That might be a little extreme. If any parts supplier knew they had something with an unusually high failure rate, you can be sure they would address it. They aren't going to sell a product knowing they are likely to have to replace it under warranty. They don't want that publicity or reputation.
Well, there is an exception though and that is GM. They had, in my opinion, the world's second best generator design up through 1986, but they redesigned them for the '87 model year and turned it into the worst pile ever. There are plenty of perfectly fine parts that come from China, Japan, Korea, Mexico, and many other countries. At issue with this U.S. Design is their tendency to develop huge voltage spikes. It is real common to go through four to six replacement generators in the life of the vehicle, and it has nothing to do with the rebuilder or where they're located. If you suspect a failing generator, talk with the engineers at GM.
To reduce the number of repeat failures, replace the battery at the same time unless it is less than about two years old. That is the key component in damping and absorbing those harmful voltage spikes, but as they age, they lose their ability to do that.
I'm not sure why you're running at 4,000 rpm that often or why you're watching the gauges instead of the road, but you have to consider that to have the problem only show up at higher engine speeds, it has to be due to centrifugal force or an electrical issue. I suspect we can rule out centrifugal force because that wouldn't affect multiple generators. When they're rebuilt, the only rotating part, the field winding, is not replaced. It's an original part and it obviously didn't fail earlier. Also, it wouldn't expand and hit the stator winding without causing permanent damage.
The voltage spike issue has to do with exactly the same thing that takes place in an ignition coil. The voltage regulator turns field current on and off 400 times per second, and it varies the percentage of off-time to on-time to vary the current output. When current flow is turned on, it takes some time for the magnetic field to slowly build up. When current is turned off, that magnetic field has no choice but to collapse instantly, and that is what induces a huge voltage spike. That's desirable in an ignition coil to fire a spark plug, but in the generator, those spikes can damage one of the six diodes or the voltage regulator, and it can interfere with computer sensor signals. In fact, GM generators are often responsible for elusive engine running problems that defy diagnosis.
The first thing to do when you have a GM generator failure is look at the battery. If yours is over two years old, don't blame Carquest or any other parts supplier. Blame the engineers at GM who have no interest in improving the design of their generators.
The next thing to look at is if there are external controls for your generator. If you look at Chrysler's design, the voltage regulator lives inside the Engine Computer, and thus it can take advantage of everything the computer knows. AC generators can easily take over five horsepower to run when under high load. When you're trying to pass a freight train, while going up a steep hill, and when pulling a trailer, that five horsepower might come in handy, so the Engine Computer can command the voltage regulator to turn the generator off under certain conditions. That can include wide-open-throttle, engine cranking, engine running too hot, and things like that.
That capability of temporarily reducing output and load on the engine is built into GM's voltage regulators but it isn't used very often. If it is on your car, that can explain why the output is being reduced at higher speeds. The simple fact that output is restored when engine speed comes back down to normal proves no catastrophic failure is occurring.
The other thing related to generators is you need three things to make them work. That's a magnet, (electromagnet in this case), a coil of wire, and most importantly, movement between them. That's why we have to spin the field winding with a belt-driven pulley. The faster it spins, the more voltage spikes are being generated. Those spikes could overwhelm the voltage regulator and send it into a momentary shutdown to protect it.
A less-common problem has to do with what the engineers did to get higher output current capability out of a small package. They did that in part by decreasing the distance between the stationary stator winding and the spinning field winding. You were lucky to get 65 amps out of the old design. The current design is much smaller but 140 amps is not unrealistic. The first problem is when just a little play develops in the bearings. That lets the core hit the stator frame. In real bad cases it locks up. I've seen two where the generator stalled the engine, and people cut the belt to allow it to be restarted. Even when they don't lock up, you have to consider how an expanding core will change how the magnetic field interacts with the stator. Reducing the magnetic field is exactly what the voltage regulator does when it is desirable to reduce output current and voltage. Reducing the output due to a disturbed magnetic field is not intentional but it isn't a permanent failure either.
If you still think there's a problem, have the generator tested, but always do that with it on the engine. Since movement is a factor in determining maximum output, testing for maximum current is always done at 2,000 rpm. That is more than fast enough to provide the full rated current. You should be able to get close to 100 amps. If the most you can get is 35 amps, there is a shorted diode, and the common suspect is the battery and the resulting excessive voltage spikes.
Sunday, February 22nd, 2015 AT 8:50 PM