We have a great big alternator. 210 amps at 12 volts – over 2500 W. Huge.
It’s driven by two large V-Belts off the main crankshaft. In the olden days, you would buy a matched set of V-Belts to use in a dual application because it’s important that they are the same size. Exactly.
Modern manufacturing methods mean that the premium belts are uniform enough that you don’t need to buy them in pairs.
We’ve had a problem where the tach would jump around when the alternator was producing maximum power. I always figured it was electrical noise and would go wiggle the connectors. Eventually the tach would settle down.
I never suspected belt tension, since I regularly check and re-tighten the belts. However, the belts aged differently. The inner belt stretched slightly, and under heavy load would slip slightly, heat up, and transfer all the horsepower into the outer belt. Eventually that belt would also heat up and slip – just enough to show up in the tach needle (diesel engines get their tach signal from the alternator, since there is no distributor producing an electrical pulse). You can see the belts are now differently-colored from the aging.
So, two new belts from spares and everything was right in the world.