Electrical Smoking Wiring Issue After GT40 Head Swap

[Grapes]

I recently finished a GT40 head swap on my 87 5.0. The car ran normally before the swap. After putting everything back together, it started and ran for a few seconds, then I heard a small pop the car shut off saw a little smoke and a wire/group of wires in the main harness getting extremely hot. the area is right after the ecc wiring comes out of the firewall on the passenger side ill include a photo although I don't think it's much help. IMPORTANT I think it's only doing that under a load. I have a multimeter and thermal imaging camera, so I'm about to start digging deeper. I'd appreciate any help normally im pretty good with this electrical stuff but im a little overwhelmed and kinda stumped as of right now. I always try to stay away from making posts but I think some collaboration would really help me out today so thanks in advance.

Edit 1: no fuses popped and hardly any wires that ive traced down are actually melted checking continuity at the circular black and tan harness connector seeing interesting readings if anyone knows the pinouts for these connectors that could be a major help.

 

[Mcmahst]

What on earth is that red wire bundle? That doesn’t appear to be stock. I would unravel that bundle and check each wire. I wouldn’t use any thermal imaging, unless you just want to confirm for the insurance where the fire started lol.

What year is the car, and do you have either a wiring diagram or the EVTM manual for your vehicle? Good luck.

 

[rednotch]

Probably a short to gound with that rats nest someone's been in the harness before, you probably pinch a wire or one of the exposed wires is touching metal some where and grounding. Would definitely trace those red wires, is it a Maf swap pre 89 car or something? If you heard a pop, you let the magic smoke out of something electrical.

 

[Grapes]

What on earth is that red wire bundle? That doesn’t appear to be stock. I would unravel that bundle and check each wire. I wouldn’t use any thermal imaging, unless you just want to confirm for the insurance where the fire started lol.

What year is the car, and do you have either a wiring diagram or the EVTM manual for your vehicle? Good luck.

yeah those red wires weirded me out saw 5 taped around one so i had to investigate I actually belive it could be factory even though it seems odd there are a couple other wires soldered similarly. the car is an 87. currently looking for the pinouts of the circular black and tan connector in the bay checking continuity for shorts

 

[Grapes]

Probably a short to gound with that rats nest someone's been in the harness before, you probably pinch a wire or one of the exposed wires is touching metal some where and grounding. Would definitely trace those red wires, is it a Maf swap pre 89 car or something? If you heard a pop, you let the magic smoke out of something electrical.

red wire splices egr solenoid area 3 of the wires go to connectors there and the merge into one goes into the center of the black connector that i added to the post the rest 2 red and 1 orange wire go through the firewall to the ecu it seems. visually none of the wires are hardly burnt, i see no cracks which was my initial idea because when installing heads and intake ive moved the harness a ton to get it out of the way thought maybe they made a connection somewhere under the loom but so far ive almost ripped it all out and haven't seen a thing. sorry if I seem all over the place definitely a pain in my butt not being able to voltage drop. Thanks everyone for sparing your time

 

[nicholase]

Those red wires are a factory splice. S119. Also in that area there is a fuse link J for your HEGO. It should be taped to, or inside the harness right around there. That could be smoking I guess. Check it. Also check your HEGO harness isn't melted on your exhaust or something.

The AC pressure switch is clearly corroded. Fix stuff as you go. What's the third wire taped to it?

 

[Grapes]

Those red wires are a factory splice. S119. Also in that area there is a fuse link J for your HEGO. It should be taped to, or inside the harness right around there. That could be smoking I guess. Check it. Also check your HEGO harness isn't melted on your exhaust or something.

The AC pressure switch is clearly corroded. Fix stuff as you go. What's the third wire taped to it?

Thanks for the information im still digging around. been dealing with corroded wires and connectors ever since i got the car the reason I even started the swap was because the ground strap going from head to firewall was so green and nasty and had so much resistance the starter would struggle to get enough amps to turn over. now that ive put a massive ground strap It will turn over just not trying to set a flame to all my work ha.

 

[AeroCoupe]

87 & 88 are fairly close in wiring so try this:

Resource icon

1988 Mustang EVTM

Jan 18, 2024

Electrical and vacuum troubleshooting manual

• Mustang5L5
• Mustang Reference Material