Questions arise on a periodic basis about “clutch” type alternator pulleys. Hopefully this thread can be a consolidated source of information about these sometimes misunderstood devices.

The “cheat sheet” short answers are:
1.) YES, they are important.
2.) NO, you shouldn't replace one with a solid pulley.

It is tempting to avoid the additional cost of a new one by using a solid pulley instead, but that is an ill-advised course of action. Some rebuilt alternators may come with the incorrect pulley too.

An overrunning alternator pulley is specifically designed for quelling harmonic vibrations that would otherwise be induced into the belt drive system. Without the correct overrunning pulley, there would be excessive drive belt vibration leading to greater loads that wear out the damper and the tensioner pivot point more quickly - not to mention the NVH increase. The additional load & stress is not good for the bearings in the other belt driven components too.

YES, these pulleys do fail. They typically fail in functionality, while remaining intact. But that is when you really want to catch it – before it fails catastrophically. I would consider them to be a “wear item” that if not replaced periodically, should at least be inspected on a regular basis if it is getting old. (Personally, I replaced mine proactively while going through my “new” old car so that I wouldn't have to be concerned about it for a long time forward.)

Argument: The early 606 engines had solid pulleys, so why not put a solid pulley on the later 606's? The reason is that the early 606's had a smaller alternator (less spinning mass) so they didn't have as much of a vibration problem as the later 606's with their higher output alternator. The 603's use the same design for their belt drive system, but (like the early 606's) they had smaller alternators. (Another source of information states that overrunning alternator pulleys were not invented until 1995.)

Relevant videos:
https://www.youtube.com/watch?v=jKMf-_Yl-AI
https://www.youtube.com/watch?v=rXehCcNzq_A
https://www.youtube.com/watch?v=4EXYP1CmL9Q
https://www.youtube.com/watch?v=hrFcPqZuO3A


Can one be retrofitted to an earlier application? That I don't know for sure, although it seems logical that one could. However, it should be done with the caveat that each pulley is designed for a specific application. (And the “if it ain't broke – don't fix it” rule also applies.)

Another thing that comes to my mind is that some people have been retrofitting larger alternators to earlier applications. I would surmise that if belt drive problems become an issue, installing an overrunning clutch type pulley would seem advisable. (Those problems may manifest as belt vibration/slap, recurrent damper failures, broken tensioners, premature wear on other driven accessories, etc.) Again, the same caveat applies.

An esoteric piece of additional information: There are two types of overrunning pulleys. One is simply a one-way clutch, typically known as an OAP (Overrunning Alternator Pulley). The other type is often called an OAD (Overrunning Alternator Decoupler) which adds a torsional spring to the one-way roller mechanism to further damp vibrations.

Hope this helps some people down the road...

__________________
Current rolling stock:
2001 E55 183,000+ Newest member of the fleet.
2002 E320 83,000 - The "cream-puff"!
1992 500E 217,000+
1995 E300D 412,000+
1998 E300D 155,000+
2001 E320 227,000+
2001 E320 Wagon, 177,000+

Prior MBZ’s:
1952 220 Cab A
1966 300SE
1971 280SE
1973 350SLC (euro)
1980 450SLC
1980 450SLC (#2)
1978 450SLC 5.0
1984 300D ~243,000 & fondly remembered
1993 500E - sorely missed.
1975 VW Scirocco w/ slightly de-tuned Super-Vee engine - Sold after 30+ years.