Pingti3, sounds like you don't have a good connection from the inside of the battery cable connectors to the battery posts.
Clean both parts with sandpaper and put it back together. Also check the ground connection from the minus cable to the body.
Claude1225, sounds like you have an ignition switch problem if the starter starts spinning as soon as you connect the jumper cables to the battery post under the hood.
Should do the same thing if you connected the jumper cables to the battery because those cables go to the same place.
2000 540/6 Slightly tweeked...everywhere