Hey everyone, I had a few questions about what I need to keep in terms of wiring while doing my ls swap.

The person I bought this car from cut out a lot of wiring already when he swapped the factory 5.0 to a 5.7 he built. He wanted everything to be mechanical so he just hacked it out with reckless abandon.

First question, do I need to keep any wiring from the stock ECM? I have a Haynes manual and an original GM service manual for 86 firebirds and it doesn't seem like anything is needed but I wasn't sure.

Second question, does anyone have any recommendations for where to mount the new ECU and fuse box? I attached a picture of the harness I'm using. Its the stock harness and ECU from the LQ4 I bought and had someone trim all the wiring I don't need for the engine and he added a mini fuse block that has wiring for fans, fuel pump, 12v IGN, starter relay, trans control, and some other stuff. I was thinking about mounting it under the passenger kick panel but I will have to do some figuring out for that.

Lastly, I was wondering if anyone could tell me what these harness bundles that come out of the firewall are for? I was hoping to remove what I don't need but I'm not exactly sure what these are meant to control. The first one is on the passenger side of the firewall and goes into the fender. I'm guessing this is the harness for the ECM but I'm not sure. The second one is right behind the exhaust manifold on the passenger side which I have no idea what it was for. The third is right next to the brake master cylinder and I think it has the headlights, ignition switch and some other important stuff but I don't know what to keep.

 

You can eliminate everything coming out of passenger side kick panel except check for a ground wire that might be in that bundle. I'm not sure if an '86 has the ground wire or not. My '89 had it and it was necessary for various things like the speed buffer box, VATS box, and some other stuff. I think my door lights were acting funny until I figured that out, but it was many years ago and I might be mixing stories in my head.

The bundle behind the engine is your HVAC controls. It's needed. And it really sucks that someone cut it. It also has 2 vacuum lines, one for operating vacuum modules in the cab that move HVAC doors, and the other for vacuum control of heater bypass valve in engine bay.

The thick bundle on driver side is all chassis wiring. Keep it. Literally almost everything in your car depends on that to operate.

 

Thanks so much, I will look into the ground wire. And yes he cut out all of the AC including wires, hoses, compressor etc. so that sucks.

 

We have a forum at this site dedicated to LS swapping, https://www.thirdgen.org/forums/ltx-lsx/
Might get more eyes on your questions if you post there in the future.

 

Ok awesome, I saw one that was for engine swaps but I wasn't sure where to post because this was mostly wiring. Thanks for the feedback

 

The wiring coming through the big rubber grommet near the heater hoses is the HVAC. 4 for blower speeds, 1 for AC compressor. There should be at least 1, maybe 2, vac lines too. 1 supplies vac to the selector switch, which routes it to the vac motors (diaphragms) that control which dash or floor vents the air comes out of, and it will be grey; the 2nd, if there is one, goes to the heater control valve, and will be purple. I only see 1 grey but I could be wrong.

Your last pic has the headlights, parking lights, turn signals, horn in the fattest harness. Put that one back. Not sure what the other 2 are.