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Square Repair Ep. 4 - LS Swap Wiring

Updated: Mar 27

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As you may know I have been doing this for quite some time now. As you may also know I'm the kind of guy that will spend 10 hours to save $100. In the past I've tried cutting down stock LS wiring harness from the engines I've pulled or straight out of the junkyard, and although its doable and pretty cheap, its messy, time consuming and if its your first time you will most likely mess it up at least once and spend several hours tracing your steps to find out where you went wrong.

Now the easy answer to that mess is to just buy a shiny new standalone harness from a number of companies, PSI Harnesses, Summit Racing or one of my favorites, LSX Specialties.

These companies have plenty of options to choose from for 4l60e, 4l80e, drive by wire, drive by cable, EV1 injectors, multec, and the list goes on and on.

For the fancier builds I Personally prefer LSX Specialties, as they will make you almost anything you want, and what you get for a more premium amount is some of the nicest harnesses I've used. They recently built me a custom harness for another project and I was very impressed with the quality for the price.

But this series isn't about calling a custom wiring company and saying "I need this and here's my credit card."

This series is about how YOU can swap a square body chevy, that's reliable, and more importantly affordable!

And that leads us to this...

This is the CHEAPEST way to wire up an LS swap for your square body, or any other classic car for that matter...

You Can Get a COMPLETE LS1 Style Harness for just over $100.

(This one even includes the necessary Injector adapter pigtails for Vortec engines here:

Now obviously your main concern right now is probably "yea but its probably some Chinese junk that doesn't work". And while yes, its obviously made overseas, and maybe you are taking a gamble on the quality control but for just over $100 these things are actually pretty solid!

If you are still unsure, remember that Summit Racing harness i just showed you?

tell me from the pictures which is which:

Thats right, you probably cant and if i had to take a guess, they are probably both coming from the same place. So with that in mind is it worth the extra $200? I don't know, order both and tell me.

So with that being said:

How do you install these harnesses?

Now right off the bat, I will tell you, with most of these there are NO INSTRUCTIONS.

So youre probably thinking oh great, now i have to figure it out on my own. Well you don't because last time I bought one of these they sent me a PDF of the instructions which is located here:

Now that you have instructions on the color coding, sensor info etc., I've already Written a complete guide on these harnesses!

That should be enough to get you started, you pretty much just need to plug all the sensors in where they need to go, connect the red power wires to the starter battery lug, provide a switched Ignition source for the ecu to turn on, one ground behind the block and one somewhere else and bingo, you've got a running engine (If you have a properly programmed ECU.. More info below)

But what about the vehicle harness?

Since this series is about square bodies, i will provide you everything you need to know about connecting your vehicles existing harness to the standalone engine harness:

Located on the firewall below the Brake master cylinder is a electrical connector that looks like this:

To remove the plug its a single 3/8 (I Think) bolt, once the bolts removed yank it out.

Don't freak out when you see a bunch of tar like goo all over it. That's just the factory water sealing method. were only going to need to worry about the passenger side portion of this plug and use this diagram as reference:

I know that it can be a bit confusing because i made the diagram as if you pulled off the plug and were looking at the pin side, this is incase you were de pinning things you didn't want.

But by looking at this you'll quickly see that the only thing we really need from this right now is the PINK wire labeled switched. On gas trucks this will be your wire that goes to the distributor. You can grab this wire cut/extend crimp or whatever you have to do to supply it to the IGN wire on the harness. This wire is hot while cranking, which is important because we need the ECU to fire when your cranking.

Another common question is what to do with the starter wire. Well, in our case nothing, that purple wire should already be going down to the starter, and on the LS starter you'll connect it to the Little post. This provides your starter relay to crank the engine.

Now you'll also see the oil pressure wire, and the water temp wire. If your truck has actual gauges you can retro fit in a SBC oil pressure sender to fit the block using something like this:

This will thread in place of the original LS oil pressure sender behind the intake, now you can either add in a mechanical oil pressure gauge as I have, or if you can make it work, an SBC oil pressure sender. Then simply connect the old oil pressure wire to that and it should work to control your gauge.

For the water temp there's an even cooler way to do it.

Now obviously, this one shows unable but all you need to know is GM Part number 12551708. This is what came stock on 1998 LS1 Camaros.

Also grab a pigtail:

The reason why we need this is this take are 2 wire coolant temp which only provides data to the ecu, and adds a prong for an analog temp signal!

The third prong (the single prong by itself) can now be hooked up to the green coolant temp wire from our square body harness and provide us a working coolant temp gauge inside the cabin. All you need to do is cut and splice the standalone harness to that 3 wire pigtail.

Now at this point if you have been following along you should now have a fully wired engine ready to hook up the ECU to.


For this harness we will need to use a Red and Blue ECU or commonly known as a 0411 ECU. These Ecus can be found in basically any truck and LS cars from 2002 - 2002. The Red and blue is designated by the color of the ecu plugs on the harness side.

In order to run this ECU you MUST have VATS disabled. VATS is the Vehicle Anti Theft System installed on all factory ECU's. This prevents the engine running when the required modules are not recognized.

There's tons of ways to go about this including several vendors on the web and Ebay who will sell you a pre programmed ECU with VATS disabled but my reccomended approach is always to acquire an ECU either from a junkyard, or ebay, then find a local tuner to disable whatever you need, this allows you a way to go back and make changes if needed later on.

If you are more of a DIY type there's both paid and free ways to do this as well. HPtuners is a great pro level software if you do multiple swaps you can buy credits ($100 Per vehicle) to program and even tune the vehicle to your liking. Now this is again PRO level software and has a steep learning curve.

There's also free ways to do this. I've only tested them out but PCMHammer, LSDroid and Universal Patcher are 3 programs to look into. These are opensource software that will require a OBDII module like the OBDX Pro GT. Again these are opensource, meaning you're somewhat on your own when it comes to figuring it out, but luckily there is some Youtube videos on it if you just give it a quick search.

Whatever route you choose removing VAT's in critical in your engine running. And once it is removed you should eb able to start your engine!

As stated in the video its always best to dry run everything before modifying the harness in any way. This will also help you confirm everything else is good, such as the engine, trans etc.

Got a question?

Hit me up:

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