Why do torque convertors unlock during coasting?

[Fsharp]

Originally Posted By: NHGUY
You get unlock under deceleration to aid in braking.The computer sees you off the gas pedal,it assumes you may want to slow down or (soon) stop.It cannot predict if you ARE going to stop or brake so it guesses.Imagine if it didn't unlock...all road RPMs would be fed to the engine,not assisting in slowing.


If the torque converter didn't unlock, the transmission would have to turn the engine at a higher speed when coasting, which would aid in braking.

 

[Hokiefyd]

The Chryslers I used to own would unlock the converter when coasting, the engine would fall to about 1,200 rpm, and it would coast along without any engine braking. I think the two Cadillacs I owned were similar to that as well. This makes for extremely smooth driving, as when you tip back in, the engine can flash up a little bit before it locks back in smoothly. One drawback to this is the PCM generally can't cut fuel, or the engine will stall.

My Toyota Camry and all of our Hondas keep the torque converter locked during coasting. I prefer this, for two reasons: I use less of my service brakes, because I get engine braking, and the PCM can cut fuel to the engine (because it's being turned by the transmission). The drawback is you have to be more careful at tip-in, or you'll get a surge...you have to drive them more like manual transmissions.

I prefer the latter, but the former works, too. It sounds like your Tundra is set up similar to how my Chryslers and Cadillacs were set up.

It's neat to watch my PCM on Torque Pro app on my phone. Coasting down in 3rd gear (for instance). Commanded equivalence ratio is 2.0 (29.4:1 air:fuel ratio, which I believe means fuel is actually off), instant fuel economy is in the upper 100s, spark advance goes to a fixed -10 degrees BTDC (which is actually 10 degrees ATDC), and you can feel the engine braking (no fuel).

As soon as the speed falls to about 20 mph, the RPM is close to 1,000, and the PCM has to "catch it" by unlocking the converter and giving gas back to the engine. You feel an immediate reduction in engine braking (as if you pressed in the clutch), the spark advance resumes to condition-based variable advance (usually about 5-10 degrees BTDC at this point), instant fuel economy returns to about 50-70 mpg, and the commanded equivalence ratio returns to 1.0 (14.7:1 air:fuel ratio).

 

[HerrStig]

Originally Posted By: NHGUY
You get unlock under deceleration to aid in braking.The computer sees you off the gas pedal,it assumes you may want to slow down or (soon) stop.It cannot predict if you ARE going to stop or brake so it guesses.Imagine if it didn't unlock...all road RPMs would be fed to the engine,not assisting in slowing.
What are "road rpms". In years of driving manuals I have found that "engine braking" is a worthwhile benefit of KEEPING the engine and transmission locked together until just before stopping. The idea with a manual is to shift to neutral just before you stop. Many EFI systems have a fuel cutoff while the throttle plate is closed and the car is "coasting". The thermodynamics of semi tractor "Jake Brakes are interesting. Diesels, as we all know, don't have a throttle plate.

 

[supton]

Originally Posted By: NHGUY
You get unlock under deceleration to aid in braking.The computer sees you off the gas pedal,it assumes you may want to slow down or (soon) stop.It cannot predict if you ARE going to stop or brake so it guesses.Imagine if it didn't unlock...all road RPMs would be fed to the engine,not assisting in slowing.


It's pretty easy to predict--I can let it know my intentions why I tap the brake. Otherwise, I want it to coast in gear. Just like I would do on a manual transmission. Engine braking when I lift off the throttle. If I want to coast at the same speed--then just the slightest amount of throttle to maintain speed.

 

[OVERKILL]

The Charger drops gears and engine brakes even when I'm using the brakes, it is kind of interesting, similar to the behaviour I'd use when I was braking with the 6spd in the M5. when it gets close to stopping it of course unlocks.

 

[supton]

Originally Posted By: OVERKILL
The Charger drops gears and engine brakes even when I'm using the brakes, it is kind of interesting, similar to the behaviour I'd use when I was braking with the 6spd in the M5. when it gets close to stopping it of course unlocks.


