did you ever find the problem, I’m having the exact same issueI have.
I have no idea there is adequate voltage throughout the vehicle and all of the Electronics work 100% just won't turnover until a direct Battery jump is connected.
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What’s a secondary power source, and does that mean I need a new starterIf you can hook up a secondary power source, and it works, then it has to be problem in the chain between the battery and the starter. Some Durangos have had issues with relays in the starting chain.