Why do torque convertors unlock during coasting?

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supton

supton

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This really bugs me, and had me literally cursing at the trans (albeit after a long day of driving). In heavy traffic I will let off so as to open up to a safe distance, then roll back on the throttle to maintain. Never had a problem in 20 years of driving manuals doing this.

Do that in my automatic, and now the convertor is locking/unlocking two, three, four times per mile. Maybe more. Hitting the Tappan Zee during rush hour had me swearing that it was the last automatic I'd ever buy.

If I'm stupid slow on rolling onto the throttle it seems to stay locked--or it locks as I'm doing it. Whatever. It's infuriating!

Thankfully in my sedate back road travels I can avoid traffic. But this automatic has me working harder than ever before with a manual. It'll attempt to push too tall a gear by unlocking the convertor instead, so I have to proactively downshift--ok, no clutch to deal with so that is a nice boon--but then it won't stay locked up.

Infuriating!
 
They do that to prevent the surging from the driveline being as noticeable.

Our newer fleet vans are locked up virtually all the time. My sig car locks up in every gear but first.

Haven't yet seen a vehicle that doesn't do it...
 
Which is why I like the Niassan step-shift AT's, since they have the Torque Convertor "Defeat Unlock" feature.
 
Originally Posted By: SteveSRT8
They do that to prevent the surging from the driveline being as noticeable.

Our newer fleet vans are locked up virtually all the time. My sig car locks up in every gear but first.

Haven't yet seen a vehicle that doesn't do it...


I don't quite understand your comment--locked up virtually all the time, but always unlocking to prevent surge? So they are not locked up virtually all the time?

I don't know what kind of surge there would be if they just left it locked up. I guess if tip-in is excessive there would be; but then the fix is pretty easy: use less tip-in.

[Actually I have that complaint too, both my Toyota's have heavy tip-in, makes smooth driving hard.]
 
Originally Posted By: Colt45ws
I don't know but it upsets me too. I remove that function from the programming on my cars when I can.


Nice! I recall you mentioning how much control you had over the trans, in your ability to reprogram (after your unfortunate upshift incident).

I can "hear" the engine very easily, so rev up/down stands out to me real quickly. In heavy traffic it really stands out!
 
The lack of logic of the TCM on the toyota transmission in my Jeep really makes me want to install the manual shift controller and add a switch for the torque converter solenoid.

The one on the Cherokee won't lock up on light throttle. Annoying.
 
Recently having several opportunities to drive 8 and 9 speed auto transmissions I had similar frustration.
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Originally Posted By: Miller88
The lack of logic of the TCM on the toyota transmission in my Jeep really makes me want to install the manual shift controller and add a switch for the torque converter solenoid.

The one on the Cherokee won't lock up on light throttle. Annoying.


Mine has "variable" lockup, so it's a PWM signal. My money says 10% duty cycle is full unlock, 90% is full lock. So if I were to blindly tie high (or low) it'll just throw a code and go into limp mode. Maybe yours will be a bit more "dumb" and content.

I know someone somewhere did do this on a Toyota, and said they got really great mpg in doing so. Probably a Tercel. Read it on the web so it must have been true.

Mine won't do lockup in the first three gears either. Only 4th (direct drive) and the two overdrives. Seems really odd to me to not allow in 3rd as I'd think long hill climbs while towing would have fluid temps broiling hot otherwise.
 
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Mine is an older Toyota transmission so it's just a simple on/off for the lockup torque converter. It will lock in all gears (including first). I discovered this when I was towing at 60MPH in 2nd gear on the highway.
 
Sometimes the clutch is not as strong as the open TC.

My 91 volvo turbo wagon had no TCC on its Aisin-Warner tranny, but the non-turbo version locked up. Same with some FWD dodges from the 80's.

To answer the question, I think most trannies have an "overrun" feature where they freewheel (in OD, not D,2,1) and I bet the TC interferes with this or causes internal wear.
 
Originally Posted By: supton
Originally Posted By: SteveSRT8
They do that to prevent the surging from the driveline being as noticeable.

Our newer fleet vans are locked up virtually all the time. My sig car locks up in every gear but first.

Haven't yet seen a vehicle that doesn't do it...


I don't quite understand your comment--locked up virtually all the time, but always unlocking to prevent surge? So they are not locked up virtually all the time?

I don't know what kind of surge there would be if they just left it locked up. I guess if tip-in is excessive there would be; but then the fix is pretty easy: use less tip-in.

[Actually I have that complaint too, both my Toyota's have heavy tip-in, makes smooth driving hard.]


Sorry I wasn't clear. Our trucks follow the same logic as yours when you let off the throttle.

The variable lockup can be done so well you can barely feel it. The Chrysler does it amazingly smoothly.

Re: surging, many vehicles do not close the throttle to idle when you let off the gas. This is but one of the reasons they unlock the converter. There are more based on your mfgr's emissions strategies...
 
Do you have a trans temp gauge on your truck? FWIW I can be stuck in traffic and my trans temp never gets much more than normal. Maybe a few degrees. I don't think it will shorten the life of your tranny, as Toyota makes a good transmission from everything I've heard.
 
Yeah, I'm not going to mess with mine. I'm sure there are good reasons, and I don't know 'em.

I'm still in the mood to get another car, but will be cheap and I strongly suspect I'll wind up with an automatic. But man was it bothering me in driving conditions that make most people "love" their automatics!
 
Originally Posted By: Silverado12
Do you have a trans temp gauge on your truck? FWIW I can be stuck in traffic and my trans temp never gets much more than normal. Maybe a few degrees. I don't think it will shorten the life of your tranny, as Toyota makes a good transmission from everything I've heard.


Don't get me started...

I have a factory "guage" for the transmission, as part of the tow package. Around 160F it will read middle of the range. I know 230F on my Scanguage will read... middle on the factory guage. ??? I programmed both SG codes in but seem to only be able to read one trans temp, and I suspect it's convertor outlet, not pan temp. So I'm not shocked when I see 230F, not when the convertor is unlocked. And... we all know Toyota's break, and are not what they used to be, etc etc etc. But Aisin transmissions DO have a pretty good reputation.

So I'm not (overly) concerned about wearing it out any time soon. Don't get me wrong. I'm not thinking it's going to melt down anytime soon. I just don't understand the logic....
 
I think it unlocks while coasting so the engine can rotate at a lower rpm saving gas/reducing drag. My truck does it too. I worry about the longevity of the torque converter lockup clutch so I try to maintain as smooth a foot on the gas as possible.
 
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.
 
Originally Posted By: Miller88
The lack of logic of the TCM on the toyota transmission in my Jeep really makes me want to install the manual shift controller and add a switch for the torque converter solenoid.

The one on the Cherokee won't lock up on light throttle. Annoying.


Toyota transmission? Toyota owns 51.9% of Aisin AW. They are categorized as a Toyota Group supplier, not part of Toyota AFAIK.
 
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