Detroit Diesel Engine not starting

[David Carrol]

I had to jump start it about a week ago, it started easily, Now I can't get it started. Battery is showing 12.7 volts. Terminals have been cleaned. It barely turns the engine over. If I jump it there is no difference.
It's acting like the battery is dead/low.

What should I check?

David Carrol

[NewellCrazy]

Test your battery first and If the battery is good,look for a bad ground wire from the chassis to the motor/transmission. For a quick test take your battery cable and jump from the frame to the motor.

[rellick]

If what Sean said still doesnt work, could possibly need a starter, good luck!!