|Most likely this is due to a defective alternator. Occasionally a faulty battery will give the same symptoms (generally, an intermittent cell). For Delco alternators, this sometimes occurs if the sense wire on the alternator plug is loose or broken.
Before replacing any parts, I would check the following:
"Clean" in the above means metal that is free from any oil or corrosion. Usually a bit of wire brushing followed by degreasing with carburetor cleaner will suffice.
- Using a hydrometer, measure specific gravity of electrolyte in each battery cell. This is usually a definitive test to identify a faulty battery.
- Fan belt should take up a roundish shape (not triangular or oval) if removed from engine, and should be properly tensioned when installed.
- Battery connections must be tight and have bright shiny metal-to-metal contact. Usually it is necessary to brush or scrape both terminals and connectors.
- Clean wiring terminals and contacts on starter solenoid.
- Clean wiring terminals and contacts on alternator (check both ends of the plug).
- Clean wiring terminals and contacts inside plastic junction box located on main cable between battery and starter solenoid.