How does an auto transmission decide when to shift?

Discussion in 'Fuel Economy' started by Smile-n-Nod, Jul 15, 2008.

  1. Smile-n-Nod

    Smile-n-Nod Well-Known Member

    I have a 2000 Grand Caravan. I'm curious how the transmission decides when to shift. Does it shift at certain RPMs, or when the load on the engine reaches a certain point? How can I use this information to improve my FE? Thanks.
     
  2. mintsk8er

    mintsk8er Well-Known Member

    I think the car somehow detects the rate of acceleration and determines what rpm is the best to change gears to meet that acceleration requirement. In short, high rpms usually mean bad FE. And if you're accelerating fast, the car is programmed to shift at a higher rpm than if you were acclerating slowly.

    So basically accelerating slowly is the only thing you can really do in an automatic.
     
  3. MaxxMPG

    MaxxMPG Hasta Lavista AAA-Vee Von't Be Bach

    The transmission in your van is the "41TE" transaxle. It is fully electronically controlled, and is adaptive. This means the shift timing and firmness are controlled entirely by a computer via a solenoid pack, and the computer learns how you drive and adapts to your "style" of driving.
    The computer relies on several sensors to decide when and how to shift -
    Input shaft speed sensor - Engine speed on the transmission side of the torque converter
    Output shaft speed sensor - Road speed
    Throttle position - How far the accelerator is pressed
    MAP sensor - Engine intake vacuum, which is a function of throttle position and engine load.
    Temperature - Shift firmness varies based on transmission temperature to account for the fluid's properties over the wide range of operating temperatures.

    Chrysler transaxles are fully adaptive - sometimes to a fault. Many consumer complaints surrounding this UltraDrive transaxle are the result of two drivers with wildly different driving styles sharing the one car. The computer can become confused by the constantly changing and widely varying driver inputs and can seem to have a mind of its own.

    Hypermiling with the Chrysler automatics will be the same as with any other computer controlled automatic, but give it a little extra time to learn the new driving style. When you see the tach (if you have one) go a bit past 2000rpm, back off the throttle a bit and it will upshift. Over time, it will anticipate that input and will shift accordingly.

    My little LeBaron convertible has the same transmission, and I am amazed at how it has learned to hypermile itself. It will even hang onto second gear until the car is just about stopped, since it has learned over time that I rarely come to a complete stop but rather slow down to a crawl and then accelerate very gently. Taking it easy with the right-foot, I can see it shift to overdrive and begin the partial converter lockup at just over 35mph.

    That's probably the best way to use the info to improve FE. Just drive very gently and give the transaxle time to adapt to this driving environment. If you're tuned into the shifting patterns, you will see a difference after a few trips.
     
  4. gfdengine204

    gfdengine204 Well-Known Member

    Maxx,

    Where can I find whether my HHR will "relearn" my driving habits and adjust?

    Dang, cars are gettin' so smart nowadays!
     
  5. Smile-n-Nod

    Smile-n-Nod Well-Known Member

    Wow! Fantastic information. I had no idea my tranny was so brainy. ;)
     
  6. MaxxMPG

    MaxxMPG Hasta Lavista AAA-Vee Von't Be Bach

    The HHR's transaxle is also adaptive, but not to the extent of the Chrysler design. With gentle driving, you will notice that it will start to shift at lower than expected mph/rpm and the torque converter clutch will "hang on" when accelerating from as low as 25mph. That will feel a bit like the engine is lugging just a bit as you get back on the gas after coasting down.
    The transaxle has a "pressure control solenoid" that modulates line pressure during shifts, so you will know that you're being gentle when the shifts become almost imperceptible. If you have a heavy foot, the computer won't let the solenoid bleed off much pressure and the shift will feel firmer. My Maxx has the same transaxle and I can say that it has changed its personality since I started getting serious about hypermiling.
     
  7. mtbiker278

    mtbiker278 Biotech Researcher

    I know this works for Hondas, but it might work for Chryslers. If you want to reset the car's computer disconnect the negetive terminal to the car's battery. Depress the brake pedal to deplete any capacitors in the electrical system. Wait 5 mins, and reconnect the battery. Reset your radio and clock and you should be good to go. Other method is pulling the fuse to the car's computer and waiting 5-10mins.

    Generally people would do this when they've added some kind of performance modification that would alter they're air/fuel ration. By resetting the computer it forces the car to relearn driving patterns, air/fuel ratio's, and general operating conditions.

