I have a 6.0L 4L80E swap into an older 4x4 K5 Blazer. Been working awesome for years. Running a custom PCM and harness by speartech. Made some changes lately, installed long tube headers and new full exhaust, moved PCM and tac box, the fuel pump relay and 3 fuse block inside cab. I took it for a spin after that and everything was fine until I hit a bunch of bumps in the road then it died. Noticed that the check engine light wasn't comming on when I turned the key on....20 amp ignition fuse was blown. Replaced it and as soon as I turned the key, blew again. Jiggled wires and replaced the fuses until I ran out and had to get towed home. Been troubleshooting all damn day and does it no matter what. It's usually the 20 amp on the K5 fuse block that blows. I have the ignition input wire on the 6.0L harness plugged into the ignition input slot on the K5 fuse block. It's always been fine this way.

The 20 amp ignition fuse on the speartech harness will also blow occasionally also. The ignition fuse has a single pink wire that goes to a splice that connects to about 15 other pink switched ignition wires on the harness. Disconnected the PCM, fuel pump realy, TAC box connections, MAF, injectors on the drivers side, o2's tranny connector, and a bunch of other connections and same thing. Fuse immediatly blows. Don't even get a fuel pump sound or noise comming from the throttle body. It's like one of the ignition wires is stripped bare and resting directly on bare metal but I've checked all the places where the wiring goes through the firewall and nothing seems to be stripped. I've carefully inspected all the things that I changed around recently and they all look fine. I have a pile of electrical tape and loom that I stripped off and removed to search. Problem is somewhere in that mass of pink ignition wires but I can't find it by looking at the wires. Any other way to find the short? I have to find the problem soon, the truck is parked in a place where the city is going to give me a $300 fine unless I get it movable soon.

 

I was doing that in my car and found that the oil pressure wire had rubbed on the fire wall.. pulled it away and problem fixed..

 

I have autometer gauges. The only gauge wires that are hooked up out of the harness are the tach and speedo. Neither are touching anything.

 

Is everything properly grounded?

I missed a ground one time and always popped the IGN Fuse.

 

Yep, all grounds fine. If there was a ground missing I probably wouldn't have been able to start it and drive it in the first place.

 

Have you checked the heat shield on the starter? When doing headers it can move and ground out the starter to the header.

 

Yes it is. You can see where it sparks on the inside of the shield causing rust to form.

But here's an update. I kept unplugging things until it got to the point where almost everything was unplugged so I just said fawk it and yanked the whole harness out. I noticed that the length of wiring that goes back to the tranny was stuck. When I was driving it I was on a really rough road and the harness must have got wedged in between the transmission housing and the body. As the body bounced back and forth sawed the harness a little. Sure enough I can see one of the pink wires is damaged a little. I'm going to hook up the ignition power to it with the harness away from the body and see what she does tomorrow. If that doesn't work then I'll have to pay that $300 fine to the city for being parked in front of my own house on what the city code enforcers are calling a sidewalk (that's actually my driveway). ..................And then I'll have to set the blazer on fire.

 

good catch. I was going to tell you to unplug the fuel pump and see if it still blows (if the pump is wired through the PCM). We have alot of vans come in with the same issue (PCM fuse in stock harness) and when the pump goes south it will short out and blow the fuse. But a chafed wire will definately do it as well.

 

forgot to update. Harness was repaired and all is well. Take care when routing the harness on a 4WD. You don't want to be in the middle of the Rubicon when **** like this happens.

 

Good work. the thing that really sucks is you found it the only way it can be found, with lots of effort and swearing.

One suggestion, rather than using electrical tape to put everything back together, consider that split flexbraid wire wrap. If you go one step further and use velcro cable ties rather than zip ties then you will have a much easier time trouble shooting things like this in the furture.