Help! Motor In--Dead, No communication

The 2003 3500 HD we have had a 6.0 that let a rod loose and went thru the side of the block.
Since this truck is only used on the farm in a limited role we opted to buy a really low mileage 5.3 that was
complete (less than 5k miles) for 450.00 and put in it.

We have completed the swap, everything plugged up identical, no differences in wiring harness. The motors
even call for the same # ecu.

Now that we have it in and everything hooks up it is dead, won't crank. When hooked to a scanner it shows
no communication with the engine and tranny. It will read the dash, air bags, etc, but not the engine or
tranny.

The dashlights sometimes want to stay on with the key pulled out.

We know that the ecu is good, even tried putting a new one in, but have the same results.

Any ideas on what the issue could be? Any help would be appreciated.

did u put both ground wires back on?

Yes, lower ground wire connected to front drivers side of the block, upper ground wire that's hooked to firewall hooked to the rear of the block on the drivers side. I also thought that might be the issue, but both are firmly attached.
Thanks for the reply.

there are 2 ground wires on the harness as well that go on the back of the block...

Thanks, I've got three of the ground wires hooked up for sure, the two on the drivers side of the block and the one that is grounded about half way down the passenger side valve cover to a stud. Where is the fourth one located?
I must have missed it, any help on it's location would be appreciated. Sorry to be so ignorant, this is the first LS motor we have ever done, we've done plenty of the old sbc's, but all this wiring and computers is tough on us old guys.

there is a ground on the drivers side and passengers side rear of the block that will be part of the engine harness, the other two ground wires under the hood (if theses aren't' grounded the engine will not start) are not part of the engine harness, one will come from the battery and the other will come down from the firewall

could it be the VAT's?

Quote:

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Originally Posted by Red03SilSS

could it be the VAT's?

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probably not, you kept the same pcm initially and it would not start, correct? The computer has no idea what engine is in there, it will just try to run it based on its current programming. once you get your power issue sorted out you will need a tune. The 6.0 programming will run a 5.3, but not the best. The VE, fuel tables, spark timing, all that is going to be out of whack because of the smaller cylinder volume in the 5.3. The pcm will not auto-correct. It will "expect" a certain amount of air and thus inject a proportianate amount of fuel but it will actually be taking in less air and adding the same amount of fuel. The MAF will not correct it untill higher rpms as the engine does not use the maf for fuel calculations at low rpms, if it even helps. Being that the truck sees limited use, you can probably just drive it the way it is, but you might as well through a v6 in there if you arent going to tune it.

sounds like a ground to me to, when i did my 98 5.3 obs swap the ground to the relay was not connected so the key would turn the gauges would work but i would get no starter when turning the key. DOUBLE AND TRIPLE CHECK the grounds...

did you ever disconnect the pcm? i had a similar problem, it was the connectors.

Thanks for all the help! It turned out to be the ground on the passenger side not being grounded to the block in the proper location. My son had grounded it to a stud on the valve cover. This created a 12.5 volt back feed on the ground. Once the ground was properly located it started and ran. The computer I had the local parts store order by VIN and engine info did not regonize the wireless throttle set up. I stuck the original 6.0 computer that a local shop assured me that was fried back in the truck and it has worked flawlessly ever since.

I have not noticed any issue with the 6.0 cmptr running the 5.3, idle is good, throttle pickup is good, runs out perfectly.
This truck being a 3500 4wd with a 4:10 ratio, I thought it might be a little under powered, but it seems plenty waspy for a farm truck.

I hats off to those that have answered all our questions. Being close to fifty and not the most cmptr/electronic literate guy it seemed a little overwhelming compared to the old sbc, but after getting this one done it really might be a little easier in the future.

I tore down the old 6.0 with my youngest son. It was a mess internally, two broke rods, one punched a nickle size hole in the block. Amazingly this truck would start and run with the two broke rods, it would just shake you out of the cab lol.

I bought the truck out of a salvage pool for 2400.00, the 5.3 I gave 450.00 for and have 400.00 in a hood and grill.
So for less than 3500.00 I have a 2003 K3500 4wd with a 5.3 with approx 4000 miles. The whole truck had less than 80K miles on it.

We put a 9' flat bed on it with a 6 ton hoist run by electric/hydraulic pump which we already had off another old truck ready for the crusher. It has turned out to be a very nice looking, excellent performing truck for the money.

Again thanks to ls1trucks site and its members for helping it be a sucessful project.

glad you got it going!!