    Most adaptive transmission (I think they don't make non-adaptive autos anymore) will eventually re-learn your driving habits, but it's hard to say how long it would take. I would also guess that the result would be some kind of half and half transition of a different driving styles.
     
  8. yddraiggoch1

    yddraiggoch1 Burning100

    AHA! That explains things. I had achieved a 200-mile half-tank in my 07 Fit and was well on my way to a 400-mile tank, when my wife drove the car for a 75-mile afternoon. When I got back in it the next day, it was like the car didn't recognize me. Acceleration response and shift points seemed light years different from what I had been used to the previous day. It has taken me a few days of commuting to get things back to comfortable level.

    I may have to try the battery disconnect reset thing.
     
  9. nissynis

    nissynis NC Attorney

    Is this true of most automatics, or is this peculiar to Chryslers? For example, would my 2002 Mitsubishi Montero Sport auto be this smart?
     
  10. MaxxMPG

    MaxxMPG Hasta Lavista AAA-Vee Von't Be Bach

    The Montero Sport should have the R4A51 transmission, and it is also adaptive. Considering the alliance between Mitsubishi and Chrysler, I would suspect (but not confirm) that some of the Chrysler transmission "behaviors" would be observable in the Mitsubishi if they borrowed some of the algorithms from the Chrysler design. So it is entirely possible that your Montero Sport is indeed that smart. :)
     
  11. AlphabetBackward

    AlphabetBackward ZYXWVUTSRQPONMLKJIHGFEDCB and A



    Is this why when sometimes I approach a red light while braking to avoid stopping completely, but the light turns green so I step on the gas but but the car will lurch before accelerating?
     
  12. aca2983

    aca2983 Well-Known Member

    This works for most cars. I did it to my Taurus this weekend and it shifts better, and the idle and idle speed are better. Also did it when I had my Subaru (but that transmission could never "learn" anything right).

    It's CTRL-ALT-DEL for your car.

     
  13. JusBringIt

    JusBringIt Be Inspired

    I also have the 41te chrysler transmission in my dodge avenger...this transmission is good and also bad in certain cases.

    Maxx is completely right about all the learning that takes place with the 41te, however I have the A604 which depletes most of the power that is produced at the crank before it gets to the wheels...the power isnt really what bothers me right now as it used to, but it's the loss of gas mileage to the transmission.

    There is a website that I frequent that has members working on a swap to a 5 spd manual transmission to be done. I will be doing that as it will be beneficial in every way.

    This manual transmission comes from the older 5 spd chrysler/dodge vehicles in the early 90's.
    my lowest shift points are:
    0-10mph 1st
    15mph it changes to 3rd
    and doesnt change to 4th/overdrive until 39/40 mph. It will however, hold overdrive as low as about 32 mph.

    The fact that it changes to overdrive so late diminishes my city mpg.

    This issue was corrected in the 01 sebring/stratus where it goes into overdrive at 30 mph.

    So having a manual transmission in fifth gear by 30 mph, I could cut down a lot on city consumption. My rpms have to go over 2000 to shift into overdrive, not for any of the other gears and is a waste IMO
     
  14. MaxxMPG

    MaxxMPG Hasta Lavista AAA-Vee Von't Be Bach

    That sudden 'bump' or lurch is one of the disadvantages of the adaptive automatics. After 'x' number of times of slowing normally to a stop, you accelerate from a slow speed and confuse the transmission controller. During coasting/deceleration, it is normal for the car to hold 2nd gear until about 5mph, and return to 1st just as the car stops. You can accelerate from that speed, but unless you ease into the accelerator, the sudden change in throttle position and the sudden drop in manifold vacuum tells the transmission "GO NOW!" and so it returns to first gear using full fluid pressure to prevent slippage.
    The GM and Toyota transaxles handle that shift without too much drama. The Chrysler transaxles can give you quite a kick in the pants when they suddenly grab first gear to haul out. I taught my wife how to listen for the ratcheting/ticking noise of the solenoid pack to know when the car is in first gear before really tromping the accelerator. If you don't wait, the car gets into a momentary limbo where it's not quite ready to go, and then first gear comes along with a firm thump. It only takes a second to catch up with you, but it's funny to feel the computer getting flustered when you suddenly do something unexpected.
     

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