Was driving and suddenly lost power steering, all warning lights popped on etc. figured the serp belt had broke. Not the case, the pulley on the alternator is flopping around. I'll have to take a closer looks later to see how it failed exactly and will take a picture of it.

A serious downside to serpentine belts--one accessory bearing/pulley fails and you're dead in the water.

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1977 300d 70k--sold 08
1985 300TD 185k+
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Do you have an overrunning pulley?

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Could be the spring on the pulley/tensioner. It snapped into half. t happened to me.

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1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

my guess is that the pulley would be void of a woodruff key, thats the safety feature to save massive destruction should the alternator lock up internally.

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1995 E300D - The original humming machine (consumed by Flood 2017)
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not sure yet, had to get to work, but I'll look at it in the morning to see whats needed to fix it. and get those pics.

https://youtu.be/_naCODCIEe8

That's a quick look at issue from this morning. I'll have to take it off to see any more and see if the pulley can be saved etc. might be a good time to also finally replace the tensioner and shock for the serp belt.

If not, I'd suggest installing one now. I don't know if the alternators with an external fan have the same spacing though

The video looks like the nut holding the pulley on came loose.

-J

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All that's happened here is that the bolt has come loose. As long as the shaft threads aren't stripped, you're ok. Take the alternator out, tighten with a shot from an air or electric impact driver, and be happy.

There is no woodruff key on this generation of alternators. The shaft may still have a keyway cut and the fan would definitely have one. So you may be able to add a key if you really want one. But once the nut is jammed down tight, it should be unnecessary.

An overrun decoupling pulley would be a waste of time and money on a 70A alternator. You'd spend a month researching it, because no such thing was ever installed at the factory. You'd also have to buy the installation tool. By the time you're done, you could have purchased a new alternator for the money. And overrun pulleys have a half life of about 75K miles, so you'd be replacing it again down the road. A 70A alternator doesn't carry enough momentum for an overrun pulley to make any difference. If you had a 150A alternator, that would be a different story.

I think that covers everything.

The woodruff key was eliminated quite a few years ago although there is still usually a slot for it so it could possibly be added.

The usual problem is that when an alternator is rebuilt, the pulley nut might not get tightened enough. The torgue spec for the collar nut on the internal fan alternator is 80 Nm. I would imagine that the external fan ones are similar.

Good. I'll have to get it out on sat. don't have air tools to go to 80nm but have a big breaker bar style torque wrench that should do the trick.

While the belt is out, make sure that none of the other pulleys is sticking. Also make sure that tensioner bearing is sound: the tensioner should rock up and down, but not forward and back.

The reason I suggested an air tool is that it's tough to hold the rotor while turning the wrench. A hit from an air gun is so quick, inertia keeps the rotor from spinning away.

Went ahead and ordered a new belt, tensioner pulley, tensioner and the little shock. If the alt is not saveable for some reason they are available local for same price so I waited till I can see what I'm dealing with there. May just go with a bigger alt and so I can upgrade sound system later. (a system I already have sitting around.)

And the good news.

Turned out the locking nut had just come loose. Removed the Alternator, secured it and tightened it down with my breaker bar.