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Sedans Toyota Avalon 2008 Toyota Avalon
639 messages, Last post on Dec 04, 2009 at 2:01 PM
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Replying to: amauhry (Feb 09, 2009 1:35 pm) Its not that these transaxles will not downshift during a "full throttle lift" coastdown, just that they will not downshift to a ratio wherein a significant level of engine compression braking will be felt/incurred. In reality in this situation an upshift is much more likely and it is those inadvertent upshifts that Toyota and Lexus are having the most trouble with(***). Not the upshift itself, mind you, but the need, often randomly ocurring need, for a quick return to acceleration. An acceleration that requires a downshift but now with the engine already at idle (little/no ATF pumping capacity) and that previous "inadvertent" upshift just having EXHAUSTED all of the ATF line pressure. And be aware that while the transaxle will not "downshift" during a full lift throttle coastdown, even the slightest touch of theh gas pedal will often result in a downshift into the most appropriate gear range for your current road speed. Now...the lock up clutch. Almost all, if not all, automatic transaxles will release the lock up clutch the INSTANT you apply any level of braking as otherwise the "solid" engine/gearbox coupling might result in stalling the engine. And.. you are right but for the wrong reasons. The "feeling" you have is much more likely due to the lock-up clutch being engaged, resulting in a "solid" coupling rather than the "soft" one with the torque converter in the drive path. If you have carpet mats be sure they are well secured and cannot slide forward over time. There are instances of un-intended acceleration due to the mat sliding forward and getting trapped behind the brake pedal but lying over the top of the gas pedal. Apply the brakes severely and the mat forces the gas pedal down. In your situation that might even be the reason your car is downshifting inadvertently. Move your foot to the "wrong" place on that carpet mat and the gas pedal gets "nudged" slightly down and the transaxle downshifts accordingly. *** I first noticed this anomally in my 2000 AWD RX300. As I slowed or braked ligthly to come to a stop at about/below 10MPH the RX would feel as if someone had bumped me slightly from behind. And in slowing, braking lightly or not, coastdown, from 40-30 MPH the RX would get a slingshot feeling, a feeling of "surging" forward as if I had let up on the brakes. This was all the result of a transaxle design modification Toyota made late in the last century which resulted in a flaw for which they have yet to come up with a satisfactory fix. With the engine at idle the ATF line pressure cannot be sustained to/at a level to keep the clutches tightly enough engaged to support even the slightest level of torque for engine compression braking so that issue is eliminated by simply upshift the transaxle.
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Replying to: wwest (Feb 09, 2009 2:35 pm) And.. you are right but for the wrong reasons." Don’t go so fast my friend. I’m talking about a specific design, that of the U660E transaxle found in my car You are, however, talking in general, which it may or may not apply to that transaxle. Here’s a fact about the 3rd generation Avalon transmission that you are not aware of: 1. The torque converter clutch lock-up mechanism is engaged in all gears regardless of vehicle speed as long as: Here’s another fact you are not aware of about the U241E transaxle found in my daughter’s 2003 Camry I4 and the U140E found in my old 2004 Camry V6. 2. The lockup torque converter mechanism is not engaged in all gears. Requisites for engagement: Whoever has owned the Camrys above know for fact these cars’ tremendous ability for coasting. Can I relate this with the metal-to-metal coupling between torque converter and transmission? I have plenty of privileged information to find the answer. I just need the time. Facts 1 & 2 are neither my imagination nor my invention. Rather, they’re Toyota’s. Amaury ’08 Limited ps: thx for the carpet mat thing. It was a very good piece of advice.
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Replying to: amauhry (Feb 10, 2009 9:08 am) Your statements verify mine... The lockup clutch will remain engaged provided: "..you don't step on the brake pedal.." And I'm quite sure the lockup clutch remains disabled in the lower gear ratios wherein the converter's torque multiplication aspect is more often required than not. There is also the case that were it actually locked up, say in 1st or 2nd gear, you might stall the engine with a quick release of the gas pedal.
