2008 Ford Explorer won't turn over most of the time.

Firstly make sure the battery terminals are clean, both the cable and the batter posts. Check also the amperage draw on thee main battery cable to the starter. If the Amperage is too high on the main starter cable you will have this problem. Check also the condition of thee main earth cable from the battery to body earth.

Thanks for getting back to me, firstly. I checked those two and everything was clean and metered well. It was just a bad ground. When I hooked up the negative terminal to the engine using jumpers, it started every time. When I disconnected them, the truck didn't shut off, but when I went to turn it back on, I got nothing. Thanks again though!

Pleasure.