I have an '87 Ford F-150. I just replaced the alternator,battery and selenoid. I did that because something was draining the battery. Now,I hook up the positive to the battery and it gives a spark,starts welding the clamp to the post and begins to start the motor...even with the key in the off position! I let it start with the key in the on position and disconnected the positive,and the alternator would not hold the motor. What do I look at first?