the way I got ECU's to troubleshoot was to buy a 400 dollar 740. I then learned that my 88 has a different ICU than the 1990 I bought so I did what seemed reasonable to a volvo addict and bought a 1988 volvo 740 for 400 dollars.. ;-)

I took one all apart , thee 1988 so I have spares for my 88 740 wagon. the other looks good enough to fix, headliner fell , windshield cracked up but I got a windshield. it pukes oil up the dipstick. maybe plugged flame trap or oil separator. I did ok with it all Ive used lots of the parts and I like having spares to troubleshoot. it seemed like they actually went up in value over covid so I'm thinking about fixing the 1990 740 up. the headliner seems like the worst part of it. I have a 1990 240 , all that stuff is similar to the one I kept intact. problem with that is it blew the head gasket. I think Ill fix that in the springtime. the 740 developed a rather serious oil leak so I'm basically running myself in circles. Instead I'm driving my 88 ford van and it's been ok.. such is life ;-)