So I have an 88 Trans am GTA, factory Auto 700r4, and 350 l98 with digital dash. Engine harness is cooked. I'm pretty sure rats or squirrels were living under the hood and ALOT of the wires are chewed. The previous owner swapped a bunch of connectors, hacked and swapped a bunch of random parts, and wires are generally brittle and every connector i touch, keeps breaking.(For context I bought this car from a repo lot, so I don't have any information from the previous owner) I think the engine harness is beyond saving without ALOT of headache and chasing wiring gremlins.

Question #1
I have a complete engine harness, and ECU from a 1989 Firebird formula, factory auto 700r4, and 350 L98 with analog guages. This harness is very nice and complete. This was a Maf car, just like mine. Can i just swap the harness over to my GTA? Do I need to use the other cars ECU or do I leave the one that is currently in my car?

Question #2
If I can't use the harness from question #1 I am strongly considering a fitech 38350 swap. The car will have mild mods like Edelbrock intake and runners, Dougs headers and y pipe, cat delete, EGR delete, and I'm swapping the early 9 bolt 3:27 gears with a 9 bolt with 3:45 gears and thirdgen PBR brakes. Possibly doing a 700r4 to WC T5 swap aswell. I'm sure the mods will eventually snowball into a cam and heads, etc... Would this be a better option in the long run?

 

I don't think there is much different in pinouts on the c207 or c100 connectors between 88 and 89. Probably nothing that isn't insurmountable by a couple pin swaps. You'd have to use the 89 computer though as you'd the cold start injector wasn't used in 89.

FI tech computer is a big ecu upgrade and takes tuning into ez mode. You don't need to know much for it to be a pretty good tune. The oem ecm isn't nearly as user friendly and you need equipment to tune it. Not to mention the fact that the new ecu is 35-40 years worth of technology upgraded. But there is a caveat, you need to make a stand alone C100 and C207 so power/grounds/gauges/ac(if you want it) all function. Which means a service manual and a donor harness will drastically help in making what you need. Not saying it's super difficult but can be overwhelming to make your own wiring for some enthusiasts.

 

The engine harness between the '88 and '89 should be pretty similar, at least for both years with analog gauges. Not sure how/if the digital dash on yours vs. the analog on the '89 is much different. Certainly the dash harness is different, but that may not matter. I'd suggest getting the service manual wiring schematics for both '88 and '89. I would probably start with the '88 ECM and modify the '89 engine harness as required (ex. cold start injector).

I agree with trying to avoid the aftermarket harness/ECM route. That's going to be a lot more work.

 

The guages not working would be a turn off for sure. The digital dash was definitely part of the appeal in the car. How bad of an idea would it be to cut all of the wires from the 89 harness and just splicing them into the 88 harness as needed? The Ignition coil harness, a little bit of the injector portion... basically anything near the passenger head is where most of the issue is. Maybe I just Splice in some connectors and send it and see what happens.

I did a full ls swap on my 87 GTA before I sold it, and wiring is not my favorite part of a build, but i'm not afraid of wiring a motor up, wouldn't be my first time. Not good at reading schematics, I am a bit dyslexic in that regard. When I look at schematics my brain just blurs a bit. With thorough instructions i can do it.

 

I'd have to see a fsm to tell you if there's a difference for the digital gauges on the engine side, but I doubt there is. If you are indeed going to need to change tune, then I wouldn't horse around with factory ecm. Too much hassle for updated, 'hard to get things for' electronics. The harness isn't all that difficult, just takes some patience and making notes as you sort out what you have and what you want. A v6 or tbi harness is a great donor.
Now if you aren't for sure making changes, take the 89 stuff, check the pinouts on diagrams that are probably posted, and install. Run it on the 89 computer.