![]() The most difficult thing in matching a transmission to a particular car and engine is to get that shift to be positive without slip but still be smooth. Only on the shift between second and third does a hydramatic, and for that matter many other transmissions, actually release one gear and apply another at the same time. To downshift from fourth to third or from second to first all the transmission has to do is to let go of the highter gear and it is in the lower gear. The same thing happens between third and fourth. You are still in first but there is an over-run clutch that allows it to be in two gears at once without binding. It just applies the clutch, brake or band that activates second gear. Wq93 has some good insight on this but I'll add a bit more.Ī typical Hydramatic transmission, like many of us have driven for years in gas trucks and cars, doesn't actually shift out of first to go into second. In that case it was a common set of conditions that caused it which was easily addressed but the conditions which set up your rough shift may be much more specific which makes it more difficult for them to be identified and addressed through a software update. My 2016 Corvette Z06 has a similar "clutch to clutch" architecture 8 speed and right before mine was produced a software update was issued that addressed a very rough shift at lower speed when you rapidly went from on throttle to off throttle and then back to on throttle and many described the shift as feeling like someone had run into them. Make sure that the TCM has the latest software version next time it is at the dealer because it may have an older calibration. My 20HD took some time to unlearn a slightly rough 3 to 4 shift that only occurred under light throttle with the AC compressor engaged so the conditions that lead to a rough shift usually are very specific. towing + regen + sudden change in throttle position in a specific gear). Most of the time the shifting is seamless but since real world conditions are virtually infinite the odds are that once in awhile there will be a rough shift and the odds of this are much higher when the transmission is new and/or when it first experiences new conditions (i.e. ![]() ![]() The TCM is on a high speed data bus with the ECM so that the Allison can utilize what they refer to as SEM or shift energy management so that engine power can be briefly reduced at critical shift points or if slippage is detected and this slippage will also cause the TCM to choose a higher clutch apply pressure. High accuracy sensors measure input shaft speed, torque converter output shaft speed, intermediate shaft speed, and output shaft speed to allow the transmission to learn and adapt for slippage and/or rough shifts and some learning has to occur as the engine/transmission combination experiences new conditions. The current automatic transmissions are truly a marvel of electronic and mechanical integration with the electronics providing what are generally perfectly sequenced shifts where one or more clutch packs are releasing simultaneously with one or more clutch packs engaging and it has to occur with perfect timing so the transmission doesn't try to engage two ranges at once. ![]() It is good that you documented it but probably you hit the perfect storm of operating conditions to experience a very rough shift. ![]()
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