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Replying to: wwest (Feb 10, 2009 2:42 pm) The tranny is delicately complex: way too many hydraulic circuits, sensors and actuators. But I’ll see if I can find a common denominator here regarding the dragging issue This stuff certainly defies just about anyone’s common sense. Amaury ’08 Limited
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Replying to: amauhry (Feb 11, 2009 9:40 am) These days that would be the source code for the engine/transaxle controlling ECU/microprocess(s). And I don't think either of us will have access to that. The best, and maybe the only, close alternative will be the manufacturer's disclosed information about the dynamic operation, shift pattern, etc. The new RX/Venza F/awd system's electrical schematic, as a "for instance" includes the OAT (Outside Air Temperature) as an input. That could/might mean the outside temperature is being taken into consideration to determine when to engage, or how hard to engage, the rear drive clutch. Or it could mean the new F/awd system is subject to overheating(***) and the system operates accordingly. *** The Ford Escape was an early adopter of this "new" F/awd system and actually had an instrument panel warning to indicate that the F/awd system was overheating (rear clutch pack was implied) requiring the driver to pull over long enough for the system to cool. The Mazda CX-7 uses this same system and has added water cooling for the PTO (Power Take Off) to prevent it from overheating. My point is that it often requires many resources, sometimes somewhat obtuse resources, in order to try to figure out how these microprocessor controlled systems operate.
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I just got gas for my '08 Avalon; it had 16 miles to go and the low fuel indicator was on. I was only able to squeeze in 16.005 gallons and I really tried hard. Why is it with all the electronic gadgets and things there can't be an accurate gas gauge?
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Replying to: tfelton (Feb 11, 2009 6:44 pm) |
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Replying to: wwest (Feb 11, 2009 10:12 am) "And I don't think either of us will have access to that". Sitting on my desk are blueprints of stuff that ranges in the million of dollars (electromechanical, hydraulics, logic controller diagrams; you name it); would you like to take a peak? Cars’ blueprints are proprietary information only NOT top secret information. Not available in Google of course, but obtainable nevertheless. In this arena, I have the advantage mi amigo. "And I'm quite sure the lockup clutch remains disabled in the lower gear ratios wherein the converter's torque multiplication aspect is more often required than not. There is also the case that were it actually locked up, say in 1st or 2nd gear, you might stall the engine with a quick release of the gas pedal." Looking at this stuff, it’s clear to me that the automatic transmission has its own set of clutches; so even if it is engaged to the torque converter clutch (lockup), provisions can be set in place to engage/disengaged whatever set of planetary/sun gears depending on unique trait designs. The fact is, with such design characteristics logics can be implemented to have the transaxle downshifts even when YOU let go of the gas pedal in whatever gear wheather you're on flat or going downhill (have you ever heard about automatic engine braking when going on a downward slope as the tach needle shoots sky high?). This is all against your perception of how tranny works. Amaury ’08 Limited
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Replying to: amauhry (Feb 12, 2009 1:44 pm) "..The fact is..." Yes the whole idea of computer firmware control of the electric solenoids which in turn control the various clutches via porting hydraulic pressure is to have total control and viability of the transaxle. But just as you would NEVER declutch a stick shift in a low enough gear ratio that would result in a high level of engine compression braking absent being sure, certain sure, of the roadbed traction coefficient these transaxle should never also. No blueprints, source code, just common everyday horse sense. |
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Replying to: amauhry (Feb 12, 2009 1:44 pm) No, and I'm quite sure you haven't either. Modern computer controlled transaxles (assume FWD) will only downshift in the above circumstance provided there are first "armed" via the driver applying some level of actual braking, even light braking. "sky high" is a pretty subjective term and so it is not one I would use in describing the level of engine RPM rise that would be likely to occur with an "automatic"(no shifter or paddle movement) FWD downshift. "Automatic" FWD downshifts, even RWD downshifts, dare not be that dramatic in the described circumstance, automatic downhill engine braking. A driver induced, shifter movement, downshift, maybe, but NEVER an automatic one.
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