Interesting. Does it get more aggressive if you brake more aggressively? Like, drops gears faster so as to obtain higher engine rpm, if you are on the brakes hard?

It would make sense, get the engine closer to the rpm you'd want, if you were out flogging the car. Me, I'm a pretty sedate driver so that'd bug me too.

But I could see how I'd be impressed in something that I'd enjoy tearing up corners in. Brake hard going into a corner, then it'd be in the right gear coming out.

 

[OVERKILL]

I don't know, I'll try that out in the spring and see if that's the case. Also, it won't change gears if you are into a corner, something SteveSRT8 pointed out to me that they do and I've noticed, it will hold whatever gear you came into the corner with as to not break traction.

 

[splinter]

No help with your Toyota but I do like to fiddle with and 'improve' my TCC characteristics and programming.

700R4 rigged up a simple toggle switch to ALDC. Full manual control at any throttle or speed (except 1st as it lacked hydraulic circuitry to do so).
4L60E PWM was easy enough to tweak with EfILive or HPTuners' hacks.
6L80E standalone TCU proved a lot more challenging but certainly do-able. Fun and rewarding if you're into that kind of stuff.

Now I'll get to learn the nuances and intricacies of the 8L90 in due course.

If you're still with me check out post #6 in this thread for clever GM workarounds...
http://www.hptuners.com/forum/showthread.php?37626-6L80E-TCC-Lockup-Questions

 

[SteveSRT8]

Originally Posted By: OVERKILL
I don't know, I'll try that out in the spring and see if that's the case. Also, it won't change gears if you are into a corner, something SteveSRT8 pointed out to me that they do and I've noticed, it will hold whatever gear you came into the corner with as to not break traction.


One of the neatest features of the LX cars is the trans behavior. Very dynamic performer with an incredibly competent stock tune.

Wait till you get a Diablo tuner, you'll be amazed what was "left on the table"...

 

[Garak]

Originally Posted By: SteveSRT8
They do that to prevent the surging from the driveline being as noticeable.

Driveline surging can cause problems in EPA testing, too.

 

[Jetronic]

Originally Posted By: OVERKILL
I don't know, I'll try that out in the spring and see if that's the case. Also, it won't change gears if you are into a corner, something SteveSRT8 pointed out to me that they do and I've noticed, it will hold whatever gear you came into the corner with as to not break traction.


mine does the same, also no upshifting.... going well over redline if needed

 

[TiredTrucker]

This gets even more bizarre. On the newest auto shift transmissions for heavy commercial trucks, the computer controlled clutch disengages on coasts during downhill operations. These are computer controlled auto shift manual transmissions, not automatics with torque converters. Only if the engine brake is needed or driver presses the accelerator does the clutch reengage. Imagine going down a slight grade with 80,000 lb of truck and your RPM's drop to 650. When speed of truck gets back down to cruise control setting, the clutch reengages. This is all in the name of fuel economy. I bet that is a lot of the motivation for some OEM's to have an automatic TC go out of lock up on coasts and such.

 

[Kiwi_ME]

Well, clearly conventional automatics have been doing this long before extreme fuel saving measures were implemented. From what I've read, in general, the lockup clutch is opened only for the sake of smooth driveline dynamics as the driver goes on and off the throttle, or at higher loads when engine pulses are more prominent, or when the transmission changes gear. Keeping in mind that there is no other soft torsional coupling like that found in a conventional clutch plate.

Regarding automated manuals, in the owner's manual of my VW Polo it says that the DSG will open both clutches on overrun - in the interests of fuel economy - but only until the brakes are first applied. I can't say if I notice this feature in practice but certainly the DSG downshifts to assist with braking.

I have to wonder how coasting in what is effectively neutral complies with existing laws regarding coasting and having "control of the vehicle."

 

[Garak]

Yes, those pulses of which you speak were no end of trouble for Japanese automakers trying to deal with EPA regulations on their performance manual transmission cars. That's the reason why things like the manual versions of the RX-7 and the Supra died in North America at the same time.