5.3L Chevy Motor Swap – Wiring Harness

This is the most confusing part of this project… there are SO many wires in the stock motor wiring harness that came with the motor/transmission setup… it’s CRAZY!  First things first… if you’re doing this project, do yourself a favor and at least checkout one of the guys who has done more work on documenting this stuff that pretty much anyone, at least from what I’ve found… his Name is Brendan Patten and he has a website at //www.lt1swap.com/ .   Be sure to check it out!!!! I couldn’t do this part of the project without his help.  As a matter of fact, I actually sent him my PCM and had him re-flash it for me for all of the specs I have for my Jeep and with all of that, he only charged like $75 which included shipping it back to me.  Sweet!

So, instead of trying to figure out the wires in the garage, I took the whole harness down to the office so I could work on the floor with it and take my time, and have good light, and plenty of reference computers nearby for looking up the information on Brendans site.  Here’s what I’m working with… and the way it looked on my floor on 8-10-11.


All I know is, this project would be a LOT more difficult if I didn’t have something to go from, and Brendan has done an amazing job with documenting all of the stuff on these motors, I can’t tip my hat to him enough!

A list of items to get for this part of the project was graciously provided by Brendan.  Here is what he suggests:

Here I will outline building your own fuse block and attach relay sockets.  This is a general guide that will work for most any wiring harness.  I use this same basic setup for LT1, LS1, Vortec Trucks etc.First off, the parts you will need to build the fuse block and OBD2 Diagnostic Port.

OBD2 Diagnostic Port Parts Vendor Part# Qty Needed
OBD2 Port Plastic Connector, 16 Pin www.mouser.com 829-12110250 1
Terminals – for OBD2 port www.mouser.com 829-12129373 4
Terminal Lock – secures terminals www.mouser.com 829-12160437 2
Lamp (for check engine light) www.allelectronics.com check lamps/indicators, inventory changes
Fuse Block & Relays Vendor Part# Qty Needed
Fuse Block Housing kit (terminals included) www.rockauto.com 85668 1
Relay Socket www.allelectronics.com SRLY-2 2
Relay 30 Amp www.allelectronics.com RLY-351 2
Relay Terminal 14-18 AWG www.mouser.com 571-42238-2 8

I got a few little extras, miscellaneous parts that will be needed to do this, based on input from Brendan…


Ok, so the next step was to tear the split-loom off the harness, and then tear all of the electrical tape off of it to see what I am up against, and what I have to work with.

So, I know that I’m going to be pulling wires out of this, because I had my PCM re-flashed by Brendan to remove all of the EGR, which means 2 less O2 sensors to deal with, among other things.   First thing, once I tore off all of the loom and tape, I had to get the clips and connectors out of the PCM connectors so I could pull wires out of it.  It’s pretty easy, but be gentle because the red and blue clips are a little on the brittle side, especially for older vehicles I would imagine.




The amount of wires you are able to strip out of the harness depends on how much of the original system you’re needing/wanting to keep.  I decided before even getting into this that I wanted to convert the acceleration system from drive-by-wire to drive-by-cable.  I like mechanical cables from the gas pedal to the throttle body…so that’s what I decided to do… and in doing that, I first had to find that DBC throttle body.  I got a good deal on an LS1 mechanical throttle body from a 1998 camaro that’s in great shape…so I put that on my intake and noticed immediately after getting into the wiring harness that there are no wires in the current system, and that I’m going to have to “ADD” wires and connectors to this system in order to get this to work with the PCM.  I found the IAC connector and the TPS connector on ebay, shipped for $25 bucks…done.   I’ll show pics of the finished product when I have them, but I have those connectors on the way, and I’ll be able to splice them with some previously used wires into the harness so that the PCM knows the position of the throttle, and so that it knows and can control the idle air circuit, which is essentially a controlled vacuum leak designed to allow the computer the ability to control this “leak” and thus, keep the motor running, even when the butterfly valve on the throttle body is completely closed.


