I have the same problem! a 1989 740 wagon, no turbo tho. Starts fine all the time until the weather gets warm here in So. Cal. I've got the same wire running to the battery to start it with the key on, and I hate doing this! When I try to start it, the dash lights come on and stay on, but you get nothing, no click or grind, nada. My hubby says I should replace the ingnition switch. I see no problem with the starter because it fires right up when you touch the wire to the pos. terminal. Could the problem be in the switch? I might just start changing out parts till I hit on the solution. Any advice would be greatly appreciated, thanks