[dinho10]

2010 f250
Clicks once and that's all, doesn't try to turn over
Both batteries tested. Does is sound like starter or?

 

[mlgjellybean123]

Assuming the base engine is good, starter or starter relay. Can test the relay by finding the starter wire (see attached image) and connecting the "right hand" side to postive battery post with the key in your pocket. If the relay is bad, starter will engage. If the starter is bad, it will not crank. We don't have many 6.4 guys left on here so specifcs will be hard to come by. What voltage are your batteries reading when isolated from eachother?

 

[GregWork]

Check starter and ground cables for tightness and corrosion.