2JZ-GTE in an AE86 GT questions

[Fii]

There are 3 related boxes in Supra - Main ECU, ABS ECU and TRAC ECU.
ABS ECU has inputs from wheel sensors, processes it and feeds data to TRAC ECU and also does the usual ABS stuff with brakes.

I know for sure that you can remove the TRAC ECU and everything else still works. I have not verified it myself, but I believe that you can also remove the ABS ECU, since it uses data from main ECU, but not the other way around.

So the short answer is - the main ECU should be able to work without the other boxes.

EDIT: I am not sure at the moment from where the main ECU gets the SPD signal, have to look at the wiring diagrams to find out.