Help with sons RC snowmobile
 

So I have this Yamaha 1:6 scale snowmobile that my son got for Christmas that he loves, it has a 6.4volt battery pack and is 2.4Ghz, its been working fine up until a couple days ago when I removed the battery pack to charge with the supplied USB charger. when it was done charging as soon as I would connect the battery to the RC it would start spinning the track even with the On/Off switch turned to off. no response to the controller. I'm thinking a relay might be stuck? I have a spare Rc the same as his new one but older that we stopped using cause the ski spindle has broken off. but I cannot get the old one to pair with the newer remote even tho its the same 2.4Ghz. is there something I can swap from the old one to make the new RC work properly again

id check out that on/off switch. if power is running through it no matter what it may have given up the ghost. could be letting power through when its not supposed to and limiting power when it is supposed to. idk, that's just where i would start the trouble shooting.
also not all 2.4 gig radio's/controllers are the same. if one snowmobile is newer it may have a different system and not be compatible with the older one.

Anyone?