I’m trying to figure out a problem with my sons, 1991, 240 Volvo . It will start when you jump it but then it’s completely dead if you turn it off and try to restart it trying to figure out the problem.

 

did you test the battery? What's the voltage across the terminals with the engine off? once you jump start, what's the voltage across the terminals with the engine running? When you say, completely dead, what happens when you turn the key w/o the jumpers? ignition lights come on? can you turn on the headlights? do you hear any sound when you go to crank it over? My guess is your battery has a dead cell and can't even make 12V.