I THINK WE MAY HAVE IT
@Turdle may be the winner!
I installed a new Bosch alternator. I compared diode tests between the new and the old. The new alternator had about 33% less forward bias voltage drop through the diodes compared to the old. Whether that is of significance, I don’t know.
The AC signal on the scope was MUCH cleaner than the old alternator. Much less AC voltage.
I took her out for a long test drive. Almost two hours. I normally start to have driveability issues and uncommanded shifting by about 20-30 minutes into the drive, accompanied by significant spikes on the OSS circuit in Forscan.
I drove for a hair under two hours. Around town and highway. OSS trace looks great in Forscan. Transmission functioned flawlessly. No codes thrown.
My theory is that at least one diode in the alternator was beginning to fail/leak when heat soaked. This was pushing a good bit of dirty AC into the truck. The OSS, being a variable reluctance circuit, sends an AC speed signal to the PCM for conversion to digital to determine shaft RPM. Alternator gets hot, AC leaks, OSS circuit picks it up, PCM thinks it’s legit, transmission starts shifting funky, OSS signal gets so outrageous that the PCM realizes it’s noise and triggers the P0721.
Very insidious. Old alternator pushed out good DC voltage, no obvious issues.
I’m not going to call it solved yet, as these gremlins do often come and go. More driving required. I’ll report back.