Here are the connectors you need (I got mine on Amazon.com, WAY cheaper than GM parts online):

iac-connector tps-connector

more coming… once I get the connectors and splice them into the harness, I’ll post pics of that too so you can see everything.   I’m waiting on these connectors as of 8-22-11.  After months of not being able to work on the harness, I FINALLY got it finished (more or less) AND took it out to the Jeep tonight and started hooking it up! (On February 26th, 2012).   I have been going crazy not being able to work on this thing, but due to job changes and tons of stuff going on with family, not to mention personal business stuff, I just hadn’t had time to devote to my project.  However, that is all behind me for the most part and I’m pressing forward with this thing, and hope to finish it before the summer is over.   My kids are dying to take a ride in it so I gotta get it going!.    Ok, so here are some pictures of the work as it went for a few months of sitting on the floor of my office/studio.   NOW its finally sitting on the Jeep!


After I took off all of the loom, I proceeded to identify every wire in the harness and then tape labeled them so I would know what I was dealing with and what needed to have replacements or extensions.


A nice ball of scrap wire… yeah, all of this was taken out and thrown in the corner.  I used a few of these wires for extending other wires but basically I had  a nice big mess that was able to be pulled to clean things up some more. Always good to simplify!


Labeling the wires is SO important when you go to make your fuse and relay block.



I pulled all of the power wires from the endpoint devices like coils and injectors and spliced them together into 3 single power lines to be soldered into the fuse block, it made it a lot easier to deal with the soldering to have fewer wires.


Ended up getting new connectors for the transmission neutral safety switches as well as O2 sensors (shown above).  Got this stuff on Amazon for cheap!  A fraction of the cost of ordering from GM parts online.


Fuse block and bulkhead PCM connectors, ready to be loomed.


The back side of the fuse block and diagnostic (ODB II) port.


The harness, loomed up and ready to be taken out to the Jeep… YEAHHHH!!!  After hours of checking and soldering and taping and looming….ugh, but hey it was worth it to learn and make it exactly how I wanted it!


Starting to connect it to the motor, should be finished connecting everything by the first weekend in March, 2012.


After much work, and testing with a 6 amp battery charger, I got all of the circuits tested.  I believe everything is working like it should.  The REAL test will come after the exhaust pipes are put on and I start it up!

Here’s what the fuse block looked like after it was mounted…. not a bad fit, and easy to get to and see the relays and fuses!  OBD2 port is there ready to use as well!    Pretty awesome, once you get the main motor harness figured out, you only have 3 primary wires to hook up, +12v for the fuel pump, +12v battery and +12v switched.   There were 2-3 other wires that I hooked up as well, like the reverse lights…. for that to work you need to run the wire coming off the harness where it plugs into the NSS, run that into a relay on #85, run power to #86, and then run the power from the battery or switched source into #30, then take #87 and run that to your reverse lights.  Basically, the transmission NSS feed for the reverse lights provides a ground when you shift the transmission into reverse, so you can use that to trigger a relay to power the reverse lights.  Simple!

I needed to come up with a power distribution block so that I could power various items and relays, as well as run power to the motor harness that was separate from the power that is feeding the chassis harness.  Here’s what I came up with.  I went to the local auto parts store and got this “terminal strip”… which needed something on there to make all of the feeds able to provide power.  So I simply got some sheet metal and a dremel tool and went to work.  This is what it ended up looking like:

Not bad, and it should hold plenty of power, the terminal strips are rated at 30amps continuous each.  I’ll not be running that much through them though, the biggest thing I have hooked up to this is a 75A bosch relay that powers the high speed for my Taurus radiator fan.

One of the other items I decided to go ahead and wire up was the check engine light.   This is very simple… the PCM provides ground on a pin that is part of the motor harness, so just run a line from there to one side of an LED and the other side to +12v switched power, and if there’s something wrong with the motor that the PCM picks up, this LED will be on… most guys I’ve seen do this actually have the LED next to the OBDII port, but I wanted it right in front of me, so I wired mine up in the dash with a big ole RED LED!

AND HERE MY FRIENDS, is the FINISHED (kinda) PRODUCT!!!!!!!!!!

This motor runs amazing, TONS of power and it’s smooth, and gets better mileage (I’ve gauged about 18-20 mpg so far) than the 6cyl did previously!

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