Its been a long, long time since I did my 96 DOHC into my 88 coupe using a 91 ECM/Harness. I did this b/c I had the low milage 96 motor and wanted to use a manual trans. I didn't want to go thru the process of figuring out all the A/T wires and "specific auto trans" wiring (yellow/purple wires come to mind). I found a 91 Cutlass w/ manual trans in my local Pick n Pull and grabbed the harness/ECM and the trans too.
So now I had a manual harness and 91 ECM and had to graft it with the 96 harness and the 88 Fiero harness. I created a excel spreadsheet w/ all the specific wiring from --> to with color codes and everything. The most important thing I can pass to you is you have to have the Factory Service Manual w/ wire diagram. 91 diagram is a little different than the 92-93. I found this out the hard way by using a 92 wire diagram and things did not line up. Once I got the 91 diagram it made sense.
Bottom line, I had to trace each wire (i think there were 51 of them) from the ECM to the connector. Some of the sensor ends were specific for the 96 motor and not the same as on the 91 harness, so those had to be grafted together. The injector wiring is one part I remember, I think its either the injector ends were different or maybe the batch fire vs. sequential thing??
It sounds like a lot, and it is, but its doable and kind of fun. I did it all on the kitchen table and it took about three days of on/off working. I had Ryan reflash the ECM with all the specific data needed and it worked perfect. No codes and ran great.
In the end I had no issues building my 3800 harness, and that one I did on the motor making each wire the correct lenght and all wires hidden as much as possible.
[This message has been edited by qwikgta (edited 02-06-2020).]