I've got a 1969 Mercury 50 HP that won't turn over when I hit the key switch. Put in a new switch, checked the cables, they appear to be good to the motor. Battery is hot, I can use jumper cables and go direct to the starter and it spins great. When I turn the key, it just clicks like a low battery. Any suggestions. Some of the wires in the cable are losing insulation (cracking) but checking with ohm meter doesn't appear to have any shorts.