The only difference is one of the plugs under the dash. You need to swap 2 red wires. One is red with a black line, the other is red with black squares instead of a line. When you swap the 91+ harness into a 90 car, you need to swap these 2 wires on the plug, or the car will not start with the ignition. I've done it in more then 1 1990 car. I don't have pic's on this computer, but you'll see exactly what I mean when you do it. All the plugs plug in. The plug I'm talking about is over by the ECU. It's a white end. When you plug the 91+ engine bay harness into the 90 car, you'll notice that the 2 wires I'm talking about are swapped. You just need to match them back up.

Shoot me a PM if you have any more questions.

Read this and then ask questions: http://www.dsmtuners.com/forums/articles-electrical-wiring/412841-1990-engine-control-wiring-harness.html

thanks

when i did mine i did both harness just to make sure it was a plug and play swap instead of hacking up a virgin harness to make it work.

If i do swap the harness and the wires in the white plug can i still use the 90 computer

Sure. Switch pins 6 and 14 on the harness for the ECU and you'll be fine. Course, if you read the article you'd know that.

There is a dash plug on the passenger side footwell that needs to be shaved in order to plug in. Also if you swap a 91+ wiring harness in you'll need a 91+ tachometer swapped in. The plug everyone thinks of can be wired in color matched, there will be several wires that aren't used. In the engine bay the CAS has a different plug from 90 to 91+, as well as the TPS I believe. It's been quite a while since I did it so my memory might be sketchy but that should about cover it.

i think im gonna just swap in both harnesses the only proublem is the 93 harness i have is for a front wheel drive not all wheel there shouldnt be much differance tho right

Between AWD and FWD turbo, assuming the same options and transmission type, there will not be any differences for a specific model year.