I realize that the input shaft must be able to spin when the transmission is in gear and the vehicle is moving...what I am asking is whether or not there is a specific dedicated mechanism solely dedicated to just one sole function: stopping the input shaft's rotation when the transmission is shifted into Neutral (and maybe Park) when the vehicle is at a standstill.
The ONLY purpose of such a mechanism would be to provide the smoothest possible shift into gear when the vehicle is being shifted into gear at a standstill: Stopping the input shaft would mean that the input and output shaft speeds would be equal (both stationary), providing the smoothest possible shift into gear when at a standstill. The mechanism would have to be able to disengage whenever the output shaft was turning, since having it engaged when the output shaft was turning would likely result in its destruction.
My (probably bogus) theory was that if such a mechanism exists, and mine was inoperative, then the clunk/lurch that I am experiencing when shifting from Neutral to Drive (with a pause in Neutral) would be due to the fact that my input shaft was being allowed to spin up in Neutral, creating momentum that causes the clunk/lurch when the spinning input shaft is then connected to the stationary output shaft when the vehicle is stationary.
I strongly suspect that such a mechanism doesn't exist (that the turbine and input shaft are ALWAYS completely free-wheeling with respect to the impeller) , but I just want to make sure so that I can eliminate the failure of that (imaginary?) mechanism as a possible source of my problem.

The fact that my clunk/lurch when shifting into Drive can be completely 100% avoided by simply always shifting directly from Reverse to Drive without pausing in Neutral (pausing in Neutral before shifting into Drive is what causes the clunk/lurch to occur when then shifting into Drive), makes me think that the problem HAS to be from the transmission input shaft FORWARD to the engine, because that is the section of the system that only ever rotates in one direction, such that shifting from Reverse to Drive quickly (without pausing in Neutral) goes from an under-load state directly to another under-load state, where any slack in the system would not be felt, whereas pausing in Neutral causes it to go from under-load to un-loaded to under-load, where any slack in the system would be then felt.
Multiple shops have looked at everything from the motor mounts and throttle transmission control cable (line pressure) to the differential and everything in-between, the transmission/torque converter have been rebuilt/replaced, the idle speed is right on spec at 730 rpm and the line pressure is right on spec at 60psi, and at this point all I have is a whole bunch of very puzzled shop mechanics, including two transmission shop mechanics.
I also have a front differential on this 4WD Jeep, and so I am going to remove the rear drive shaft, put the transfer case into 4H, and then see if the clunk/lurch still occurs. I don't understand how even the worst possible sloppiest rear differential and/or rear driveshaft, suspension, etc. could somehow NOT clunk/lurch every time I simply don't pause in Neutral, but I need to grease my slip yoke, anyway, so not a waste of time, regardless.

I just performed the experiment I think that I can say for sure that the problem is not the rear driveshaft, differential, etc. The symptoms never fit for the problem to be behind/beyond the transmission input shaft, but never know.
The clunk/lurch is pretty much identical using only the front differential, including the "rebound" clunk that you can see in the my old posted video of the rear driveshaft movement.
Using the front differential only, when I shift directly from Reverse to Drive without pausing in Neutral, I get one minor clunk, and when I pause in Neutral before shifting to Drive I then get the same "violent" clunk/lurch (and also the same rebound clunk) that I get when using only the rear differential.

Regarding my question about whether the transmission input shaft turns when the transmission is in Neutral, I tried this: With the rear driveshaft still removed, I used 2H and Neutral to see if the rear transfer case output shaft would be turning at all, and sure enough it spins VERY slowly and steadily...and I can easily stop it with my hand. So I'm assuming that this means that MY torque converter turbine and transmission input shaft ARE turning with the transmission in Neutral, because otherwise what would be turning the transfer case rear output shaft? One would think that Neutral would completely 100% disconnect the transmission input from output, but either that is not 100% true or only MY transmission behaves this way. I'll need to find out if other people's XJ's behave this way or not, I guess, to find out it this is normal or not.

(This post was edited by soyjer on Jun 12, 2019, 7:01 PM)

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