13.8 volts is just squeaking into the acceptable range of 13.75 to 14.75 volts, but there's more to the story. First lets be sure the circuit is wired properly. One side of the dash light should get 12 volts from the ignition switch when that switch is in the "run" position. The other side of the bulb goes to the generator on that brown wire. The current going through the bulb is what tells the voltage regulator to turn on.
Once the generator develops output current, the voltage regulator puts 12 volts on the brown wire to turn the dash light off. You found the charging voltage is higher than battery voltage so you know the generator is doing something, but this only means it's ready for the rest of the tests. That is full-load output current and "ripple" voltage. You need a professional load tester for those tests.
AC generators put out three-phase output. If one phase is missing, ripple voltage will be very high and the most output current you will get is exactly one third of the generator's rated current. This is a huge problem with '87 and newer GM generators. Due to their design, they develop huge voltage spikes that can damage the internal diodes and voltage regulator, and interfere with computer sensor signals. The battery is supposed to dampen and absorb those spikes but as they age and the lead flakes off the plates, they lose their ability to do that. Failing to replace the battery, (unless it's less than about two years old), is the main reason for repeat generator failures on GM vehicles. Many people go through four to six replacement generators in the life of the car.
In some applications the voltage regulator responds to the loss of one phase, which is caused by a defective diode, and turns on the dash warning light. Some regulators see the drop in voltage from the missing phase and bump up the output voltage. The battery smooths that out, and that average is what your voltmeter is seeing. The missing phase may be why the regulator isn't turning the dash light off.
If the dash light circuit is wired correctly, have a load test done on the charging system.
Sunday, October 25th, 2015 AT 7:51 PM