Alternator blowing up!

No, AC generators, (alternators), are physically incapable of developing more than their design current. They are self-regulating in that respect. If you have repeat failures, it is more likely you're overlooking a wiring problem related to the generator. Any circuit that has a short and draws excessive current will blow its fuse.

First you have to take apart the generator to see why it failed, assuming it is indeed defective. It's not uncommon to have a rash of failures from one rebuilder, so if you really do have multiple bad units, try a different brand.

Some generators are mounted on rubber bushings and not directly to the engine. If that applies to yours, check for a corroded ground strap or wire. From what you described, you could have nothing more than a drain on the battery. That doesn't have anything to do with the charging system.

To check if the charging system is working, use an inexpensive digital voltmeter to measure battery voltage while the engine is running. It must be between 13.75 and 14.75 volts. If that is what you find, that proves it is okay to do the second half of the tests, but that requires a professional load tester to measure full-load output current.

A good, fully-charged battery will read 12.6 volts with the engine off. It's the voltage with the engine running that is important. 12.75 is much too low for that.

Two in a row is more typical of a pattern failure. That occurs when a new employee at the rebuilder is doing something wrong on every unit he works on, or the rebuilder got a bad batch of a certain part. That's why I mentioned switching to a replacement generator of a different brand.

Other than that, it's not common to get a defective rebuilt generator. Repeat failures that occur after hours or days are often caused by the battery. Some generators develop voltage spikes that can damage its internal voltage regulator and diodes, and interfere with computer sensor signals. The battery is the key component in dampening and absorbing those spikes but as they age and the lead flakes off the plates, they lose their ability to do that. This has been an especially big problem with 1987 and newer GM vehicles. When their generators are replaced, you must replace the battery at the same time unless it is less than about two years old.

If your replacement generator doesn't work as soon as it is installed, it is more likely you have a problem with the wiring. With the engine not running, check for 12 volts on the large black / red wire on the back of the generator. If that is missing, check for a blown very large fuse. It may be bolted into the under-hood fuse box. Back-probe the light green / black wire for 12 volts. If that is missing, check fuse 36, a ten-amp. With the ignition switch turned on, the charge warning light on the dash should turn on. If it doesn't, check fuse 14, a ten-amp inside the car. That is the "turn-on" circuit that tells the voltage regulator when to start running the generator.

To be valid, any testing has to be done when the charging system is not working. You've identified that by the 11.95 volts. If you find intermittent operation, it is extremely unlikely the generator itself is the cause. That is real possible on older units with worn brushes, but not on new / rebuilt units. Look for stretched terminals in the electrical connector and fuses that are loose in their sockets or have corroded terminals. You're more likely to find that in the fuse box under the hood.

No. Think of the battery as a bucket for holding water. You put 12.6 inches of water in the bucket, the weight of that water is what provides the pressure. (Voltage is electrical pressure). What you've done is replaced that bucket with one that's much wider in diameter, but it's still filled to 12.6 inches. The pressure hasn't changed, but you can do work for a longer period of time with the larger volume of water.

The larger bucket will take longer to fill and will take longer to empty. Your larger battery will take longer to become fully-charged, but it will last longer before it becomes discharged.

Installing a generator with a higher current capacity won't change anything either. While it is capable of producing higher current, it will still only provide the amount of current needed by the electrical system and to charge the battery. The battery won't charge any faster.

Have the new generator tested at the store before you take it. AC generators are self-regulating when it comes to current. They are simply incapable of producing more current than they're designed to produce. The only thing in that respect that might cause a problem is if it is forced to produce its maximum current for a long period of time. That could cause the diodes to get hot and short. That's where knowing how the previous ones failed is helpful. Normally the only way to get it to charge wide open is with a full-field load test, and that is only done for a couple of seconds.

The only thing the battery can do is if it's over two or three years old, enough lead has flaked off the plates that it will lose its ability to dampen and absorb any voltage spikes the generator might produce. This has been a real big problem on GM vehicles, 1987 and newer. It's worth mentioning for other brands, but it's not as common. Again, you have to know how the other ones failed to figure out what's causing it.

The rebuilder would have to determine that. They need to see which part failed. With high-mileage units it's usually worn brushes. With an old battery, it will be a failed voltage regulator or a shorted diode. With one shorted diode out of the six of them, you'll still be able to get up to exactly one third of the generator's rated current. That can be over 30 amps which can be enough to run the electrical system.

Thanks for the thread, it will help others, nice work CARADIODOC :-)