Thanks for adding this fix, but there is more to the story. GM has had a real big problem with their generators starting with the 1987 models when they were redesigned. They develop huge voltage spikes that can damage the internal diodes and voltage regulator, and interfere with computer sensor signals. It is real common to go through four to six replacement generators in the life of the vehicle.
The solution is what you already did; replace the battery. The battery is the key component in damping and absorbing those harmful voltage spikes, but as they age and the lead flakes off the plates, they lose their ability to do that. Any time a generator is replaced on a GM product, replace the battery at the same time, unless it is less than about two years old.
I must add to some of your comments too. 12.4 volts means the battery is good but partially-discharged. A fully-charged battery will read 12.6 volts after any surface charge is removed. A totally drained, good battery will read near 12.2 volts.
Next, you only said the generator tested "okay" and it was a false reading, but you did not include any numbers. Testing off the engine is almost worthless. A good AC generator can easily require over five horsepower to develop its full output current. In-shop bench testers rarely even have a one horsepower motor to run the generator, so there is no way they can run it to its full potential. That full potential is the "full-load output current" test, and is critical in evaluating the performance. The only thing bench testers are good for is to verify the voltage regulator is working properly, and the generator is capable of developing some output.
Of concern in this case is that full-load output current. Every AC generator has at least six diodes, which are one-way valves for electrical current flow. As I mentioned, it is real common for them to be damaged in these generators. With one failed diode of the six, you will lose exactly two thirds of the generator's capacity. 30 amps from the common 90 amp generator is not sufficient to run the entire electrical system under all conditions. The battery will have to make up the difference until it slowly runs down over days or weeks. Most bench testers do not test for a failed diode. You need a professional on-car load tester. Some actually print out the "ripple" voltage, which will be high with a failed diode. Most just show a relative reading from "low" to "high" on a digital bar graph.
A lot of people falsely blame the quality of their replacement generator when they fail in a few days and/or multiple times. You said the charging voltage dropped from 14.9 volts to 11.5 volts, but you did not say how long it took for that to occur. The 11.5 volts indicates the battery was badly discharged, and there was a high load on it to run the electrical system. Testing the generator on the test bench only lasts a fraction of a minute. How does that compare with how long it took to fail on the vehicle? Diodes are almost always good, shorted, or open, with no in between. When they fail, they do so instantly and permanently. The method of failure you observed is typical of how a voltage regulator can act as it gets hot. On any other generator with a built-in regulator, they are fairly easy to replace separately. On 1990's Ford products, for example, they can be unbolted and replaced within a minute or two. With GM's miserable design, it is not practical to try to repair anything because they were not designed to be taken apart. Repair parts are available now from aftermarket sources, but the diode block is almost always damaged in the process, and there is no way to test the voltage regulator. Given the high difficulty, you do not want to do this a second time, so you must replace any parts that could cause the problem. You are better off just replacing the assembly with a rebuilt unit.
Also, be aware that for many GM vehicles, it is common to find quality rebuilt replacement generators with a one or two-year warranty, for a very high price, and one with a lifetime warranty at a much lower cost. That would have been a good deal for my friend if he would have found it at the time. He is on his third replacement generator on his 1999 Suburban.
Tuesday, January 17th, 2017 AT 2:22 PM