Another V6 to 5.3 wiring question

I'll be working on a 98 chevy vortec 4.3 truck. I have a 5.3 (cable TB)/tranny/harness/pcm.

Converting the harness to be standalone is fairly simple, is this the first step in wiring up this swap myself? If so, what do I have to do to make the guages etc on the truck end of the wiring work once the engine runs standalone? It seems to be the most simple method of getting this done, and I can do the standalone wiring, but I don't know what to do after that.
Buying a harness isn't out of the question, but I prefer to learn these things and not rely on outside sources if I don't have to.

It looks to me like most companies that make you a wire harness basically send you a standalone harness, but it is modified to use the old V6 computer to control non engine/tranny functions. I'd like to know how to make the standalone harness do the same thing if that's the best way to go about it. I'd also like to know which computer controls the guages, cruise control, etc.

Keep your stock vortec harness and don't just rip it apart like most. Remove it from the truck in one piece. You'll need to remove components like the injectors and stuff you know the LS1 harness already has because it'll run off of the new 5.3 pcm. I have a 98 manual so I can guide you with what you need to remove. You'll want to keep things like the oil pressure/fuel switch that bolts on the back of the block and it will be moved down to the oil filter bypass on the side of the block by the oil filter which will need to be tapped to 1/8" pipe if I remember correctly, transmission wiring, gauges, etc.......you can use HPTuners to set the tach resolution to 3 to keep from buying a tach module. You'll want to keep the vortec plug for the altenator because it will plug into the 5.3 altenator for the gauges. All of your gauges will work correctly if you don't destroy the vortec harness and I can't stress that enough. You can add the electric fan relays, A/C and fuses with a small stanalone harness like I did which was very simple. You'll need a weather pack terminal connector kit if you want to make factory connections. Remember to disconnect the vortec harness at the C100 connector on the fire wall, unplug the connectors below the cruise control unit and unplug the blower connections in the cab on the passenger side and pull them through the firewall then the whole harness will release. Wherever you mount your PCM will dictate the length of your standalone harness the will need to be added.

Good luck!

Great reply and it is very much so appreciated. Your name was mentioned as someone to talk to.

Here's the interpretation I have as of now as to what I am going for:
~Convert the LS harness to standalone as if the engine/tranny were going into any older carb vehicle. The 5.3 harness controls all engine and trans functions.

~Remove only the engine control portions of the 4.3 harness, basically remove the parts that are left on the 5.3 harness once it is standalone.

Old harness goes to:
- alternator for volt guage.
- temp sensor installed in passenger head for guage. 5.3 harness uses the one on the front of the driver's head
- starter
- oil pressure location that you said to tap into oil filter bypass
- a/c compressor

I don't understand where the speedo and tach get their signal, or what changes have to be made going into the cab. I am almost getting discouraged at this point, lol. I know I can do this, but it isn't my truck and I just don't want to get too far in and realize I'm stuck. I've never been "intimidated" by a project before this one, but i've never not been unable to figure out and finish a project either.

I think my plan for now is to pull the 4.3 engine and harness and just start mapping things out. I have the workspace and I have the time with the truck, I just wish there was a little more easily laid out info on what exactly to do.

Again, thanks for your post and anything I got wrong or left out up there(I know there's alot missing) please let me know.

Electric fans are on the back burner, and I can easily install some standalone if I want them later I've done that before with just a temp controlled switch and a few feet of wire.

The tach wire coming out of C100 on the firewall will tap into the tach "white" wire on the pcm. The speedo will get it's signal from the VSS. It will be a challeng but you'll enjoy it if you have a little patience.

BTW, the VSS is pin #15 (Blue connector) on the VCM and is the yellow and black wire. There is also 2 other VSS wires for the Radio and another output that I believe is green.

I have a schematic for my 96 GMC K1500, I assume it will more than likely be the same. I also have the schematics for the 2002 corvette and 2002 escalade. Let me know what I can do to help also.. I am working on an E3D schematic in CATIA after I finish my CNC plasma table.

Rick, in 98 they changed the PCM up so there are a few differences. This is why HPTuners can tune a 98 and not a 96 or 97 truck. They are close though..........

I was under the asumption that the 5.3 was from a 2000+ vehicle. What year make and model is the engine from?

You're correct, the 5.3 is but he'll need the schematic for the 98 to tie in his wiring to the 5.3 pcm.

Ah, gotcha.. I can get it this weekend if you need it. We have Mitchell1 OnDemand5 at the shop. RC

I've got the factory manual but if you have it electronic I know he'd appreciate it.

It would be great if someone had it in e-form.

My confusion on the tach and speedo stems from reading that the old computer controls the speedo, and I assumed that both pcms needed the signal.

I have some materials coming from S&P, and hopefully they will line me out a little more. I am positive that everything will start to make more sense if I just pull the truck into my shop and get started.

I think I'm making a bigger deal out of it than it will turn out to be, but I'd rather be overprepared than get into it and find out I should have asked more questions. I never found a ~great~ writeup on this, so once I figure it all out I will try to put it all together in a way that really explains what you are trying to accomplish as far as which computer will control what etc.

When you're done and have it all figured out you could write up a detailed e-form of how you did your wiring for the next guy and he will be greatful. :thumb: I'm not too good at doing it while I'm doing the work, I usually write them up after the fact from memory, kind of like when I do my LS2 intake install.

Writeups like yours make the mechanical aspect as easy as could be by explaining what all needs to change and even how a person should go about it. I've worked on several projects of that nature before, and the specifics you provided about this particular swap make it child's play. You and S&P make sure there are virtually no hangups to that side of it, and I'd like to get a wiring writeup that gets as close to that as I can. Granted it varies so much between vehicles and engines that it can only do so much, but I'd like to help the people like us that just want to know what is entailed in doing it yourself.

I've done the LS2 intake swap if you run into problems. It's pretty straight forward though. Seems nobody can post a decent writeup about the swap without people ruining the thread by talking about they should have just gone LS6.