Harness Rework Tips
Thread Starter
On The Tree
Joined: Dec 2004
Posts: 189
Likes: 1
From: Austin, TX
Harness Rework Tips I have decided to tackle my harness myself, and have a few questions that I'm not finding answers to... I have the stock harness & PCM that came with my 00 WS6 motor and 4L60E, basically keeping it stock except for LT headers. Drivetrain is already in the car, and close to all hooked up mechanically except for the PS and trans cooler hoses. I have a Dakota Digital VHX dash, and a new American Autowire Classic Update harness in the body, mostly installed from the dash back.
I have enough experience with wiring practices that I'm not really intimidated by the project, but I haven't had to rework this many circuits at once before. I've read stevesnovasite and lt1swap.com, and all that makes sense. Please excuse any dumb questions, though.
- The Dakota Digital stuff came with their sensors and adaptors. Should I use them, or the ones already in the motor?
- Is it worth the trouble to reloom everything? I'm not above just chopping plugs off. I'm building a daily driver.
- What is a good work strategy- do it mostly on a workbench, or in situ? I am thinking it would be best to do most of the work in the engine bay... Lay out the harness and hook everything up, and then start working backwards from the things are gone.
- Where can I source an OBDII port for under the dash, and where does it hook up to the harness?
- Should I add relays for the headlights or any other big power draws while I'm doing the fuse block and fan & fuel pump relays?
- Is it worth the trouble to mount the PCM behind the dash?
Thanks for any help. I'm sure I'll have more questions as I work through this.
I have enough experience with wiring practices that I'm not really intimidated by the project, but I haven't had to rework this many circuits at once before. I've read stevesnovasite and lt1swap.com, and all that makes sense. Please excuse any dumb questions, though.
- The Dakota Digital stuff came with their sensors and adaptors. Should I use them, or the ones already in the motor?
- Is it worth the trouble to reloom everything? I'm not above just chopping plugs off. I'm building a daily driver.
- What is a good work strategy- do it mostly on a workbench, or in situ? I am thinking it would be best to do most of the work in the engine bay... Lay out the harness and hook everything up, and then start working backwards from the things are gone.
- Where can I source an OBDII port for under the dash, and where does it hook up to the harness?
- Should I add relays for the headlights or any other big power draws while I'm doing the fuse block and fan & fuel pump relays?
- Is it worth the trouble to mount the PCM behind the dash?
Thanks for any help. I'm sure I'll have more questions as I work through this.
Use the Dakota digital oil pressure and temperature sensor.
It's time consuming, but worth it to re-loom your harness. It will make for a much cleaner install. The F body harness you have is damn near stand alone as it is and easy to convert over.
I tend to lay the harness out over the motor and identify and label everything. I then remove the harness and set it out on a table where it will be easy to work on. Do yourself a favor and buy a roll a pvc harness tape. Amazon sells the tape cheap and you can find the high heat split loom there as well if yours is too old and brittle.
OBD ports can be found on ebay, amazon or your local wrecking yard.
Relay for the headlights are not a need, but always a nice way to do them.
Mounting the PCM under the dash all depends on your harness length and mounting location. It makes for a clean install, but if you've got to make the trunk of the harness longer it's a lot of work.
It's time consuming, but worth it to re-loom your harness. It will make for a much cleaner install. The F body harness you have is damn near stand alone as it is and easy to convert over.
I tend to lay the harness out over the motor and identify and label everything. I then remove the harness and set it out on a table where it will be easy to work on. Do yourself a favor and buy a roll a pvc harness tape. Amazon sells the tape cheap and you can find the high heat split loom there as well if yours is too old and brittle.
OBD ports can be found on ebay, amazon or your local wrecking yard.
Relay for the headlights are not a need, but always a nice way to do them.
Mounting the PCM under the dash all depends on your harness length and mounting location. It makes for a clean install, but if you've got to make the trunk of the harness longer it's a lot of work.
tracetrimble, 1964SS is right, relooming the harness makes a cleaner install. I have a truck lq4 so, I basically left the wires in the harness. I went for the functionality purpose. I like to make sure everything works before it looks cute. Also, I would like to lay the harness, out on the engine, while its in the engine bay, and modify it to the desired length. That would make for a cleaner install also. Maybe someday, ill replace and redo the harness, once the truck is taken apart for a frame restoration.
1964SS nailed it. On my 68, I couldn't figure out where to mount in under the dash with the factory harness length, so I ended up lengthening my harness to locate the ECM under the passenger front fender. Making an f-body harness into a stand alone would be pretty easy, I chose to re-use the camaro underhood fuse boxes and have them located under the drivers side front fender. That way my install has all the same fuses and relays as a stock car. I also removed any fuses or relays I didn't use. For me, I used a bunch of zip ties to bundle wires together that way I could pull the harness off the car for lengthening.
