Motor will fire with key,as soon as you let go it will die, if you hold key to start it will stay running but starter stays engaged
If that engine is a Cummins. Then look at the fuel shut off solenoid on the injection pump. There are three wires on it. One should only have power while cranking and one should remain powered when the engine is running. The third is ground. I suspect that the hold wire is not getting power. If that solenoid doesn't pull all the way in then it will pull too many amps and blow the fuse.
I’ll check, thanks
That’s it, I will start looking for a fuse or wait for some help to keep solenoid open while running and check power at plug on 3 wire