04 Silverado 4.3 to 4.8 Swap Need Help

I hate to do this but as I cannot find any info where it seems anyone has had a similar problem...I posted on performance trucks with no help however I have a 2004 Silverado started out as a 4.3 5 Speed truck I scored a 4.8 out of a 2000 with low miles for cheap and also came with the harness from the 2000 maybe someone can help me out with a few problems i've ran into. We currently have it in the truck and all hooked up every ground and every plug minus a couple that go to the transmission because apparently this harness out of the 2000 is out of an automatic is it possible to use this harness or will I need a different one? As of right now none of my gauges are working in the truck and when you turn the key on and try to start the truck as soon as you engage the clutch without even turning the key it tries to start itself however it is not getting fuel and will not start. Hopefully someone can guide me in the right direction here in what i'm doing wrong or what I need to change. Everything off the 2000 harness seemed to plug into eveything on my 2004. Are the fuse blocks different is why i'm receiving these problems or do I need to get a newer style harness? I want to retain the cable style throttle body as the truck was drive by cable is why i've refrained from picking up a newer style harness. Ive read the nbs v6-v8 threads over and over trying to get a solution. Also I forgot to mention the pcm has already been flashed for the 4.8 and mods by nelson performance so once i can work out all the small problems it should be good to go.

with the truck being an 04 u need a harness for DBW i dont think u can use the harness for a DBC with an 04 but i may be wrong

Well my truck is currently drive by cable as the 4.3 was when I pulled it out

i know this but with the v8 they are DBW

Yea I understand...Nelson has already set up the pcm (red/blue connectors) for the drive by cable i'm thinking my biggest problem with the harness is that its a 2000 harness and with my 2004 fuse block the pin outs are different in that harness causing numerous things not to be working. If i'm not mistaken the 03+ are blue and green which are different pin outs as well

the pinouts for red/blue and blue/green are for the pcm....i think your problem is the bcm in your truck isnt liking the 2000 model computer and thats causing it not to run..you might be able to swap bcms but you would have to swap gauges too... im not sure that would work though

Chick spent a month trying to make his work like that and finally gave up. Save yourself trouble and headaches. go spend $150 on a 03+ harness,tac,and pedal assembly and be done with it

Yep so much easier to convert to DBW and use the same year harness and PCM.

yup plug and play except maybe O2's. few hours at worst

What about the 2 plugs on the trans since mine is a manual truck not an automatic and will my current tuned PCM from Nelson work with the 03+ harness? I know theyll have to retune it for the drive by wire

What about the 2 plugs on the trans since mine is a manual truck not an automatic and will my current tuned PCM from Nelson work with the 03+ harness? I know theyll have to retune it for the drive by wire. And I'm guessing I'll have to get a different throttle body as well

Also my buddy that's doing the swap works for gm and we located a part number for an 04 4.8 engine harness for a manual truck with drive by cable and with his discount I can get it pretty reasonable I may go that route to keep from having to buy so much extra stuff. Should all the sensors from the 2000 motor work on an 04 harness or will some have to be changed.

Not sure why no one mentioned but for how much work this is, please put a 6.0 in. The 4.3 V6 and 4.8 V8 have too similar power curves to make this worth the effort. ASSuming stock to stock engines with no mods you are gaining 20 HP and 10 TQ.

That would of been fine if I hadn't of gotten the low mileage 4.8 complete with everything for $300 and it's not stock it now has a cam, springs, pushrods, ls2 timing set, stainless long tubes and some other stuff and eventually it will be boosted. I had considered a 5.3 or 6.0 but I got a good deal on this and I've seen people make good numbers on a 4.8

Hey, good try. Better than not trying.

Go drive by wire. Please. If your buddy is a GM tech, then he can show you what wires to pull from your harness for the 5speed. There's even a thread by some Frenchy guy showing what wires to pull that shows what to pull from the auto harness. Then you just cut the clutch switch pigtail off your 4.3 harness wire it up to what's left in the V8 harness and call it titties.

