Yes, as well as every time you lift the throttle on a 5-speed to change gears, the rpms drop, the serpentine belt must pull hard to slow down the big amperage heavy spinning mass TDI alternator.Because of its high compression the diesel stops rotating in an instant when it's shut off. The clutch prevents damage to the belt/pulley/alternator by mitigating this shock.
Right on! And, the "big" 120A Alt has far more rotating mass than the 90A and smaller alts.Because of its high compression the diesel stops rotating in an instant when it's shut off. The clutch prevents damage to the belt/pulley/alternator by mitigating this shock.
The B4 model came without a clutched alternator pulley and did just fine throughout its lifespan. I just switched mine to the Gates AOD at 440,000 miles and the original crank sprocket was fine. I switched the wagon at 325,000 miles and it was also fine. My brother's B4 with 332,000 miles is still going strong on the solid pulley and he has no plans to change since he's cheap.
I'm not saying there wasn't an improvement, just that it's not an end-all if the pulley is solid.
The need for the alt clutch isn't so much the engine stopping as it is the pulses of power from each piston causing the crank to slow during compression and then accelerate during combustion. The crank does not turn at a continuous speed, especially at lower RPMs when the actual injection event lasts just a few milliseconds. This basically hammers on the crank pulley constantly.
Incidentally this is the same reason why single mass flywheel clutches tend to chatter. Same cause, same forces.
Sent from my HTC One XL using Tapatalk