There are two potential problems to be aware of. First of all, if you can determine the current rating of the old unit, and the new one is the same, you have nothing to be concerned with.
If the car came with a 130 amp generator and you install a 90 amp unit, there may be times when it won't deliver enough current to meet the needs of the electrical system. That is a very low-level worry, so to speak, because the battery will make up the difference, and you won't need that high current for very long. Head lights, fuel pump, heater fan, wipers, rear defogger, and radio might require a total of 60 amps. Anything else like power windows and mirrors would just be for a few seconds, so that won't run the battery down. The 130 amp generator was installed to cover any combination of electrical systems on your car but there's no chance it is going to have to actually develop that much current in normal operation.
The bigger problem can occur if your car came with the 90 amp generator and you install one of the larger ones. First you must be aware that 130 amps refers to maximum capacity, not what will actually be developed while you're driving. If the car's electrical system needs 73 amps, the generator is going to develop exactly 73 amps; no more and no less, and it doesn't matter if it's capable of 90, 120, or 130 amps. The only time it will develop its full maximum value is during a load test with a professional load tester. The test involves loading the electrical system down just enough to make the generator develop its highest possible amount of current, but only for a few seconds. The test is done just long enough for the mechanic to get the reading.
Where the problem comes in is there is some type of fuse in the fat wire going from the generator back to the battery. Very often the size of that fuse is selected based on the capacity of the generator the car came with from the factory. On older cars that was usually a "fuse link wire". That was simply a few inches of wire with a smaller diameter so it was the weak link in the chain, and the insulation was designed to not burn or melt. Being a wire, it takes some time for excessive current to burn it open. If yours is sized for a 90 amp generator, it will not burn open during the load test because 90 amps is the most you'll be able to get, and the link can handle that. But now, if you install a 120 amp generator, it will still be fine during normal operation, but doing that load test is not "normal" operation. If the test is done properly so it just takes a few seconds, the fuse link won't burn open that quickly, but if an inexperienced mechanic dilly dallies around and takes too long, the link could burn open. Most of the time the rest of that wire can handle the higher-output generators so the fix would be to just replace the burned fuse link with a larger one sized for that replacement generator.
Most newer cars use a large fuse that's bolted into the under-hood fuse box and those aren't as forgiving as a fuse link wire. During the load test, if the replacement generator is capable of developing more current than that fuse is rated for, the fuse will blow instantly. No further testing is necessary because now you know the generator is working. All that's needed is to replace the blown fuse. In this case, unless I learned differently, I would install the same size fuse that was removed. The car's electrical system isn't going to want more current than before, so even though the new generator has a higher capacity, it's never going to develop enough to blow the fuse during normal driving. The reason I wouldn't install a larger fuse is because I don't know if the circuits in the fuse box can handle that higher current. With different options on the car that demand that higher current, that may include a different fuse box. By sticking with the original size fuse, you'll know everything is protected properly.
Monday, March 30th, 2015 AT 6:44 PM