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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Powertrain and Drivetrain Discussions > N54 Turbo Engine / Drivetrain / Exhaust Modifications - 335i > Alternator Pulley



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      04-19-2015, 07:21 PM   #1
KiwiRD
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Alternator Pulley

Hey all. Pretty sure my alternator pulley is toast and is making a knocking noise so I've ordered a new unit from ECS. But can anyone describe what's actually going on inside the pulley for it to make that kind of sound? Seems like a hell of a racket for such a small object! I've heard there's some kind of clutch system in there so is that likely to be the issue?
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      04-20-2015, 12:17 AM   #2
Miko226
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Also check your tensioner but the clutch inside the alternator pulley is probably the problem of the noise it is likely spinning too freely and needs to be replaced.
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11 335d - Mod's Coming Soon!
08 n54 135i Alpina-AT 105k Miles
Engine: JB4 e40/93, AFE DCI, VRSF 5" FMIC V2, VRSF Upper/Lower CP, BMS Tstat Bypass
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      04-20-2015, 01:36 AM   #3
N54Bob1988
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Its like a sprag clutch. It freewheels one way. Locks the other. So when you let off the gas going 5000rpm the 14k rpm alternator can spin down at its own pace and not tied to engine rpm. They do this so you dont need uber tension on the belt. Longer pulley bearing life. Fuel savings ect. When the pulley locks both ways the added load makes the belt squeal keep in mind also a special tool is required to remove the pulley

Last edited by N54Bob1988; 04-20-2015 at 01:49 PM.
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      04-20-2015, 08:42 AM   #4
scotthureau
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I just had a knocking sound from this same area (07 335xi). Shop fixed it by replacing the belt tensioner. It was in the same general area as the alternator. They described the tensioner as wobbling back and forth. I would imagine a part of the tensioner is physically hitting something metal in order to make a knocking noise. A bit scary as the noise sounds a lot like a valve knock, only at a higher rate/frequency. Mine only did it at low speeds below around 2000rpm. I think the repair was around $275- $300 including labor.
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      04-20-2015, 04:13 PM   #5
KiwiRD
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Good advice. I'll definitely get the shop to take a look at the tensioner as well. I did the screwdriver test by resting the tip of a flat bladed driver on the alternator housing and could hear the knocking from within as clear as day. It's amazing how well that works. So I'm only guessing that it has to be something connected directly to the alternator itself.


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Originally Posted by scotthureau View Post
I just had a knocking sound from this same area (07 335xi). Shop fixed it by replacing the belt tensioner. It was in the same general area as the alternator. They described the tensioner as wobbling back and forth. I would imagine a part of the tensioner is physically hitting something metal in order to make a knocking noise. A bit scary as the noise sounds a lot like a valve knock, only at a higher rate/frequency. Mine only did it at low speeds below around 2000rpm. I think the repair was around $275- $300 including labor.
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