1995 Chevy Suburban won't start unless jumped

I have a '95 Chevy Suburban 454 dooley that won't start unless its jumped. Batteries are good, cables cleaned, alternator changed on autozone's suggestion(they installed it). It dies immediately if you disconnect the battery. Any suggestions?

I have a '95 Chevy Suburban 454 dooley that won't start unless its jumped. Batteries are good, cables cleaned, alternator changed on autozone's suggestion(they installed it). It dies immediately if you disconnect the battery. Any suggestions?
If the battery tested good and the connections are good and the starter is good, a jump would be unecessary, piece of the puzzle is missing. You need a full electrical test of battery capacity, alternator output and voltage drops on the pos and negative cables.

Dude, you got bigger electrical problems than needing a jump. (Suburban Dually??) This is one messed up electrical system. I assume you jump the engine correctly, negative to the dead engine's ground. When you disconnect the jump, it dies. I wonder if your battery is finding a good engine ground. I assume you cleaned the ground cable at the battery, how about the other end, at the engine. Undo this connection and ohm it to make sure you have good conductivity. Clean this connection, the cable and the post and nut at the engine. Do not add grease to this connection unless you live in a single wide trailer still on it's wheels with 27 members of your family. ( This grease inhibits oxidation, because it also inhibits conductivity.)

You may also want to check to see if there are any fusible links that could have burned out. Also check alternator output with a voltmeter to make sure it's working. If everything is working as it should, it should still run with the battery disconnected. A wiring diagram would probably be helpful for tracking this down.

Consider this thread closed, I suppose. I sold it to a man who had more money than he had sense. He wanted it; I didn't. I warned him. He still wanted it. I sold it. He loved it. That was about 4 years ago. It's still parked in his back yard. What else can I say? Thanks for the help.

On any modern vehicle, NEVER (did I state "Never" ??) remove the battery cables from a running engine to see if it's charging. This was somewhat OK back in the generator days and before solid state electronics, not on more-recent vehicles.

Pay attention, or live with the consequences.

I 'preciate you posting that bit of information. I've learned that a long time ago...but I'm glad you put that bit of caution on here.
Thanks