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If all other shifts/behaviors of the transmission remain unchanged you can most likely rule out engine speed and throttle as probable cause as those are used for all shift determination. You?d likely find everything messed up if it were one of those two things.
Update: Unless the trans solenoid connector is bad (And I cleaned the crap out of it) the electronics/wiring are not at fault.
I ran the truck while monitoring the low active outputs for the TCC (Or/Bk, PCM C2-21) and the 3-4 (Overdrive) shift (Bn, PCM C2-21) solenoid.
I was able to put the truck into a OD cruise condition (Low throttle, over 80kmh) with the overdrive off. As soon as I turned overdrive on, the PCM overdrive went low to enable the 3-4 shift to OD. However, the trans did not shift.
Similar action for the TCC, but it locked in when the PCM output went low as it should.
Seems I have a trans issue, perhaps a 3-4 solenoid failure. We'll see. I'm surprised and happy. Surprised because of how the problem seemed to be linked to the write of the ECM, and happy because a trans issue will hopefully be easier to deal with than a PCM failure.
Chay
Update: I measured the pressures as per the service manual...all good except the OD pressure. Switch to OD on and pressure barely rises to 15psi.
Possible OD solenoid but probably a cooked OD-Seems some have the piston seal tear for different reasons.
I did energize the TC solenoid with the engine off by grounding the orange/black wire at the trans...heard a click.
Same for OD solenoid-Brown wire...no click. Not conclusive but maybe a lead.
I'll change the OD and TC solenoids and retest.
Chay