mrvedit

I too am confident that the PCM is commanding the 1->2 shift by turning off Solenoid A. I cannot think any VB problem that could cause a 1-2> shift without turning off Solenoid A.
Granted, an intermittent Solenoid A or its wiring could cause it, but that would cause random time 1->2 or 4->3 shifts.

joecar

See attached pic showing EFILive (HPT is similar), shows M1 1->2 MPH parameter...

earlier calibrations had graph/curve for this...

if the M1 1->2 mph is achieveable, then it will cause the 1->2 shift while the lever is in manual 1st.

mrvedit

Joe, those are the fixed WOT values and I think the OP is asking about partial throttle.
The part-throttle shifts are controlled by a table of values depending upon the throttle position %.
If the TPS is defective and reading low, the trans will shift at very low speeds; unfortunately it will also command too low a line pressure with the result of ruining the clutches.

joecar

Ted, you're right, there is a table for M1 1->2 for the whole range of TP.

mrvedit

Here is a typical part throttle shift table as seen in HP Tuners.