1984 Cadillac Deville Ddiesel

The PATS security system would prevent the engine from cranking if the system does not "see" the right chip in your key. If the system is working correctly, the light should go off after you turn the key switch on.
You should have power on the small starter wire when the key switch is in the start position.

Won't start as in no rotation of the starter motor and no clicking sound? Or starter turns slowly with the engine? Or the starter and engines turns normally but will not start? Or starter turns normally but engine will not turn?

With the starter removed, have someone slowly turn the engine with a wrench (you may need to remove sparkplugs) while you check the condition of the ring gear around the flywheel. You may have teeth missing from it which would just let the starter free-wheel. Also check the starter's gear if you didn't do it before.
If those both check out OK, you may just have a corroded cable or clamp in the path to the starter. It draws 100-150 amps when loaded with the engine and any tiny bit of resistance will drop so much voltage that the starter won't fully engage.

Check the starter/solenoid - sounds like either one or both need replacing. If you can get the starter off - you can take it to an Auto Zone or an O'Reilly's to get it tested. I'm not sure of any tests for the solenoid (sometimes it's part of the starter) - some cars also have a starter relay switch. If that's the problems - it's usually cheap & easy to replace.

Since you replaced your battery, do you hear any clicking when you try to start the car? If not, replace your Starter and Starter Solenoid. You also have a Starter Relay, but these relays don't go out that often.

I’m happy to help further over the phone at https://www.6ya.com/expert/jeremy_69f3cc28d95bf514

It could be the starter. It could be a bad battery cable connection. Have you tried jump starting the engine. If it starts, then most likely it's to do with the battery or charging system.

your starter is under the intake manifold. Before replacing starter, recommend you read my post at http://www.fixya.com/cars/r5973094-starter_replace and let me know if you have questions.

Sounds like you blew fuse. Fix fuse and borrow a car starting switch they use at car repair shops. As long as you can turn tumbler to on ,you can start using the starting switch from repair shop..There should be a way to run a start button to start car and your off/on key should be OK
Ron

seems like you have yourself an engine misfireing or you may have a defective starter.
your description was not clear,

sounds like jumped timing belt maybe chain ,no spark is my guess if it turns over and dont even make a pop noise..