Wiring original truck harness to LS

[C.I.Blazer]

Starting to work on the wiring phase of my LQ4 swap into my 71 Blazer. The original truck wiring is a complete mess. So many things have been modified and modified poorly (wires twisted together with tape, wire nuts, loose wires, etc.) I am trying to make sense of the wiring in the engine compartment and what i need with my standalone LS harness.

On the passenger side of the bulkhead connector, I will hook up 12PPL to the new starter and run 20W/OR/PPL (or a replacement wire since mine is trashed) to the PCM for 12v ignition power. I will keep 20T and 20DG as I am trying to use my stock temp gauge. I do not have 20DBL (have mechanical oil pressure) or 18LG and 18DG.

On the driver side of the connector, I think I can remove 16BRN as well as all the wiring to the alternator and voltage regulator since I am using the LS harness to control the new alternator. Obviously I will keep all the wires for the lights, horn, etc.

I am unclear on 12R and 18B that connect to the junction box at the battery. It looks like they both supply power to the rest of the truck system so I assume I still need them? Can I simply run 12R all the way back to the connector and remove the junctions to the alternator and voltage regulator? Do I still need the fused 18B/W that attaches at that junction?

I am posting a low res pic below, but I am sure you all already have the high res version. As always, I very much appreciate the help. I have done my best to research prior to asking this question.

[C.I.Blazer]

Quick update after playing around poking around this morning. I am going to remove the 20W/OR/PPL from the connector and pull the wiring for the PCM from inside since that were it will be mounted.

Still looking for feedback if anyone can help.

Thanks.

[mongocanfly]

I'm definitely not familiar with all the wire numbers you have listed...
But the LS harness is pretty much stand alone...only needing a constant hot, switch on hot, and grounds..
Constant hot I ran from the junction block that's on the firewall underhood..
Switch on hot I used the wire that went to the distributor..
Then ecm needs the water temp sender that's on the drivers head..for temp gauge I added a sender to the pass side head...the ecm doesn't need to see oil pressure..but you'll need a metric to standard adapter to use your sender...
Are you using a aftermarket LS harness?

[C.I.Blazer]

Thanks Greg! Yes, I am still settling on a vendor but likely to go with a PSI harness. I think I am hooking up similar to you but pulling my switched hot directly off the ignition switch inside rather than the distributor wire in the engine compartment. Just seemed like less routing through the firewall since my PCM will be inside.

Driver's side temp will be used for the LS but will add one to the passenger side that I am hoping to use for my factory gauge.

Guess my main question is about getting rid of the excess (old) alternator wiring (since that will be done with the LS harness) and the old ammeter wiring since I will not swapping for a voltmeter instead. Just my OCD getting the best of me wanting to remove any excess wiring.

[mongocanfly]

Yeah I dont blame you for cleaning up the wiring...on the "switch on hot" wire theres no issue coming from your switch..the distributor wire I pulled back in the cab, so it doesn't go under hood and back into cab...
On using the pass head temp location..I drilled and tapped the metric hole to npt...
On the alt wire I ran the charge wire from the alt to the firewall junction block...not sure if you have that or not..
PSI harness is a good choice..

[LS short box]

On your constant 12 volt I would pull it right off the main starter stud which comes directly off the battery cable. Makes for a real nice clean 12 volt to the ECM. It looks like you are using your original chassis harness and fuse block. It's old and may not provide a solid 12 volts to the computer. Also you can't have to many clean grounds.

[Andyking1203]

Very helpful info guys!!! Thanks

[nsb29]

Don’t know if you have a voltmeter in mind I just added one to my dash the first one I bought If I remember right came from an early 70s truck would have had to do a lot of modifications as it turned out it it didn’t work anyway so I found one on the Internet that was an 1 1/2 diameter for $15 I took it Out of its case unscrewed the face plate and where the rivets had been on mine screwed directly on to the volt meter I only had to clearance the needle and it perfectly in the space of the old amp meter

[pilotXC]

The ammeter works on my swap. I went around and around to get to the right wiring set up for the alternator to function with stock gauges and new PCM... I posted a diagram here, post #49:

http://67-72chevytrucks.com/vboard/s...=796095&page=2

As Mongo said, pull the distributor wire into the cab so you don't have to add one more Frankenstein splice into your system... My ignition wire actually reached the plug on the fuse box in the cab, so I just removed the old HEI wire and put a connector on the PCM wire and plugged it into the fuse block. The wiring for the PCM is really simple, I ran the battery PCM wire to a buss on the core support, it's in the diagram.

[MalibuSSwagon]

On my swap I pretty much stripped out everything except for the sensor wires for the gauges and the power wires. I dropped the Silverado fusebox under the hood and used it to power up the truck harness, and then of course the wire for the coil to key-on power the block, starter wire to the starter relay, and thats about it I think.

[nsb29]

I went back and reread blazers post and pilots one has a 71 the other one has a 72 they both come with voltmeters. I think they switch from amp gage in 68, an amp gage won’t take the new alternators my old one looks like toast from 100 amp my LS is 140

[C.I.Blazer]

Thanks for the help all. I think I am clear now on the connection to the LS harness and will follow the great advise given above. I am going to swap from my ammeter to a voltmeter. The swap looks pretty straight forward and I can pull all the needed wiring from inside the cab. I believe I can get rid of the wires I listed above and am at least going to give that a try. If course i will keep them in place until I get everything running, then unpin and remove them if all goes well.

Looking forward to my first start...