I don't have a good answer about the stress related to speed, but I should add another chapter to my sad story that might give you some insight. First of all, in an attempt to use correct terminology, I'm calling this an AC generator as that is the term the industry chose recently to standardize many things. The alternator was developed by Chrysler and first used in 1960, and they copyrighted that term. Regardless, everyone will know what you're referring to when you say "alternator".
The first important point to understand is that unlike the older DC generators from the fifties and before, AC generators are physically incapable of generating more than the maximum current they are designed for. To build one that is capable of producing more current than the other one, you simply have to add another loop of wire to each stator winding, and use diodes that can handle that much current. (A diode is a one-way valve for electrical current).
The next and more important detail for your problem is the voltage regulator is going to command the generator to deliver exactly the amount of current needed to run the electrical system and recharge the battery, and no more. System voltage is an indicator of the system's needs and is what the regulator uses. Notice that engine speed is not a factor in determining the generator's output.
If it helps, think of the engine's oil pump. If you double engine speed, you double what the pump can deliver in terms of volume, but not pressure. The pressure relief valve dumps the excess oil right back into the oil pan, so system pressure stays the same, the resistance to oil flow stays the same, and therefore oil flow to the engine stays the same, regardless of speed.
The same is true of the generator. Lets say, for the sake of an explanation, at maximum output the generator takes five horsepower to spin and it develops 100 amps. If your electrical system needs 50 amps, the generator is going to take two and a half horsepower, but again, engine speed is not a factor. In theory the faster movement between the magnetic field and stator winding would result in more current being developed. More current getting pushed into the battery and circuits that use it will result in voltage, (electrical pressure), going up. The voltage regulator will see that higher voltage and reduce the small amount of field current going through the spinning field winding. That results in a weaker magnetic field which results in less current being developed in the output stator winding. That lower current will be, ... Tada, ... 50 amps, and two and a half horsepower.
To say it a different way, the generator puts the same load on the tensioner regardless of engine speed.
Now, there is another part to the story that I personally have not been involved in except to have two friends tell me about it, and that is that Chrysler is using a spring-loaded pulley on their alternators for quite a few years. I don't know what the problem was they found a solution for, but it's possible it had something to do with preventing vibration from the tensioner from transmitting to the alternator, and that will lower the stress on the belt. That's just a guess though.
You made reference to marks on the tensioner body. I'm not aware of any tensioner that has marks that are there for a useful purpose. Serpentine belts don't stretch, and they don't sink into the pulleys' grooves like the older V-belts did over time, so the tensioner arm shouldn't change position over time either. I think the clue you need to look at is that the tension is becoming real weak suddenly since you said you can push on the belt with just one finger. To me, THAT is what would point to a manufacturing problem since you said this happened more than once. The spring inside it didn't beak because then you'd have no tension at all. Rather, it sounds like one end of the spring popped off whatever it was hooked onto so not it doesn't have as much tension. You might consider comparing a new one at the parts store with one of the failed ones to see if you can tell if something happened to that spring.
Monday, February 23rd, 2015 AT 11:35 AM