Yup. You successfully performed the charging voltage test, but that only means it is okay to do the second half of the testing, and that requires a professional load tester. The generator needs to be tested for "full-load output current" and "ripple" voltage.
All AC generators use three-phase output and a minimum of six diodes. Diodes are one-way valves for electrical current flow. With a single failed diode, the most output current you will be able to get is exactly one third of the generator's maximum rated current. Thirty amps from the common ninety amp generator is not enough to run the entire electrical system under all conditions. The battery may have to make up the difference until it slowly runs down over days or weeks.
Also, the loss of one of the three phases results in a momentary drop in output voltage. On some models, the voltage regulator sees that and responds by bumping up charging voltage in an attempt to get it back into the acceptable range of 13.75 to 14.75 volts. This variation is that ripple voltage. The battery absorbs much of that ripple voltage, and your digital meter responds much too slowly to see it, but a professional load tester will measure it. High ripple voltage is simply a clue pointing to the cause.
The voltage regulator also turns the dash warning light on, and it does that in response to the drops in voltage caused by the missing phase. The fact the warning light goes off after a while is most likely the result of higher engine speed where the voltage dropouts occur too quickly for the regulator to see them. Also, the battery becomes fully recharged within a few minutes after cranking the engine, so it puts less of a load on the charging system. At that point it is easier for it to maintain a smooth and steady charging voltage in spite of the missing phase. The clue is you may see the warning light turn on again when you turn on multiple high loads like the head lights, heater fan, and rear window defogger.
Wednesday, January 4th, 2017 AT 5:07 PM