Alternator Pulleys 101: Clutch type / one-way pulleys - PeachParts Mercedes-Benz Forum

RunningTooHot · #1 02-04-2015, 01:45 PM

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...

vstech · #2 02-04-2015, 02:22 PM

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...

compu_85 · #3 02-04-2015, 02:34 PM

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

RunningTooHot · #4 02-06-2015, 12:13 AM

Quote:

Originally Posted by compu_85

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

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.

Jarod · #5 02-06-2015, 06:22 AM

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...

engatwork · #6 02-06-2015, 07:50 AM

Pretty sure the 95 E300 has the fixed pulley.

BenzTurbo · #7 02-04-2015, 03:59 PM

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!

Zulfiqar · #8 02-04-2015, 05:18 PM

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.

compu_85 · #9 02-04-2015, 08:56 PM

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

Maxbumpo · #10 12-03-2017, 06:33 PM

Quote:

Originally Posted by compu_85

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

J,

Did you have to use a different size serpentine belt with the OAP clutch? What size? I'm installing an Ina clutched pulley wheel on the stock alternator on my '87 wagon with OM603.

fahrvergnugen · #11 02-06-2015, 09:04 AM

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

funola · #12 02-06-2015, 09:20 AM

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.

greazzer · #13 02-06-2015, 10:07 AM

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.

funola · #14 02-06-2015, 10:16 AM

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?

VT220D · #15 02-06-2015, 12:19 PM

Great post by the OP!

Quote:

Originally Posted by funola

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?

A larger alternator will place additional stress on the crank and may have something to do with crank pulley failure. It would be interesting to know if there is any correlation. The cases I've seen (on the internet) have often been OM617 engines with their typical large accessory load. I've also seen it happen to a poorly maintained OM603 3.5 (in person) that was otherwise stock. I wonder if a bad idler could add enough torsional stress to loosen the pulley.

When I was researching alternators, pulleys and belt drive systems I found that the alternator is the single largest mass in the belt drive system. The polar moment of inertia of the alternator rotor places a great deal of stress on the belt. This is especially true of diesels which have large torsional vibrations at the crankshaft. Every compression stroke slows the engine and every power stroke accelerates it. Ideally these changes occur at the alternator in sync with the engine. Decoupling pulleys allow the alternator to freewheel when the crank decelerates.

Serpentine belt systems gain their power transmission efficiency advantage from their thin cross section. This reduces hysteresis loss when the belt bends around a pulley. Compared to a 3 or 4 v belt system the fuel savings add up. Unfortunately the willingness of a serpentine belt to bend easily also causes flutter. When designing a serpentine belt system the position, tension and damping of the idler becomes critical. Decoupling pulleys reduce belt flutter in serpentine systems simplifying belt tuning. The decoupling of the alternator's mass also saves fuel. Manufacturers have adopted them almost universally.

This link has some good information on (mostly Denso) alternator output and at one time they specified the polar moment for their alternator rotors though I can't find it now.

Alternators | Electrical | Products | McLaren Applied Technologies

When I decided to update the alternator on my 220D I contemplated adding a larger alternator with a serpentine drive system and a decoupling pulley. I ultimately decided against it when I measured my stock 35 amp alternator and measured my full load power consumption. I went with a NOS 40 amp mini Denso from a Kubota diesel tractor and a conventional V belt drive system. If you need a high power alternator like Greazzer does for his project then a decoupling pulley might be the way to go.

Either way make sure your crank bolt is tight!