Follow rj713's adivce, it could be also (as he says) a bad DC in board. If you're sure the charger's good then that's what's most likely. I doubt a PMU reset would make much of a difference, but it's pretty easy to see if the charger's good or not. If you purchased from a shoddy seller then it's ''very'' likely it's bad, wires get broken easily.
If you end up needing a new DC-in board, here's a [http://www.ifixit.com/PowerBook-Parts/G3-Pismo-DC-Sound-Card/IF106-003|link].
If you end up needing a new AC adapter, here's a [http://www.ifixit.com/Apple-Parts/PowerBook-G3-AC-Adapter/IF901-013|link].
Some advice for using those pismo chargers though, when leaving it charging, either fully unwind the "Yo-Yo" or leave it next to your computer, not underneath or above. The problem with the "Yo-Yo" style (which prompted them to change the cord style in later G3s and then G4s) was that the coiled wire carrying a current turned it into a solenoid, which creates a magnetic field, which means bye-bye for your hard drive. Just some advice that'll keep your pismo around a lot longer.