Help! Motor In--Dead, No communication

[indiansprings]

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.

[banker099]

did u put both ground wires back on?

[indiansprings]

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.

[banker099]

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

[indiansprings]

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.

[banker099]

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

[Red03SilSS]

could it be the VAT's?

[TimeLimitHit]

could it be the VAT's?

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.

[Wtfan18]

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...

[lechez71]

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