Thread Starter
On The Tree
Joined: Dec 2004
Posts: 189
Likes: 1
From: Austin, TX
Thanks, great input so far. I appreciate it. I really want to hear it run, so I may do the wiring in stages - reloom it later, after I work out the bugs. I do have a factory fuse & relay block that I plan to reuse. You guys' wiring jobs look good, thanks for the inspiration. Also got an OBDII pigtail on order from fleabay. I didn't realize it only had like 5 wires.
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Thread Starter
On The Tree
Joined: Dec 2004
Posts: 189
Likes: 1
From: Austin, TX
So I've made a lot of progress on the harness, almost done really. It really wasn't too bad at all, once I started working my way thru it. Just have to be patient and methodical, like most technical things.
I was just barely able to get the PCM connectors fed thru the existing ~Ø1 1/4" hole in the firewall, and extended a couple wires to make it easy to locate the PCM inside. I haven't decided where to put the PCM yet. I may eventually add Classic Auto Air system, and don't want to have to relocate it for that. Anyone have any suggestions or pics of their setup?
I believe the best way to do the coolant temp with a DD dash is to purchase a CTS from a 97-98 Corvette which has 3 wires, one of which is a feed for an analog gauge. I have the sensor and pigtail on order, but does anyone know which wires I connect to what? Is it just a common ground, and two identical outputs?
My Classic Autowire kit feeds several circuits through the fuse box / bulkhead which could now just live inside the cab. What have folks done with this? Just join them into the harness under the hood, and back into the cab?
Thanks for the help.
I was just barely able to get the PCM connectors fed thru the existing ~Ø1 1/4" hole in the firewall, and extended a couple wires to make it easy to locate the PCM inside. I haven't decided where to put the PCM yet. I may eventually add Classic Auto Air system, and don't want to have to relocate it for that. Anyone have any suggestions or pics of their setup?
I believe the best way to do the coolant temp with a DD dash is to purchase a CTS from a 97-98 Corvette which has 3 wires, one of which is a feed for an analog gauge. I have the sensor and pigtail on order, but does anyone know which wires I connect to what? Is it just a common ground, and two identical outputs?
My Classic Autowire kit feeds several circuits through the fuse box / bulkhead which could now just live inside the cab. What have folks done with this? Just join them into the harness under the hood, and back into the cab?
Thanks for the help.
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TECH Enthusiast
Joined: Feb 2008
Posts: 503
Likes: 1
From: Ventura, Ca
If you have the stock engine bay harness you can combine the 2. It has all the relays and makes a nice way to tie into your body harness. The 98's used a 3-wire temp sensor and oil pressure sensor that hooked up both to the PCM and the gauges separately. The 99+ used the serial port for the gauges so it was controlled by the PCM. You didn't mention it but if you were rewiring the harness' hopefully you eliminated some unnecessary stuff like the egr port. On the body harness there is a lot of extra stuff depending on what you want to use but the ABS is the main one that comes to mind. I found that a pcm in the dash is not a good option if you want to ever put in a HVAC system or end up having to remove it. I extended my harness to fit in the passenger fender like a lot of others.
Thread Starter
On The Tree
Joined: Dec 2004
Posts: 189
Likes: 1
From: Austin, TX
I have the underhood fuse/relay boxes with about 6" of wire still attached, but not the rest of the harness. Planning to use one of them to control the fans. The battery will be in the trunk, so the fuel pump relay will go back there.
Yes, I pulled out all the unneeded circuits.
I'm using the 3-wire 98 CTS, just need to know which 2 pins go to the PCM, and which one to the gauge?
I like the idea of the PCM being indoors, but I will take another look at mounting it under the pass fender. I lengthened whichever sensor it is on the back of the intake that pulls tight first, so it's no longer the restriction. Would I have to lengthen every wire in the harness, or is that enough? How hard is it to remove the PCM under the fender? Any pics?
Thanks for the reply.
Yes, I pulled out all the unneeded circuits.
I'm using the 3-wire 98 CTS, just need to know which 2 pins go to the PCM, and which one to the gauge?
I like the idea of the PCM being indoors, but I will take another look at mounting it under the pass fender. I lengthened whichever sensor it is on the back of the intake that pulls tight first, so it's no longer the restriction. Would I have to lengthen every wire in the harness, or is that enough? How hard is it to remove the PCM under the fender? Any pics?
Thanks for the reply.
Last edited by tracetrimble; Jun 15, 2016 at 04:52 PM.