Older OBS to LS wire harness setup

I have. A 1994 c1500 w/t that had the old 4.3 in it. I have the 5.3 dropped I with my 4L60 behind it. The guy from LT1 swap.com already did the pcm reprogrammig. Now for the harness !! I have trimmed it down like he stated in the lt1swap.com website. I have eliminated the emmisions part. I am missing a couple plugs that were cut off but I have followed the wires one by one to this : driver side O2 sensor plug - I have another donor LS harness and will cut, splice and solder to my modded harness, the VSS plug - I was going to cut it off my old OBS harness and solder to the LS harness, is it doable ?? PRNDL plug - i was not going to use this because i believe its just to indicate on the LS cluster if its in gear, is that right?? Also how the heck do I integrate the LS harness to the OBS one? Can anyone provide a step by step procedure or which obs wire to LS wire? I can not seem to find a thread for my year truck !! By the way, my truck is a "plain Jane" meaning, no cruise control, tach, power door lock, but I got one power passenger side window with a manual driver side window !!!

all you need is a set of wiring diagrams from your truck, and a set from the donor vehicle. compare the two and make necessary changes. eveything has just about been covered on this site. all you gotta do is a lil searching.

go here for diagrams.... http://search.ebscohost.com/login.as...ile=autorefctr

username - student
password - watc

are you going with cable drive or drive by wire?

Your most simple option is to take your donor harness for the LS engine and make it into a stand alone.. you will need to put your oil pressure sensor from the old engine onto the LS with the proper adapter.. you will need to re use the temp gauge sending unit wire that was plugged into the driver side head and proper sensor onto the LS engine.. you can use the speedo output wire from the new LS pcm and run it directly to the speedo wire at the cluster,,, the new PCM should have a MIL wire too that can be ran straight to the cluster... your setup as far as wiring should be pretty simple as long as you are comfortable and can read schematics

you should be fine with putting the VSS plug from your old trans onto your new harness, I believe they are the same

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Originally Posted by 1991SSP

are you going with cable drive or drive by wire?

Your most simple option is to take your donor harness for the LS engine and make it into a stand alone.. you will need to put your oil pressure sensor from the old engine onto the LS with the proper adapter.. you will need to re use the temp gauge sending unit wire that was plugged into the driver side head and proper sensor onto the LS engine.. you can use the speedo output wire from the new LS pcm and run it directly to the speedo wire at the cluster,,, the new PCM should have a MIL wire too that can be ran straight to the cluster... your setup as far as wiring should be pretty simple as long as you are comfortable and can read schematics

you should be fine with putting the VSS plug from your old trans onto your new harness, I believe they are the same

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I have a drive by cable set up. I have already have redone done the harness to stand alone. I have some knowledge of schematics but I will you tube it just to get a refresher. Hopefully tomorrow I can start on the harness. Today I hooked up trans lines, electrical fans, radiator remounted, other small misc stuff. Radiator hoses, fluids, drive belt, harness and PCM placement is all I need now to start it.

I wanted to know how the obs fuel pump will be controlled. Either from integrating the obs wiring to the LS harness or from the original obs harness? I hkave already unplugged the obs pcm from behind the glove box. I have not cut that plug nor the original harness from the engine bay. Also what about the wires connected to power distribution block located on the firewall ?? What wires get disconnected? I was going to get the 12 volt power source for the LS pcm from there along with adding a fuse. I did a stand alone harness and followed the instructions from lt1swap.com .The thing is that I did not completely cut out the fuse block from the original.

Your truck isn't that different from my '93 1500 4X4. Let me see if I can answer some questions:

Fuel pump:I believe you will need to have a '96 or later fuel pump for the Vortec injected engines. These pumps create higher fuel system pressures needed for the LS engine to run.
You can use the original fuel pump relay. The only wire that needs to be undone from the relay is the dark green/white wire in position D of the relay plug. This is the relay actuating wire from the old PCM. You will now need to run the same dark green/white wire from the new PCM, position #9, connector C-2(red or green, depending on year of engine being swapped) to the position D of the fuel pump relay. Basically, you're swapping out the old PCM relay control wire for new PCM relay control wire. It just so happens that they are the same color.

As for the power distribution block, I left all my wires attached since they power certain interior functions which have nothing to do with the engine swap. You can pull 12V from that block and fuse it for a constant 12V source for the PCM. You will also need a switched source of 12V for the PCM.

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Originally Posted by Mickeymotormount

Your truck isn't that different from my '93 1500 4X4. Let me see if I can answer some questions:

Fuel pump:I believe you will need to have a '96 or later fuel pump for the Vortec injected engines. These pumps create higher fuel system pressures needed for the LS engine to run.
You can use the original fuel pump relay. The only wire that needs to be undone from the relay is the dark green/white wire in position D of the relay plug. This is the relay actuating wire from the old PCM. You will now need to run the same dark green/white wire from the new PCM, position #9, connector C-2(red or green, depending on year of engine being swapped) to the position D of the fuel pump relay. Basically, you're swapping out the old PCM relay control wire for new PCM relay control wire. It just so happens that they are the same color.

As for the power distribution block, I left all my wires attached since they power certain interior functions which have nothing to do with the engine swap. You can pull 12V from that block and fuse it for a constant 12V source for the PCM. You will also need a switched source of 12V for the PCM.

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Thanks!! I have swapped the fuel pump to that of a later model already. I will try the fuel pump relay mod today and go from there.

I do not have the original alternator from the 5.3 . Is it possible to use the one off my old 4.3 that had a one wire connector? If so which wire is used form the LS alternator plug?

Not sure if it's possible to mount the old 4.3 alternator to the 5.3 bracket or where the brown wire would go on your existing alternator. You may have to bite the bullet and purchase one new or used from your local junk yard. If you do buy a new/used one, the brown wire should be at pin B of the LS alternator plug. This brown wire should tap directly into your existing brown wire for the battery light.


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I finally got it running !! But with starter fluid =( I got air, spark and NO fuel. Truck is a '94 C1500 W/T that had a 4.3 V6 motor in it that has been swapped to an '02 5.3 with EFI with DBC, vats removed, underhood fuse box from the LS truck that is modded like the one on lt1swap.com http://lt1swap.com/underhoodfuseblock.htm . I have upgraded the fuel pump to one of a later 90's V8. All the grounds are where they should be, but I will double check. But it will only turn on when I spray starter fluid in the intake. I do have fuel getting to the fuel rail, I check the shrader valve. I will check the pressure with a fuel gauge this week. Any suggestions?