07 Cadillac dts. No crank. No communication at data link connector. Help please

I purchased a one owner garage kept 07 dts with a blown engine ie. connecting rod hanging out of the block 4.6 Northstar at 92,000 miles. I replaced the engine with a used 80,000 mile 4.6. Hit the key and nothing happened, jumped the starter and engine turns over. It has fuel pressure but no start. Dash cluster lights up, but when I turn the key to the start position all instrument needles sweep the gages from left to right and then back again. The cluster lights go black and then light up again.
There was a 15 amp blown fuse in the engine compartment fuse block, although I did not note the circuit. I replaced the fuse. Nothing changed. I also replaced a blown 20 amp fuse in the fuse block under the rear seat #F23 LTR/CNSL.
I have checked the engine block for ground and all grounds under hood show 12.6 volts. I connected a scan tool to DLC, and it read no communication. Just to be sure, I also connected a basic code reader which also read no communication.
I am stuck on where to go from here.
May end up in divorce court if I can't make this car run. Please help.

PEric wrote: all grounds under hood show 12.6 volts

You have a bad engine ground. Check the main ground cable between the battery and engine block.

Like pole71 said, there's a wide open ground somewhere. If all the cables are connected, start voltage drop testing at the block and work your way towards the battery. Tight eyelet bolts don't mean a good connection.

Did you ever find the fix i have the same problem

I have the exact same problem after engine replacement on a Cadillac XLR has anybody come up with a solution for what was going on here I've checked everything five times over all my grounds are good still can't get the car to spin over or communicate with the scanner

Did you try Global OBD? Start unplugging modules that share the data bus network until comms come back at the dlc.

Msuits85 wrote: I have the exact same problem after engine replacement on a Cadillac XLR has anybody come up with a solution for what was going on here I've checked everything five times over all my grounds are good still can't get the car to spin over or communicate with the scanner

Would you be willing to take some voltage measurements at the DLC?

Msuits85 wrote: I have the exact same problem after engine replacement on a Cadillac XLR has anybody come up with a solution for what was going on here I've checked everything five times over all my grounds are good still can't get the car to spin over or communicate with the scanner

Is this no comm with a scan tool or no comm with a diy level code reader? If it's a scan tool try manually entering the VIN.