Join Date: Oct 2008
Location: Seattle, WA
Feedback Score: 0 reviews
Is the 'charging system' light on? (looks like a bttery symbol on the dash)
Do you have continuity between the alternator output stud and the positive battery terminal? Between the alternator case and the negative battery post?
If all the above is fine then disconnect the positive clamps on both batteries and test for a parasitic drain (multimeter set to amps with one test lead on the positive battery post and the other on the positive battery clamp). If you get a reading over a couple tenths on an amp (excluding hood light) then there is something in the system drawing more current than it should when the truck is shut down. Pulling fuses one at a time under the dash board might be a good place to start if that's the case.
Hope that helps.
1990 F-250 XLT Lariat 4x4, reg cab, 7.3IDI N/A, T19C (swapped in place of E4OD 11/10), dana 60 front, rear grizzly locker 4.10 gears, 35" mud tires, 5" lift