Idle in neutral and drive - why are they different? Auto trans

[twogts4us]

why mess with it?

 

[propellerhead]

Good question. Too bad no one has answered it yet. I'd like to know too.

 

[dew22]

Good question. Too bad no one has answered it yet. I'd like to know too.

Answer

An automatic transmission has no load on the engine in neutral so the idle speed will be highest. When you put the car in Drive, there is a small load on the engine so the idle will come down some. It is normal.

 

[propellerhead]

Dew22: I think you're missing it too. His question is why is it set in the engine's computer to be higher in neutral vs. drive. On an old school carbureted car, yes the load answer applies. But in this case, it is based on the setting in the engine computer, regardless of the load.

When the car is in D, there is an additional load on the engine and the computer compensates for that when it adjusts the idle speed to 560 RPM. With the EEC constantly monitoring and adjusting, idle speed is what the EEC says it will be. The question is... why is the EEC programmed to hold the idle RPM at 560 in D and 760 in P or N?

PS. I never looked at the numbers. I'm just going with what the original poster posted.

Edit: Let me highlight what the original poster is asking...

I understand what you're saying, but what I mean is, the car is actually programmed to run fewer RPMs in "D" than in "N". That's the part I don't get.

I can program both "D" and "N" to 800 RPM, and it'll stay there, no matter which one I'm in.

I'm just wondering if there is a reason the car is programmed to hold a lower RPM in "D" than in "P" or "N". It's not the engine load if the car is specifically programmed at a certain number RPM, and I'm just wondering WHY it is programmed lower in "D" than "N" from the factory, instead of just a steady RPM accross the board.

I hope that makes sense.

 

[03ghoststang]

Even though the rpms are controlled by the computer there is still a mechanical load being put on the enigine by the tranny. So when its engaged in gear "D" the car is ready to go just release the brake pedal and it will take off slowly. So i still say the load factor is to why the rpms drop...Like when you turn on the A/C the computer adjust the rpm but when you take off you can feel the load being put on the engine by the A/C compressor. So its similar to the tranny dropping the rpm's to 560 because its a bigger load being put on it, but if the idle speed in "D" was 760 the car would have a tendacy to wanna roll more.. Try doing a stall speed to that rpm see if the car would want to roll more even though the brakes are applied..

 

[Red03Mustang]

Even though the rpms are controlled by the computer there is still a mechanical load being put on the enigine by the tranny. So when its engaged in gear "D" the car is ready to go just release the brake pedal and it will take off slowly. So i still say the load factor is to why the rpms drop...Like when you turn on the A/C the computer adjust the rpm but when you take off you can feel the load being put on the engine by the A/C compressor. So its similar to the tranny dropping the rpm's to 560 because its a bigger load being put on it, but if the idle speed in "D" was 760 the car would have a tendacy to wanna roll more.. Try doing a stall speed to that rpm see if the car would want to roll more even though the brakes are applied..

I'm not saying that load isn't being put on the car in D. My question is:

Why is the computer programmed to hold "D" idle at 560 and "N" idle at 760?

Those numbers aren't exact, but close. At least they similar to what mine was set to.

I know if you put a car in "D" it puts load on it and makes the RPMs drop.

I think, and what another poster mentioned sounds right, is that because of the MECHANICAL drop in RPMs that would naturally occur from going from "N" to "D", the computer is set at approximately the RPM it would naturally drop, and therefore tries to even out the RPM within the range it "thinks" it would drop to.

Properllerhead, thanks for understanding my point.

 

[jstreet0204]

Also, you want the lowest idle possible in drive to keep it from creeping, and for the best gas mileage. Because of the load, it is capable of maintaining a lower idle without stalling, which is most likely why they didn't simply program them to be the same.

 

[propellerhead]

I know if you put a car in "D" it puts load on it and makes the RPMs drop.

... and the computer adjusts it back to the programmed idle speed.

