Normally changing to a different current rating does not cause a problem, but with the addition of all the unnecessary computer controls, it is hard to know what will cause an issue. Just because the plugs and wires might be the same, there could be some other differences we do not know about.
The key points to understand is an AC generator, ("alternator" is a term copyrighted by Chrysler, but it is the same thing), is physically and electrically incapable of developing more current than it was designed to deliver. I can impress you with the theory if you want me to, but it is not relevant to my wondrous story. The next important point is the generator will only develop the amount of current needed by the electrical system, and no more. It is similar to running a lawn sprinkler on a garden hose. No more water volume is going to come out if the city installs a water pump with double the capacity. What you gain by going to the 130-amp generator is the ability to get 130 amps, but only if that becomes necessary. If the vehicle needs 53 amps right now, you could have a 500-amp generator, but it is only going to produce 53 amps.
Where the potential problem comes in is when your mechanic performs a full-load output current test. That is designed to make the generator produce its maximum current for only a few seconds. Related to my previous reply where I discussed a failed diode and getting only one third of the rated current, under this test, you are either going to get very close to the generator's maximum rating, one third of it, or 0 amps. There is no such thing as a "weak" generator that can only deliver some value in between those three values.
To continue with this potential problem, as far back as 1960 when Chrysler first used an alternator, there has been a fusible link in the output wire between the alternator and the battery's positive post. There are two sets of three diodes in every AC generator, and if one in each set were to short, you would have a direct short to ground, and a melted wire. The fuse link, or fuse link wire, is a short section spliced in that is a smaller gauge, so it is the weak link in the chain. It is insulation is designed to not melt or burn. These fuse devices also protect you if you bump the output terminal with a wrench while it is in contact with the engine. Fuse link wires take some time to burn open, so they will not respond instantly like a regular fuse will. Almost every car was available with a standard generator, or a larger one if it had air conditioning, and an even larger one for police vehicles. The fuse link was sized according to the generator that was installed at the factory. If the car came with a 55-amp generator, the fuse link was probably good for around 65 amps, but remember, you would never need that much current except during the full-load output test. The problem I'm finally getting to is if you came along and installed a 75-amp generator, it still would deliver only the amount of current the car needed, until someone performed the full-load output test. That is when it would produce 75 amps, and if you did that long enough, the fuse link would melt.
To make matters worse, in almost all vehicles since the early to mid 1990's, the fuse link wire has been replaced with a regular fuse that's bolted into the fuse box. Those do not have the time delay feature a fuse link wire has. The instant you start the full-load test, the fuse will blow if the generator is able to develop more than what the circuit was designed for. When the wiring harness is manufactured, the output wire is large enough to handle whichever generator gets installed on that vehicle. Only the fuse link wire is different, per the application. That means if you wanted to switch to a larger generator, you could replace the fuse link wire with a larger one, and the rest of the circuitry could handle it. That is not always true when the vehicle uses a bolted-in fuse. It is easy enough to bolt in a larger fuse, but there can still be differences in the buss bar under those fuses that connects their common points together. There is always a small amount of resistance in an electrical connection. (Resistance in the water sprinkler analogy would be partially crushing the hose with your foot, which would make water volume go down). Electrical current flowing through resistance generates heat. Heat in an electrical connection tends to increase the resistance, and that generates more heat, and pretty soon you're one of the half dozen people who post a photo here each year of a melted area in their fuse box. That happened on its own, but if a generator was installed that had a higher capacity, then add some modification or accessory that needs that additional current, and you have the potential to stress parts of the circuit that cannot be upgraded.
Another factor is the car manufacturers have become really good at down-sizing electrical switches and connector terminals to the point they're just good enough, with almost no margin for error. In the 1970's, Ford figured out they could save twenty cents per car by leaving off four grease fittings. The suspension and steering parts sometimes lasted long enough to get the car out of the warranty period, then it was the owners' problems. While that sounds like saving a pittance and costing a lot in customer satisfaction, they argued they would save twenty million cents when they built a million cars, and they were willing to do that for short-term profits. When you ask for a replacement fuse box today at the dealer's parts department, why is it they usually ask for the engine size, generator size, and date of production? It is possible they traced too many garage fires to a melted fuse box, so they beefed them up a little, but only for the larger generators or some other optional equipment. It could be you do not have some optional equipment, and to save a few cents, they left off some terminals in the fuse box. Look in yours and you will see some empty sockets. Each terminal not installed saves a little money. To say that another way, a smaller terminal might be sufficient for use with the smaller generator, but when you switch to the generator with a higher capacity, and then you add the "hard-of-hearing" stereo systems to play "music" for me, or you add a pile of lights, or even convert a van to a camper package, you risk stressing parts of the electrical system that cannot be or were not upgraded.
Now that you have all this exciting information, it still boils down to your warning message that appears to be related to using the non-original size generator. Since around 2000, Ford's most "intelligent", meaning complicated, computer is the instrument cluster. It is one of two computers involved in blowing the horn, and it sticks its nose into almost every other circuit and system. We do not know what it is looking at to determine when to display the warning message, but when I read the message goes away after switching back to the original generator, it does not surprise me too much.
Oh, .as a footnote of value, a "diode" is a one-way valve for electrical current flow. 1959 and older vehicles all had DC generators which were not very efficient, but direct current can go right into the battery to be stored. AC generators produce three-phase alternating current that would go rapidly in and out of the battery and generate some serious heat, but nothing would get stored in it. The diodes reroute the pulsing and changing currents to all go in one direction, out to the vehicle and the battery. Those diodes are all "reverse biased", meaning they are blocking current flow, when the engine is off. Those are the only thing stopping the battery from rapidly discharging through the generator when it is not running. Any one shorted diode of the six reduces the maximum capacity to one third of what it is designed to produce. A shorted diode in each of the two sets results in a rare but major dead short. If you ever run into that, you will be grateful for the fuse or fuse link wire.
May 5, 2017 at 4:45 PM
