06 f350 5.4
Have put a new battery and alternator in the
truck and it still only runs off the battery I’m
getting 12.5 v off the positive cables
That, as I'm sure you know, is too low. Check all your battery and alternator harness connections, look for any abnormal wear on wires, and be sure to check the fusible links.
Use a 12 volt test light and check for power going to alternator ( test wire that is held on with screw ) should have power even with vehicle not running. If no power check for blown fusible link in wire going to alternator do not believe that year vehicle has an alternator fuse but it may.
You get 12.5 volts at the battery with the engine running?
I think I figured it out my instrument cluster was fried and wasn’t telling my alternator to charge. Is this true ?