We have a 2006 883 Sportster. Turn the key lights come on, but do start dimming. Hit the start button, hear a pop and then it's dead. What is strange is the aux lights do work (tack). This happens a couple of times, so decide to take the battery down to be tested and am told it's dead. Buy a new battery and charge it up. Put it in the bike. Turn the key on, the lights are bright, hit the start button and pop. It's dead again except for the tack light in aux mode. This time the lights won't come back on at all. We did test all the fuses, the starter relay, the battery. Prior to this it had been running perfect with no indication of a problem. Where do we start? Thanks

 

Check the big ground cable from the battery to the tranny case.
Could be corroded or loose.

 

Also check for corrosion on the fuse tabs & sockets in the fuse box, also the tabs & sockets for the two black relays in the fuse box.