Bleh, trouble with my diagram is it isnt 100% accurate in terms of where the wires come from and go to on the battery wire wire. However, if your setup looks like this *see attached picture*, then that is correct and it should work fine.
Q. how did your friend have the esc wired up when he was using it? Im gonna assume he was using both battery input leads with a pair of 2s lipos or pair of 6/7 cell nimhs? If it worked alright then without any external BEC or other mods, then either the internal BEC has become damaged, or its not liking being used on 4s lipo with the suggested wiring setup. Beyond that, Im thinking you may have to send it in for non-warrenty repair if the BEC is acting up- making it work properly with an EXternal one isnt as easy as it is with an esc with only one set of battery leads ( I cooked a bec doing that once, then discovered the internal one was fine with a single 4s pack anyway, go figure
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