Freewheel coasting with an automatic transmission.
 

Is there a way to free coast with an automatic tranny ?

We've had great articles on taking control Manumatic - EcoModder and on freezing the slushbox Torque Converter Lockup - EcoModder

Shifting into N and back to D makes noises that don't sound healthy to me.

Is there a way to "disengage the clutch" as it were ?

If there's a way other than shifting to "N" I'm not aware of it.

Jason,
Good to see people are enjoying my write ups on the wiki.

To my knowledge shifting into and out of N is the only true way to disengage the "clutch" Note: there is no actual clutch on a auto.

a half way of doing it is to disengage the torque converter (if you have manual control)

I routinely go in and out of neutral on my automatic without any issue. Before i had control of the torque converter I did have to rev match otherwise there would be a good crunch as the wheels span up the engine.
so for me if i was above 60 kph while in neutral i would rev up to 1.5k then shift into drive. or if i was at 100 kph i would rev up to 2.2k then shift into drive.
This stopped any crunching or noises.

after i got control of the torque converter i stopped rev matching as the unlocked torque converter will soak up any rev mismatch as its not a direct mechanical connection.
so now at 100kph i have the torque converter unlocked then i shift direct into drive, the engine comes up to speed based on the wheels backpowering through the torque converter (the car slows a little bit) then i switch the torque converter lock on and away i go.

Hope that helps, good luck

So with rev matching, it shouldn't be any different than a manual.

Is there a way to depressurize the TQ so it free spins?

That would probably take too long to refill.

correct rev matching is the same in auto as it is for a manual

regarding the TQ (torque converter) free spinning by depressurizing. This is a bit above my head, it may be possible to depressurize the TQ but i would expect it would be increadibly hard and possibly dangerous for the TQ.

Having said that i have noticed once or twice that if i am in drive and say traveling at 60 kph then turn off the TQ lock up the engine will only take as much petrol as if i was in neutral. This indicates that if the wheel side of the TQ is spinning faster than the engine side then it will not back power the engine when the TQ is unlocked.
Note: I have not checked this so it may not be too accurate you may need to check with your own car.

I coast with the engine on and off in my car but it doesn't make any unsettling noises, if it did make noises I didn't like I would not continue to do it.
I've finally learned to listen to my intuition, when something seems like it might not be a good idea there's probably a reason.

There should be no problem with a slush box being put in Neutral and then back into Drive again. They are designed to be able to do this as a safety feature when driving on ice and emergency stops to disconnect the driven wheels from the drivetrain. But rev matching will help the process when returning to drive.

I go into neutral at 55mph and coast down to 35mph twice each way to and from work. I do not do a rev match, I just pop it back into D. Been doing this for about 6 months now, and haven't had a problem yet.

The issue is the torque converter, small engines have relatively high stall speeds (lots of slippage at low engine RPMs, to compensate for a relative lack of low RPM torque) while V-8s generally have much lower stall speeds (to better utilize the low RPM torque they produce). So with the higher stall speed converters popping it back into gear with the engine idling doesn't load the drive-train due to the high slippage, the low stall speed converter imposes a significant load on the drive-train even at idle hence the fun noises.

I've been shifting into neutral more and more with my VW TDI. The only time it protests is shifting back into D at speeds lower than 10mph. Otherwise it's completely seamless, as long as I keep my foot off the go-pedal for 1 or 2 seconds as it re-engages the wheels.

How fast have you done this at? 60?

I do it all the time in the Odyssey with no fuss. Above 55 mph or so I do rev-match it for smoothness. All the time = multiple times every mile.

The more I look at modern automatics, the more they look like manuals with a locking, viscous clutch.
Thanks, I'll try it tomorrow.

I know this is an older post but it is very relevant to my thoughts. I feel like I am getting hypermiling down very well. The only thing I don't do so far is the coasting aspect, mainly because I fear it might be terrible for my transmission. I drive a 2010 Civic Sedan - Automatic. Should I need to Rev match or should it do this automatically? I have gathered that if I wait a couple of seconds before accelerating after switching back into D that it should be fine with no problems. Just would like a little input before doing this. I know a lot of people coast, but does it have negative effects on the transmission? I plan on keeping this car for a very very long time and would hate to have premature transmission failure.

Strikes - I know this was the case for older automatic transmissions, not 100% sure about new ones, but they don't really like to be driven in neutral for long, most in neutral end up disengaging the pump for the fluid, and can over heat (info is dated, and it may be contingent on if the engine is running). Anyone else reading may be able to verify this for you. I would look for more of a lockup device with your car, many new cars actually cut fuel when you are going down hill. In my BMW when in gear and the throttle is closed there is very little if any fuel going to the engine, where as if I put it into neutral and let it idle while coasting I am using fuel. If you can lockup your torque converter while going down hill you may be able to save some.

Neutral is fine for most automatics if you leave the engine on. Doesn't return the FE gains as engine off of course, but I've been driving my 2001 Beetle in neutral for well over 30k miles without any changes in how the automatic works.

I once did neutral-coasting in an automatic Corolla, with the engine off and driving downhill. No, it wasn't a suicidal attempt :p

My stepdad did a downhill coast with the engine off years ago and it overheated the transmission, he was stuck on the side of the road for quite a while.

217,000 on a 1997 Toyota Avalon....always coasting in neutral engine on (automatic)
275,000 on a 2002 Infiniti Q45 always coasting in neutral engine on (automatic)
Never had a tranny problem w/ either. Did run a cooler on the Q45 but that was becuase of the front grill block.

Only car I PERSONALLY could not coast in was a late (the last swoppy oval body) model Tauras. the rpms never dropped when in neutral.

(many threads on this topic success)

The only way you might cause damage is by turning the engine off while coasting in neutral.

One dark rainy morning I attempted to turn on a windshield wiper in an Army 5 ton truck. Blindly grabbing things on the dash of a military vehicle can turn out badly, and one of the things on that dash was a fuel cutoff. I coasted, but not all that far. :rolleyes:

In the dark, in the field, nobody notices when the truck behind them isn't behind them any more- until they get where they're going and say "didn't we have a 5 ton when we left?"

I have EOC my suburban for nearly 20 miles before and have done many 5 to 10 mile EOC runs.
I have a mechanical transmission temperature gauge and do not see any temperature increase or spikes when I restart.
But I know not every automatic transmission is a robust as the old THM700R4, which in its day was not known for being a tough transmission.