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Originally Posted by 559 2TD

Go drive by wire. Please. If your buddy is a GM tech, then he can show you what wires to pull from your harness for the 5speed. There's even a thread by some Frenchy guy showing what wires to pull that shows what to pull from the auto harness. Then you just cut the clutch switch pigtail off your 4.3 harness wire it up to what's left in the V8 harness and call it titties.

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If I go this route I need to change the throttle body need the tac module and pedal set up and that should be it for the dbw isnt it? I've already had nelson tune my pcm for drive by cable and its the red and blue connector the 03+ dbw is green and blue isnt it will my other pcm still work or will they need to swap it out for the right one or will they even swap it out I wonder

Yes you'll need a new pedal, TAC module and DBW TB and related wiring (between the pedal and TAC). Considering Nelson does free re-tunes it shouldn't be an issue to get the proper PCM. This way everything plugs in, everything works like factory, and it's just less of a headache.

Ok need some assistance I know a lot of people will say they don't exist but I need to find a VIN for Allen @ Nelson to flash to my PCM for my truck, it has to be for an 03+ truck that is a v8 and drive by cable. I know the truck does exist i've only seen one so far and this was before I decided to do my motor swap so I didn't think to grab the VIN however another one has to be out there since GM did offer the harness. I bought a harness through GM for a 2004 v8 truck with a manual transmission and DBC. Everyone keeps saying go DBW well its to late for that and I would of had just as much $$ in buying all the DBW stuff as I would just buying the DBC harness from GM and I didn't want to go through all the trouble of pulling wires from an automatic harness I just wanted a simple plug and play with what I have.

The Express ans Savanna vans stayed DBC. Use a tune from one of them. Or take a look through the HPTuners tune repository

Alright well got back to work on my truck again...now more problems. I ended up buying a 2004 manual trans harness from GM with dbw and then went and got the pedal and all the stuff to make it work. Got it all installed in the last couple of days I have double and triple checked every plug, wire, and ground to make sure i'm good to go. I go to start the truck and it does nothing its like it is dead from the ignition. Called Nelson today to make sure my VATS was turned off and indeed it had been. Security light hasn't been on but it was another check to make sure it had been turned off. I can get it to start for a few seconds by jumping the starter relay under the hood with a wire but it wont stay running. Tonight we went through every circuit and relay and switch in the truck to make sure they all had power when needed when the clutch was pressed and key on etc...and I still have nothing when I try to start it. Everything comes on and works accordingly but it just will not fire. If anyone might have any insight on what this might be or have had the similar problem please let me know i'm about at my whits end trying to figure it out.

Anyone?

If you're sure 100% that everything is wired correctly pay NP to ship you another PCM and see if it's just a bad load or something, cover that base.

Im more than a 100% sure it's all correct I've went over every single wire from the dash harness to the motor to pcm

Maybe ns switch? I know if that is not working correctly it won't do anything...

Reading through this real quick, but are you trying to use a RED/BLUE pcm with a 2004 harness?

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Originally Posted by PitchBlackC3

Reading through this real quick, but are you trying to use a RED/BLUE pcm with a 2004 harness?

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No, Nelson swapped it out and sent me the right one the blue/green and everything is wired just as it should be from the factory my truck is throwing a bunch different codes now and still is only able to start it by jumping the starter relay cannot get it to crank with the key. Here are a list of codes its currently throwing according to a tech2:

P0068 throttle body air flow performance
P0102 maf circuit low frequency
P0412 secondary air injection solenoid relay control circuit
P0418 secondary air injection pump control circuit
P0801 reverse inhibit solenoid control circuit
P0803 skip shift solenoid control circuit
P0804 skip shift lamp control circuit
P0856 traction control torque request circuit
P1516 tac module throttle actuator position performance
P1630 theft deterrent learn mode active

Keep in mind me and my mechanic buddy have went over every bit of the wiring we have schematics for all the starting circuits, engine harness, pcm harness and body harness and still cannot find an issue of why it will not start with a key everything has power where it needs to go and ground when needed but no dice when trying to start it.

