Looking for genuine help, not a bunch of people talking smack about how cruddy AT's are.

Car: 1993 300ZX 2+2 A/T NA (I know, NA's suck, blah blah blah)

Using a consult, the ECU threw a Code 54 - the trans is fine, car shifts up to 2nd gear but I have to let off the gas and re-apply the gas - like a manual - to shift.

I did replace the batter last year and have done all the disconnect the battery to reset the codes BS and this issue still persists. The car was driving fine for six months and then one day while driving it decided to downshift to 2nd and it has been this way ever since April of last year (somewhere around there).

Anyways, I checked all the wires going into the trans and cleaned all the connectors and still the same issue. Have not checked the filter or fluid level yet, but the original owner did put in a Japanese AT in it - also, it does have a transmission radiator as well.

What I want to know is how do I go about checking the Harness and Connectors to see if there is a short - more specifically, where is all that stuff at. also, would flashing/resetting the ECU solve any of this?

I do have FSM for the 90 - 94 but just need some kind help from all the smart ZDriver's here. Thank s for your help.