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.

I found a deal on two oap clutches and plan to install them on my 150amp alt on my 190D2.5T and my 87 300TD... I've not begun the install yet though...

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Here are a couple videos I made of the dramatic difference the 606's OAP made on my car:

Before w/ a dead shock, then just the OAP (still with a dead shock):

https://www.youtube.com/watch?v=DGI9XLuhaYY

With a new shock:

https://www.youtube.com/watch?v=fB_r4odE9hI

Smooth as silk

Before I installed the OAP I burnt through several tensioner shocks - literally. They would overheat and barf their oil out in a cloud of smoke.

VW introduced these for the 1997 model year and they greatly reduced serp tensioner wear, and reduced wear on the crank pulleys (a known weak point of the engines at the time).

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

i have a 150a alternator to go on my 603 and i think i'll order one of these. but which one? OAD or OAP? anything to minimize the risk of vibrations in the belt system and breaking the timing cover where the tensioner pivots!

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83 300CD- sanden, dual p/f condensers, 160a alternator, ect
91 300TD- 722.6, #22 head, 3.5L IP, w140 manifolds, ect

I have one on my 606.910 too, I took it to an oldschool alternator/starter shop and asked to install the "new fangled" pulleys - he pulled out one from his shelf - it was for a SAAB/VW/??? something else too and charged me USD 85/- for the ordeal including installing it.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

Mine is an OAP. It's the one for late 606 motors, but fit fine on the 110A alternator I installed on my 603.

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

Thanks for sharing those videos. It's nice to have a visual example of an MBZ application instead of the generic / promo videos that I posted.

An amazing difference.

__________________
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.

WOW!!!

Thank you for posting this!!
I've been going through my 95 E300D slowly as money permits, and the belt vibrates a LOT!! for a 606 the thing was rough, motor mounts were collapsed completely, and that eliminated a lot of nasty vibration and sound, but the belt still looks awful.

I replaced the shock damper thing when I got the car which helped a lot, but the belt still vibrates like mad at idle. raise the idle even a few hundred rpm and it eliminates a lot of it.

I'm now curious if this might be related.
I'll check once its warmer and lighter outside, if I have the solid pulley or one way, where can I find the OAP style? and how much is it going to hurt my wallet?
One day this repair ocd will cost my my wife at the rate I'm spending money...

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1982 300D (w123, "Grey Car")
1982 300D (w123, "Blue Car")
2001 Ford F150 "Clifford" (The Big Red Truck)
1997 Dodge Ram 2500 12V Cummins
1996 Dodge Ram 2500 12V Cummins
Previous Vehicles:
1995 E300D, 1980 300SD, 1992 Buick Century, 2005 Saturn Ion

Pretty sure the 95 E300 has the fixed pulley.

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Jim

FYI, most of the VW TDIs had this sort of pulley too.

Does running a bigger alternator (120 A and bigger, stock is 55 to 65 A) on 617.xxx engines cause problems in the long run and may benefit from installing an OAP? I remember reading somewhere that V belts has enough slip that it's not a problem, as opposed to serpentine belts which has much less slip thus transferring much more of the shock load.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Does anyone know if the OAD (or OAP) pulley came in the double belt flavor vs. serpentine belt for the OM617.9XX engine ?? I've looked and the only thing I see are serpentine belt applications.

I recall posts over the years on this forum of loose crankshaft pulleys and mangled pulley pins on 617 engines. Were they related to bigger than stock alternators?

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

V belt slip happens by virtue of how they work, whereas a serpentine belt works against it, yes funola.

I looked for a V belt decoupling pulley but I never found one. I think the decoupling pulley arrived too far into the serpentine era for a V belt version. I'd love to be proved wrong on this though!

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1969 220D 5 Speed (OM616)
1983 240D 4 Speed
1985 300D Auto 376K
1985 300D Auto 275K