Disengaging auto trans for coast.

[beatr911]

Our '02 Honda Odyssey FSP leaves the transmission engaged when decelerating. On one hand this may be good for MPG by enabling fuel cut when coasting above 1500rpm (per the manual) but keeping the engine speed up to enable fuel cut really reduces the length of the coast. Manual vehicles can use engine on coast to a greater benefit that those with automatics because of total disengage.

Many GM auto trannies (and I'm sure others as well) disengage when coasting at almost all speeds. This feature I like, it enables longer and more consistent coasts rather than decelerating though all the downshifts to keep the RPMs up and enabling fuel cut.

So, anyone know how I can enable disengage when coasting? I'd guess it is a PCM module modification somehow.

P.S. Using neutral makes me really nervous, there is a distinct zzzip sound of grinding gears when shifting to Drive when in motion. A replacement $2000 transmission can buy 500 gallons of $4 gas, and is a lot more hassle.

[MetroMPG]

I'd also be very wary of any strange noises shifting back into gear.

Out of curiosity, have you tried raising engine RPM when re-engaging drive?

[BBsGarage]

I'm thinking that may not be normal.
Have you check the tranny out?

[beatr911]

The fact that the gears are grinding when re-engaging leads me to two possible conclusions. 1) The transmission is not intended to do this. 2) There actually is something wrong with it.

As it shifts without fault 99.99% of the time I'm guessing that it's not intended to do this. RPM matching still produces a zzzip non-synchro mesh noise as it figures out what gear to be in. Early Ody transmissions had problems, this one has all the updates as well as meticulous maintenance. I'm hoping this isn't just an early sign.

With all the Honda ECU cracking that has taken place, I was testing the water to see if someone knows how to make the PCM disengage the trans.

[Andyman]

I guess the PCM is a powertrain control module that can shift the transmission with electrical signals. My guess it that might make the same noise if you had the PCM do the switching between neutral and high gear without using the shift lever. If you check the signals going to the transmission you might find out if it is trying to go directly from neutral to high gear or engaging a lower gear first. I know that old Mercedes transmissions (without electronic control) first go to a lower gear when shifting from neutral to drive.

If I wanted to modify the system to shift to neutral when the throttle is closed, I would start by finding out what voltage levels are on the wires that control shifting when the transmission is in the various gears and neutral. Then I would build a separate box connected between the PCM and the transmission which would monitor the throttle position sensor or the MAP sensor or a vacuum switch and send the appropriate signals to the transmission for neutral when decelerating. I would automatically cancel using neutral if the shifter is set to a lower gear or the brakes were applied. I would also have a separate switch to turn off switching to neutral. I would either use relays or transistors.

I think that the GM overdrive transmissions use an overrunning clutch to eliminate engine braking in overdrive gear. It can transmit torque in one direction only.

It should be possible to add an electrically controlled clutch behind the transmission to automatically eliminate engine braking in rear wheel drive vehicles. I've never heard of anyone selling such a thing.

[88CRX]

I put both Lexus in neutral when i coast down hills. I've had the RX300 for 8 year and no problems. I put the IS250 in neutral too no problems too. If figure since the light on the dash is green for neutral it's okay to coast with it.

but they don't make funny noises. And, every model car is different.

[apgrok1]

I would say that the zippy noise is the clutch solendoids redirecting the flow back to the clutches. They turn on and off several times a second to get the correct shifting firmness. Auto trans gears are always meshed, there is no sliding past one another. Also, I don't believe that the trans shifts in neutral when coasting. What you are probably noticing is the lock up converter disengaging. I have noticed that if you leave just a tiny bit of foot in the throttle, it will stay engaged.

[88CRX]

Originally Posted by apgrok1

I would say that the zippy noise is the clutch solendoids redirecting the flow back to the clutches. They turn on and off several times a second to get the correct shifting firmness. Auto trans gears are always meshed, there is no sliding past one another. Also, I don't believe that the trans shifts in neutral when coasting. What you are probably noticing is the lock up converter disengaging. I have noticed that if you leave just a tiny bit of foot in the throttle, it will stay engaged.

Your saying the torque converter is still engaged, only the lock is disengaged when in neutral. If so, then i should still feel the car engine brake, or accelerate when i push the gas. It doesn't engine brake or accelerate when in neutral.

[beatr911]

Thanks Andyman, I'll have to study the shop manual to see how this trans operates and how it is controlled. A switch on the dash to disengage lockup would be a help, but the overrunning clutch is the tidy solution.

Apgrok1 the noise is almost exactly like when doing a bad gear shift on a non-syncromesh transmission, like a model A or a farm tractor. I suppose I could be mistaken though as modern 5 speed transmissions are pretty complicated.

I'll study the Honda shop manual some more for some good ideas. Thank you all for your help and insight.

[claytori]

I frequently shift the ATX into neutral on my Saab 9-5 when coasting. It has a special feature that will soften the engagement when shifting back into drive. But it is better to match the rpm's when re-engaging. Eco driving techniques need practice.