check the voltage AT the alt main terminal, then compare to the batt voltage. We have had fusable like break down and would carry voltage but not amps necessary to fullfil the volt_amp requirement. Running a n10 wire to the pos post is a quick fix. IF this doesn' work as stated previously the alt is at fault.
Had same problem 2 years ago. Rebuilt the alternater but didn't help. Talked to Allan at R and R and ended up replacing console relay and that did the trick. My old relay would be so hot, you couldn't stand to touch it. Hope this helps. Talk to you later.....Dan
Don't overlook alternater even though you just had rebuilt. Some of these Delco replacement parts are junk. I wish a guy could still get original equipment Delco replacement parts. I think there are so many aftermarket suppliers that Delco quit making their own. Be sure batterys are charged up very good with battery charger before installing rebuilt alternator. This will give you better chance of rebuilt lasting. Otherwise currant trail to cab is starter to main breaker near engine to cab solinoid continuous power stud to cab solinoid switched power stud. Follow the trail to see where major drop is. Acceptable rule of thumb drop accross breaker or solinoid is .2 volts. Might want to clip battery jumper cable from good ground on cab to good ground on frame to be sure you don't have a ground problem. Good luck
Charged batteries one at a time, put in new cab selonoid(because Dan said to). Have 13+ volts with blower and all lights on........Houston we have lights! I think the weak batteries were the main problem. thanks for the help.