Some of those codes are odd, you shouldn't have AIR codes at all on a 2004 and the NV3500 doesn't have skip shift or reverse lock out solenoids so you shouldn't have those codes either. Almost makes me think you have a F-body tune loaded, but if that was true your DBW throttle wouldn't be working at all.

Also the theft deterrent learn mode code is a little strange. That should be a self clearing code after the key is turned off.

I imagine VATS was turned off, but maybe Allen will chime in with what he did for your tune. (I see where you said he checked it already now)

After talking with Allen this is the email he sent me yesterday:

Here's everything you requested we tune for:
4.8L, 224/224 562/562 114
longtubes, exhaust, cai
remove: cats/o2, egr
31lbinj, 4.10
87/89 oct
* speed limiter 150


Those codes below are definitely harness related.
When the pcm sees for example a low freq maf code, that is a ground or the
maf wiring between the maf and pcm.

When the pcm sets the tac and throttle body codes, it's wiring and tac/pedal
related.

I can turn off the air injection codes, the theft deterrent, skip shift
codes, but those codes are showing up because there are no devices present.
But they aren't causing the pcm to act up.


The theft codes is coming from the body control module, the pcm has vats
turned off due to the engine conversion.
Vats is not on in the pcm.

We are definitely dealing with wiring, ground and power issues here.

I would agree all the PO800 codes seem to be F-Body codes that don't have anything to do with my truck and not sure how I could possibly have any harness issues the harness is one I ordered specifically for my truck from GM for a 2004, 4.8, 5 Speed with DBW damn thing was $500. I have checked all grounds and circuits and still not getting anywhere with this thing. Indeed the DBW stuff is not working with my pcm and it was removed from a running driving truck at the salvage yard even the same year as mine. I'm about to go light it on fire and roll it out of the shop its getting to that point.

Did you check at those sensors and make sure you don't have gnd on the signal side when it shouldn't be there? What about the voltage on the 5v reference circuits? You checked the gnds on the back of the heads?

Try unplugging the BCM connectors and see if it will start without it plugged in.

Which sensors? I know my buddy had checked all the starting circuits from the neutral safety switch to the starter relays all the way to the pcm. I'll run out and see if it will crank without the bcm hooked up.

I would start at the MAF and TAC module and pedal since those are the ones throwing codes.

Just tried to start it with BCM unhooked still same response as before. Like it follows all the way through with wanting to start but no crank.

Three things I can think of (and I'm no expert):

1) Are you sure the connectors are connected to the right receptors? I know GM uses different connectors almost everywhere, but sometimes they have duplicates. Maybe you crossed a couple of connectors somewhere (just a thought).

2) If you can energize your starter at the relay, track back from there. Seems to me you are not getting the signal from the PCM to the relay to make the magic happen. Things seem to be working forward of there, so you can almost eliminate that. Divide and concur.

3) Make sure you don't have any bent pins on the PCM or BCM or any other plug. Those things are a true PITB and really hard to diagnose.

Connectors are all plugged in correctly, and you can jump the starter at the relay and veh will crank so that would cut the sys in half, i have the 12v from the clutch safety switch when it is depressed at the relay, have the 12v that goes to the starter at the relay also the only thing not present is the ground side that the pcm controls. Seems to me cause i have checked every thing else i know that the pcm is the issue as none of the codes present should have an affect on the veh cranking, and btw i have 12v coming from the ign switch to the crank fuse also. i have ohm'd all these wires and all test just fine and as he said the engine harness is brand new, and the dash harness was just fine when it was in the truck before so to me the only thing that has been changed is the pcm.

After much debate got my new PCM from Nelson yesterday and got around to installing it today truck ran just fine and started right up as it should with the key.