2005 sportsman 500 loses electrical after 15 seconds. Turn the key off and back on everything imediately comes back on for 15 seconds then goes out like you turn the key off. Not the ECM. New speedo and swapped left hand handlebar kill switch. Battery is fully charged. Anyone else have this problem? ECM has been updated but still swapped it with a good one with same result.

 

Sounds like the only thing you haven't switched out is the ignition switch itself. You can ohm the switch out,plus you can check the wiring leading to it for obvious bare spots or soft wires that have melted internally. The switch gets it's power from the ecm,but if faulty or it's wiring,it can trigger one of the solid state circuit breakers in the ecm. When it cools down for a few seconds you go through the same thing again. Either the switch or it's wiring or connector I would think may be the problem. One other thing is the fusible link,but if it goes don't think it resets,just melts.You can check that constant power is going through it. Located on the small wire to the solenoid I believe close to the battery. If it's a switch which isn't uncommon for one to fail,they're cheap.
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Have another polaris with same switch so I swapped it out and dielectric greased the connection. Have looked at the wiring and connections . Still the same. Key on 15 seconds and display goes blank and if running shuts off. Turn key off and back on everything is reset for another 15 secs. Cannot hear anything trip.

 

All the solid state breakers are in the ecm. Somethings causing it to trip and reset. Since you changed switches and no difference that still sounds like a wiring short.I'd start at the switch wiring and connector and work down under the steering post area.It's not fun stripping back the wiring loom the check for bare or melted wiring but that's what I'm thinking as this area,the headlight wiring and connector gets the most stress when turning back and forth.Same thing on the connectors and wiring from the left control switch down into the main harness.

 

Have you checked the stator ? Unplug the plug at the ecm { the one with 3 yellow wires } and check them to chassis ground - should be infinite .