My vehicle came with a seized clutch pulley on its alternator, so it acted pretty much like a solid pulley. When shifting from first to second gear at wide-open-throttle, the engine RPMs slow down very quickly. However, the mass of the rotating assembly inside the alternator (which is the bulk of the alternators mass) spinning at such high RPM (6500 crank rpm = unknown but much higher alternator RPM) possesses so much inertia that the contact between the alternator pulley and the serpentine belt actually slips (all of the other accessories slow down much quicker than the alternator, and the grip of the crank pulley on the belt is much greater due to the much greater contact area it has with the belt) until the alternators rotating assembly rpm matches the engine rpm again. This slippage produces a very loud squealing sound right after the shift. The 'overrunning' clutch pulley allows the rotating mass of the alternator to continue spinning at its high RPM while the engine has slowed down to a much slower RPM. Eventually (over several seconds) the alternator rotating assembly will slow down until it is once again driven by the engine. At 2:30,@alternatorman claims that the alternator is always under a tremendous load, and the alternator slows down with the motor. This not always true. In vehicles with a PCM controlled alternator (such as Ford 4G with the white regulator), it's possible that the PCM turns off the alternator at WOT (my vehicle does, and probably many these days do, precisely this - yes, it's running solely on battery at WOT, and yes you can actually see the needle drop on the voltage gauge while in WOT - but it's not a problem since I'm in WOT for no more than 20 seconds at any one time). Doing this removes the alternator as a source of drag, putting more of the engines power to the ground instead of into powering accessories. This means the alternator is under no load, and will not slow down by itself due to current loading. Shifting at low RPMs and light-to-moderate throttle, the alternator will probably not overrun the engine much, so the clutch pulley will not slip much or possibly not at all. But once you're moderately on the go pedal, any firm shift will cause the alternator pulley to slip. If you have a solid pulley, you'll hear a loud squeal. Once I replaced my seized clutch pulley with a new, working clutch pulley, all of my squealing shifts sounded more like a muted supercharger winding down - much less embarassing.

My diesel engine needs clutch pulley because the crank decelerates rapidly when it hits each compression stroke at idle. The heavy alternator does not want to slow down so quick so it winds-up the tensioner, which therefore pumps back and forth madly during each revolution and the belt flaps, making a racket. The clutch pulley smooths it all out. I don't know why anyone would bother putting them on the lower compression petrol engine except maybe the flywheels have been made very light to minimise vehicle mass and improve responsiveness so the same phenomenon has crept-in.

The outside turns freely in reverse rotation because it's supposed to sit still against the belt, with the inside mass freely rotating foward. This allows the alternator to spin on it's own accord, until the outer pulley speed reaches or exceeds that of the inner rotating mass. This is my own noob conjecture, but it logically explains the claims refuted in this video.

Actually, the reason the clutch pulley's were put on the police alternators, was to keep the belt from throwing off if the gas pedal was mashed to the floor, then let up and the brakes slammed on, while the engine was still at high rpm. Many of the cop alternators had a smaller diameter pulley than civilian ones, to get more output at idle. The smaller diameter pulley increased the chance of the belt throwing during rapid accel/decel situations. Of course, most of the clutch pulleys on the cop cars are likely seized, and are just like solid pulleys anyways.

Hi - Why would you want to do this? I ask as I'm currently thinking about going the other way to a clutch pulley. I want to do this in order to help reduce issues with failing belt tensioners which the ford mondeos are very prone to.

Hi Oliver ,do u have done the conversion on your mondeo?if u did it ,therre is any difference?

You do this conversion because clutch pulleys are sensitive mechanisms prone to failure. They contain a lot of fragile moving parts and the catch mechanisms are easily sheared off. The only benefit clutch pulleys provide is decreased belt wear and noise, but they won't prevent tensioner or other system failure.