I have a 1990 760turbo wagon that was idling looow.

She was reporting a too lean/too rich on the OBD. I did find a tear in the pipe from the filter to the turbo and installed a new pipe. Now it won't start. A test of the Idle valve showed no signal current at all but 12V on one terminal. I think I'm reading the wrong book again. There are only two terminals..
So then she starts but stops as soon as the pedal is touched. I put the old pipe back on (tinkers first rule : One variable at a time please)
Now OBDC reports fault in the ecm/u.
I go to the dealer $990.. I go to various places all at once. I wind up buying a 760 ECU off ebay, a 280 000 541. I open the panel and am looking at a 3-517 370 (which I assume is a Bosch Number?) and stop. A day of research and I now think I want a 0 280 000563.

Does a 541 work in a 1990? Whats the difference between a 541 and a 563? ($300) I note that earlier 760s don't mention OBDD fesatures. Is that the diff?