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The Overrunning Alternator Decoupler (OAD) is a new device for replacing the solid alternator pulley to decouple the alternator rotor inertia from the front-end accessory drive (FEAD) system. The OAD is used primarily to reduce the belt tension fluctuation in the accessory drive system when the alternator inertia, in combination with the engine torsional vibration, creates NVH (noise, vibration and harshness) problems. In this paper, the functional principles of the OAD will be explained and the influence of the decoupled alternator rotor inertia on the performance of a four-components system (CS, AC, alternator and tensioner) will be reported. The test data and the numerical simulation results show clearly that the amplitude of the belt tension fluctuation, the tension arm movement and the hubload of each component are reduced significantly, when the OAD is used. This study indicates as well that the FEAD system with the decoupled alternator can work with a lower pre-set belt tension.
 

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The 4 cyl VW diesel engines use this, where it presumably helps out even more with the sharp slowdown and acceleration of the crankshaft on that type of engine. The VW users tend to hate them, because they are expensive to replace, and they usually fail by exploding in place... I do agree that it is a good thing, though.
 

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I'm being a bit dramatic - but it does tend to come apart in pieces when it fails. I was lucky - the one in my Jetta welded itself together to become an "old style" pulley, i.e. a solid one. This was fine for a few years until the tensioning arm started going out. So I got to replace both...

I had since seen another link here on the forum that says the OAD is not a simple one-way clutch. The main point is to be careful match the pulley style when replacing the alternator.

But yeah, funny how these little design changes come in over the years. I had never heard of such a thing either.
 
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