I just replaced a 28100 PXO 832 M1 TCU in my 91 EX with a junkyard-bought 28100 PXO 731 TCU from a wrecked and 'alleged' 91 Accord LX. What happens: In Park: No S Light; in R, no S light, N=no S light; D4= S LIGHT IS ON SOLID, but weak; D3 = No S light; D2=No S Light; D1 = No S light. While in Park, depressing the "S" Switch on the gear selector lever DOES NOT activate the S light for Speed Shift. Trans shifts normally and at expected rpm's.

Jumping the Service Port throws NO CODES. Opened the case. R41 & R42 are OK. No visible burns or shorts on inside of case or on the circuit board, front nor back.

Would this mean this TCU is incompatible with the system of an EX? Or is the Shift Position sensor be-fouled?

How does one know from TCU part numbers if a unit will be compatible from DX to LX to EX?