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

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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.


http://i488.photobucket.com/albums/r...s/scan0286.jpg

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?

Was the fuel pump for a 96 or newer 1500?

Yes. I double checked with a fuel injector pressure gauge at the Schrader valve in the fuel rail. Its at over 55 psi plus or minus.

well to me if she's firing with starter fluid, and u have fuel pasture at the rails (which means your pump is working) it sure sounds like the injectors aren't firing. which I believe could only be a few things. vats isn't removed (which u said u did...) or bad injectors/coils.... but if she tries to start with the fluid, then it seems injectors and coils are ok. sounds like a tune issue to me. I'm still new to this but I'm just using the knowledge I have. is there another pcm with the same setup u have that u could try? a buddy or something?

I know you said you have all the grounds on, but I would double check again. I remember reading a post here where someone else had a similar situation. Turned out he was missing a ground. The engine must be grounded to the frame and body. Make sure the connections are clean and don't have rust/paint around them. Clean if necessary.

mickey is right. there is a chance u got a stray ground somewhere. that happened to me. i was missing one ground which prevented it from firing. i didnt try it with starter fluid like you did. maybe thats the issue. my lose grnd was the one up front down low beneath the ps pump. thats a common mistake aparantly. check all your grounds. theese new motors now days rely on the computer for everything which supplies a gnd to make things work. literally one ground can screw a ton of stuff up. good luck. post back with results

Thanks guys. Unfortunately, I do not have a buddy that has done this swap that I can borrow their pcm. I will double check the grounds. I made two common grounds when I redid the harness. I have them bolted to the accessory bracket next to the alternator. The negative battery cable which comes out of the battery (battery is in the obs original location) is split into to two wires. Smaller One goes to the inner fender well, thick wire to the passenger side of the block. Then I have the the two common grounds bolted on the driver side on the accessory bracket. I then have a thick ground wire from the driver side block to the frame. And lastly a wire from behind the passenger head to the firewall. Does this sound about right? Also when I would check to make sure they were grounded, I would use a test light, with the clip connected to the positive terminal of the battery and touch each of the grounds and it would light up. Was that the corrected way to test them?

sounds like your missing some... there should be like 5 grounds or so just from the harness alone.... most of them are in the back. they should be coming out of the harness with ring terminals on them so they can be connected to the bellhousing bolt studs. i know for sure there is the one down low up front on driver side....but you have two??? not sure what the second is..

yes thats a way of testing but it doesnt mean its a good ground. could be just enough to light up the bulb. you should use a meter and set it to the resistance setting and touch one lead to the neg batt and the other to whichever ground you wanna test. the meter should from 1 to 0. if you have numbers, that means that is resistance (ohms) between the two points. that means its a bad/dirty connection or there is a slight cut/pinch somewhere in that wire.

now what i just described is only testing the ground path to the battery and not the wire itself. sounds like you got a decent path on each point, but i really think your missing wires from the harness that go to the pcm. if i was you, i would crawl up in there and look down behind the motor for stray grounds dangling.

Thanks guys !! I double checked the grounds and cleaned them up cause some of them did have paint. I used a multimeter and checked the ohms and theybwere all at 0.3. . She fired right up, but I had to manually accelerate it at the cable for it to stay running. It would shut off if I let go of it. Going to check the resistance of the MAF, TPS, and IAC valve. Maybe one of them is bad. Also what about the truck intake vacuum openings? Could that be a part of the issue? The opening in the back of the intake is connected to the brake booster, I have the original purge valve still in place but the emissions is deleted and reprogrammed that way. I have an opening on the top of the intake but I have a plug over it. I also have a plug on the fuel pressure regulator.

The fuel pressure regulator senses engine vacuum to maintain fuel pressure under all engine load conditions. It must be hooked to the intake manifold. You should also have a manifold absolute pressure sensor on top of the manifold. If either of these two are missing, let alone both of them, you will have some running problems.

Thanks mickey !! Sure enough, there wasn't a MAP sensor connected, period !! It was an open hole on top in the back of the intake. I hooked up the scan tool to the obd2 connector and it verified it. I bought one and put it in, but there isn't a plug for it on my harness. I may have taken it out when I redid the harness, but I do not see it in the box where I put the wires that I took out. I will double check tomorrow. Now it starts but I just have to hold the accelerator for a few seconds, let go of it and it idles by itself. Thanks mickey also for the part number of the serpentine belt u used. I got the same one and it works like a charm. I just have to remove some washers I placed behind the custom a/c bracket cause it about 1/4 to far forward and doesn't line up with the other pulleys. I used the 5.3 upper and lower hoses and just trimmed them a bit to fit on the original C1500 radiator. Thanks again everyone, almost done, now all the finishing touches ar3 left.

Glad to hear you found the problem. Hopefully the plug for the MAP sensor was overlooked and is still lurking somewhere in the harness.