I think, and what another poster mentioned sounds right, is that because of the MECHANICAL drop in RPMs that would naturally occur from going from "N" to "D", the computer is set at approximately the RPM it would naturally drop, and therefore tries to even out the RPM within the range it "thinks" it would drop to.

I'm still not buying the idea that Ford set the P/N and D idle speeds to mimmick what the old carbureted engines did.

Properllerhead, thanks for understanding my point.

You're welcome. I thought you stated it pretty clear in your first post.

 

[Red03Mustang]

Yeah. The computer sees what the mechanicl stuff is doing, then adjusts everything to what it's programmed as being the "best" settings.

I agree with jstreet0204. I think they set it that way so the car doesn't creep too much. That makes sense to me, because neutral doesn't care what it's RPMs are, because nothing is engaged.

Now, haha... what makes them set Neutral at a higher RPM than Drive? Doesn't THAT just waste gas? Or is there something I'm missing about how the gears engage? Do they NEED a higher Neutral RPM so that it "falls" into place when the tranny engages?

Just something to think about.

 

[propellerhead]

Taking a guess... emissions? Maybe you get a more complete burn at 800 RPM than you do at 500?

 

[jstreet0204]

Now, haha... what makes them set Neutral at a higher RPM than Drive? Doesn't THAT just waste gas? Or is there something I'm missing about how the gears engage? Do they NEED a higher Neutral RPM so that it "falls" into place when the tranny engages?

The same idle in nuetral would stall.

 

[Red03Mustang]

The same idle in nuetral would stall.

Well, I don't know about that, because I can lower the neutral RPM to 500, and it won't stall. I've experimented with lower and higher RPMs, so I know how the car acts in these ranges.

The reason I'm messing with it at all is because the car occasionally acts like it wants to die right after the initial RPM burst, when the needle's dropping back down. It'll fall all the way to about 400 RPM, kinda blurb a sec, then kick up to 600-700 RPM range where it is programmed to be. Kind of hunting for the idle, I guess. I'm not really sure why it does that, so I thought maybe adjusting the RPM idle/drive would help. It only seems to if I up it to around 800 for both.

I was just wondering mainly if there is any reason that I would need to keep the span that the computer has programmed in there. For example, since it has approximately a 200 RPM difference between neutral and drive, is I up the Drive RPM 300 RPM, should I make sure the Neutral RPM is 200 higher than it sill, or can I set them evenly?

That's just my curiousity. It's not that important I guess, but I was just wondering if anyone on here had experimented with it and possibly had an answer.

 

[jstreet0204]

Well, I don't know about that, because I can lower the neutral RPM to 500, and it won't stall. I've experimented with lower and higher RPMs, so I know how the car acts in these ranges.

The reason I'm messing with it at all is because the car occasionally acts like it wants to die right after the initial RPM burst, when the needle's dropping back down. It'll fall all the way to about 400 RPM, kinda blurb a sec, then kick up to 600-700 RPM range where it is programmed to be. Kind of hunting for the idle, I guess. I'm not really sure why it does that, so I thought maybe adjusting the RPM idle/drive would help. It only seems to if I up it to around 800 for both.

I was just wondering mainly if there is any reason that I would need to keep the span that the computer has programmed in there. For example, since it has approximately a 200 RPM difference between neutral and drive, is I up the Drive RPM 300 RPM, should I make sure the Neutral RPM is 200 higher than it sill, or can I set them evenly?

That's just my curiousity. It's not that important I guess, but I was just wondering if anyone on here had experimented with it and possibly had an answer.

Well I should have said the same idle in nuetral would be more likely to stall. You may get lucky with it most of the time, but the lower it is the more likey you are to have a periodic stalling problem.

 

[bobjenkins]

the planetary gears inside the automatic transmission , while in nuetral the crankshaft spins the middle (sun gear) and the planetary gears are spinning so that the ring gear does not spin so theres no load on the engine but in drive there is