My guess is that GM never bothered to re-evaluate (in depth) the sensor tooth/tonewheel design when they switched from the old variable reluctance (AC) sensors to the new hall-effect (DC/digital) speed sensors.
Maybe they thought they could compensate/adjust for a "wrong" tonewheel (which was obviously originally designed for use with a VR sensor) in software (or hardware, adding extra conditioning circuitry in the hall-effect sensor itself)...but in the real world, it doesnt work perfectly all the time.
When you design hardware, tooth size, frequency, and tooth count on a speed sensing circuit, there are some pretty decent differences to take into account when dealing with a sensor that is just measuring a digital flip/flop/on/off/yes/no change in state (hall effect), and a sensor that produces a zero-crossing analog waveform that varies gradually in both amplitude and frequencey (VR).
Anyone who has seen an Allison C1/C2 drum knows what the teeth and teeth spacing look like (or what the PTO gear looks like on earlier transmissions), in my limited understanding of the 'theory' of operating, its not ideal and doesnt look much like a true hall-effect tonewheel.
Or maybe GM did infact change the tonewheel design to suit the new sensors on the 2015.5, and Im totally wrong.... Ive never taken apart a 2015.5 "AVF" trans to look at it. My parts catalog only goes up to 2013, and its not worth my time and effort to dick around with the dealer asking them to look up all of the exact differences (if any) between a 2015 and 2015.5 trans.
Or maybe it has nothing to do with the speed sensors at all, and GM simply hasnt quite figured out how to fine-tune the new T87 TCM yet? Who knows.
Anyone elses guess is as good as mine.
Ben
