Hi,

Maybe the shift up program works from a signal from the ICU and not the speedometer.
The ICU communicates with the ECU for timing variations in program for coolant temperature and knock sensor adjustments.

I say this because, when mine worked on my 1986 wagon, with the MPG option!
It did seemed to be RPM and LOAD sensitive.
Its ICU has a vacuum diaphragm to sense loads.
The 1989+ got a different EZK and CPS system that sits side by side under the glove box.
I’m sure they are hugging and snuggled up to work together.
I’m surprised to hear that the MPG feature was continued into 1989!

When observing the light it seemed to be RPM related but I would noticed a few seconds of lag variations, so it moved the shift point around a little bit if I played with the pedal!

The light indicated a shift when I was still interested in going up to the next gear with some momentum, so it would not feel as if I would be lugging a little.
The system was lacking some sensibility to compensate for a grade going up on the increase or the car being loaded differently.

I considered the whole thing as a nuisance feature and took the bulb out! Tape works too!
I still don’t care fir service reminders lights built on mileage only.
By the nineties we got blessed with a cursory or cussed at “CEL” that scares people off their wallets for smog inspections.

Later on they went to making ECU programs to “adapt” to a drivers habits. Supposedly to raise mileage ratings but it came with its baggage of learning quirks for those engineers.

Happy hunting the diagrams for that signal wire!
It’s got to have one or two because of the CPS system.

Phil