: 4T80E shift algorhythm logic -- what are the facts?



mtflight
07-07-05, 11:12 PM
OK, inquisitive minds want to know..

I tried the WOTs in 2nd gear on my ETC (OBD2), with engine braking on deceleration When I approached redline, it automatically shifted to third... still maintaining the car in third, then downshifting back to 2nd upon release of the accelerator experiencing engine brake (almost as if it was doing PAS, performance algorhythm shifting; however, I don't think the Eldo has that).

I hear some fellow enthusiasts use 1st instead, during jackrabbit starts.. and let the line pressure determine the shift.

Then I hear that if you put the shifter in 2nd, it will start in 1st.

To confuse things futher, if you disable ABS (pull the fuse while car is on), the car starts in 2nd... but is that when in D? what if you put the shifter in 1st? Will it really be 1st, or second after all?

It seems that this tranny has a mind of it's own, with it's best interest in mind. If you read the owner's manual, you will not get much info, other than vague messages that tell you never to downshift at above the gear's range or you risk engine damage that won't be covered by warranty. Is that a blanket statement? Or was it written before they knew the transmission would not allow overrevving?

Is it the tranny itself doing the thinking? or the PCM?

Any stories? Ideas? Facts? Bbobynski? Advanced user tips? When is it really on 1st and on 2nd... for "spirited driving" is 1st or 2nd recommended?

I usually just keep it in D and use the pedal to determine if I want to downshift.

PS on a sidenote I cut-out doing WOTs in 2nd because after a few cycles I noticed my temp gage a notch east and the readout at 238... so I put it back in D and coasted in this Texas heat for a long time until it went back in the 220s (whew!)

danbuc
07-08-05, 12:56 AM
The performance shift algorithm basically monitors various input such as Throttle postion, vehicle speed,..ect. It then takes these inputs and tailors the car's shifting to the way the car is being driven. What this means is, that if you are driving the car hard on a road coarse lets say, when accelerating through a turn, most cars would tend to upshift towards the end, cutting down the acceleration as you exit the turn. The Performace Shift Algorithm takes your drivign style into account, and will hold the lower gear through the turn, all the way out so you can accelerate quickly out fo the turn, before it upshifts. It will also hold a lower gear on hills, to prevent the need for unecessary downshifting. It basically act like what you would normally do with a stick, when driving a car "like you mean it". It will tend to hold the lower gear, and will be less likely to upshift right when you don't want it too.

I think this feature was only option on the STS/ETC/DTS models, but I night be wrong. Bbob could definitely tell you, and could probably explain this a little better than I can. Basically, it allows you to control the car shifting a
little bit more, making it more fun to drive.

edit: Manually selscting a lower gear, is not the same feature as the Performance Shift Algortihm. Selecting the lower gear basically tell the trans not to shift our of this gear, until Redline is reach. Selecting a lower gear, also raises your line pressure, resulting in much firmer shifts.

mtflight
07-08-05, 10:04 AM
edit: Manually selscting a lower gear, is not the same feature as the Performance Shift Algortihm. Selecting the lower gear basically tell the trans not to shift our of this gear, until Redline is reach. Selecting a lower gear, also raises your line pressure, resulting in much firmer shifts.

Are firmer shifts harmful for the engine/ transmission?

Randy_W
07-08-05, 12:20 PM
Usually firmer shifts keep the heat down and will benefit the tranny. That is a very general statement and not a blanket answer.;)

BeelzeBob
07-08-05, 12:29 PM
Firm shifts are actually better for the trans in most cases.

There is definitely logic in the transmission control algorithms to protect it and the engine....quite a bit of logic actually.

"Line pressure" is not determining any shifts. The PCM is controlling line pressure in the trans as well as the shift points. The PCM is looking at a variety of factors to determine when to shift the trans and what line pressure to apply. Forced upshifts are done to protect the engine from overrevving and to keep oil temperature of the engine in control...i.e...in a case where you would run top end in manual 3 the trans will go to 4 to keep engine oil temp down as the engine will be running at 6000 + if you are running fast in 3 manual.

There is so much logic in the trans controller that it is impossible to describe in a post. The PCM is taking a myriad of factors into account to determine the shift points, shift quality, etc. The trans itself is basically "dumb". It has the line pressure controller inside as well as sensors and the shift solenoids but the PCM is taking in all the data and dictating to the trans devices what to do to effect what the PCM wants.

There are cases where the line pressure will be raised slightly if the trans is in manual gear selections, especially if it is forced to shift at high engine speeds. Shifting at very high engine speeds is more stressful on the trans due to the fact that the clutches are overcoming the peak power of the engine to change gears as well as overcoming the "spin energy" or inertia of the internal trans parts and pieces to make the shift. This spin energy can be quite high and actually excede the force induced by the engine itself in some situations so the trans will bang the shift quite hard to limit clutch slippage.