Well, ran into this issue today as I was looking forward to getting my car running after being down for 4 months.. but then this happened.

I have a 1999 Pontiac Firebird Formula, which originally had the LS1/4l60e combo with about 190xxx miles. Junked that combo and ran with a built 4.8, single turbo and a built 4l80e to give some background. Now, I have the stock 99’ PCM which hasn’t been modified other than tapping the secondary speed sensor for the 4l80e trans control. I have all connectors & grounds hooked up, even checked them prior to dropping the motor in and everything looked great.

Situation #1 I hooked up the starter but made a rookie mistake and installed the ground strap that goes to the block, to the starter. Hooked up the battery cables and the starter harness on the passenger side started leaking smoke. I immediately unhooked the battery, disconnected that harness and saw that the igniter wire for the starter had been melted. As far as I know, no other wires had been affected. I even checked the MAIN harness just in case and the wires were intact. So I Went ahead and tore apart the loom for the entire harness that leads to the starter, o2 sensor & crank position sensor.. and re-spliced a brand new wire, got the starter hooked up PROPERLY this time.

Situation #2 After inspecting all the other wires closely a few days later, I finally had the guts to attempt to hook up the battery again today, went ahead and did so and same thing.. smoke show on the passenger side, however it was the MAIN harness that was smoking this time. I pulled apart the loom and didn’t see any melted wires. But I wouldn’t doubt if they began to melt further into the harness where I couldn’t reach.

keep in mind, I hadn’t turned the key at all during any of this, just simply hooking up the battery caused smoke to seep out of the harness.

Honestly really lost with this one.. wiring issue for sure but I don’t know where to begin and what’s exactly causing this issue, I had inspected everything topside and none of the wires had felt ‘hot’ after the smoke was seeping thru the harnesses
Apologies for the long post, but anyone deal with this in particular? Any info helps. Just wondering if I could solve this issue here before I decide to get a new pcm.

Emilio

 

Use a ohm meter. Disconnect all grounds and then check for grounds on each connection. If nothing than hook your meter lead each ground terminal ( 1 by 1) and put the other lead to ground, if it read than isolate it.

 

Appreciate the reply man, I’ll have to look into that. I’ll post an update whenever I get the chance

E​​​​​​​

 

Well.. finally got to ripping the engine harness out today. Cut open the loom and Inspected every wire as close as I could and found a black/white wire from Pin 76 on the Blue PCM Connector. Which is Fuel Injector #5 Control. The Wire was melted in a small segment, but further upwards into the junction (where the blue/red connectors branch off) it looks as if it was pinched, and a little bit of the wire was exposed. Might not need to install a new harness after all.

Gonna slap a new wire in there and make sure it’s secured neatly and clean off all the surrounding wires now that I have the loom cut out completely. Just goes to show that when swapping motors, always inspect those wires and make sure they’re neat. Especially after buying a used 25yr old car with 190xxx miles! I’ll update this thread after I repair that wire and put it all together again, hopefully luck is on my side. I hate wiring

E

 

Late update on this thread, but me and my pops repaired the burnt injector wire as well as a few other connectors that looked a bit shady, bought some brand new 1" loom and some heat resistant electrical tape, brought all my wires together nicely... got her all hooked up and no smoke!!
Very close to getting this crazy train up and going.

E