912 Engine Knocking

[Brett Voigt]

Last week my fan belt tore off of my 68' engine. Luckily it was only a few blocks from home, so I made it back without any issues. I just replaced the belt and now hear a knocking noise. I though maybe it had to do with the tension of the belt so I both loosened and tightened it and still hear the knock. I then took the belt off and no more knocking, so I figure it's either coming from the generator (bearing) or the fan. From what I can see the fan looks ok. Anyone have a good writeup or way to change out the generator bearing? Also, is it better to just replace the generator? Looks like the Bosch replacement is only about $175 (I really just wana drive since theres only about 2 months of good weather left). Thanks

[Brett Voigt]

If it helps... when I try to spin the generator fan, it spins freely.

[WFBowen]

Brett:
Try this: make an easily legible mark on the generator pulley. Reinstall the belt, run the engine at idle, and see if the knocking sound syncs with the revs of the pulley. The generator/fan assembly is spinning considerably faster than the crankshaft. This is strictly based on my personal experience, but that noise you're describing I've heard many times and each time it turned out to be a belt making the noise. JMO.
Also, since you have the later generator, it's probably wise to just buy a new one (VW Bosch) rather than having a shop press new bearings into the old one.
Bill

[Peter Graham]

Could be the fan nut has loosened. Since it sounds like you're pulling the generator anyway, this will be easy to check. If the nut has loosened, remove it and check for damage to the fan hub. Bad generator bearing(s) will not generally make a knocking noise unless there is other damage (bearing carrier). Bearing/brush replacement can normally be done with basic hand tools...

Peter

[Brett Voigt]

So the generator/fan spons freely woth the belt off. No knocking sound when engine runs with the belt off but starts up again as soon as I re-attach the belt to the generator. The knock does seem to correlate with rpm of the generator/fan (thanks for the mark idea). I going to take it down once I get some time. One think ive noticed is that the belt and pulley get exceptionally hot after only a few seconds (20-30 or so) of running. Is this normal?

[Jerry1]

Brett
A couple of things. You should have about a 1/2 inch of movement in the fan belt if you sqeeze it together in the middle. You adjust the belt by moving the spacers on the inside or outside of the pulley. less spacers inside will tighten the belt more will let the belt set lower in the pulley and will be looser. Sorry if you already know that but want to make sure that was clear. Also I have had my main pulley nut get loose and make a knocking sound you might check that. I think it torqs to 60 lbs.
Jerry

[Brett Voigt]

Originally Posted by Jerry1

Brett
A couple of things. You should have about a 1/2 inch of movement in the fan belt if you sqeeze it together in the middle. You adjust the belt by moving the spacers on the inside or outside of the pulley. less spacers inside will tighten the belt more will let the belt set lower in the pulley and will be looser. Sorry if you already know that but want to make sure that was clear. Also I have had my main pulley nut get loose and make a knocking sound you might check that. I think it torqs to 60 lbs.
Jerry

I was aware of the 1/2" play and the spacers. Didn't know about the 60lb torque spec though.

[NICE 69 S]

See Peter Grahams post #4 above. The two halves of the fan belt pully fit on to a carrier on the generator shaft, and have two flat sides. If the engine is run with the bolt too loose, it will wear the flat sides of the pully and cut into the carrier. Check for play in the fit of the pully halves to the carrier. If the pully halves are getting hot like you say, then they are probably rubbing against each other because the bolt is too loose and the carrier or pully halves are worn.

Bob B

[oldbusboy]

From experience if the bearings are going they don't generally knock.. they squeal. It generally is a pretty quick decent to a seized bearing. Since you have already isolated the noise to the generator I'd turn it by hand with the pulley and belt off. You should be able to hear or feel something. There is a chance it could be the bearing or the fan nut. Could be a missing woodruf key under the pulley? Could also be the generator strap.

Trevor

[Brett Voigt]

I finally had some time to get back and play around with the generator. Adjusted the belt a few times, torqued to 60ft/lbs and even played with the generator a bit. When I spin it by hand clockwise (how it rotates with the engine), it spins freely, when I spin it counter-clockwise (how it's not supposed to spin) it makes a tiny clicking noise. Either way, I attached the whole thing up again and drove around the block. The knock was subtle at the start, but at the end of the trip it was considerably louder. When I went to take the belt off (HOT), I noticed that the pulley halves are not longer perfectly square (at least the straight edges aren't entirely straight) and they are either starting to bend in from the hub or grind off, bear in mind that the pulley halves and hub are brand new along with the belt. I'm about to pull the generator, mostly because when it's in the circuit, that's the only time I hear the knock.

[Brett Voigt]

https://youtu.be/IcZo-QZl4qk

I uploaded a video of the engine running to youtube... It's my first upload, so hopefully it works.

[oldbusboy]

hmm.. I had an aftermarket pulley and because the part was cheap it ended up getting elongated on the female side. When it was together there was enough slop to make it noisy.. I believe in my case that slop ended up hurting the generator bearings. If your belt is hot I'd guess you have slop.

Check your pulley halves. They should fit together snugly and there should be very little play. If they are worn, you should find a good original to replace. Perhaps you can borrow an original for a quick test if you don't have a source for an original tight one to buy right away. If the halves are really nice and tight move on to the generator itself.

If I was going to tackle your issue on my car I'd take the belt and pulley off and run the motor briefly (keep in mind there will be no cooling, so only do it quickly) to determine if the noise is the generator/fan or motor. I suspect it is the generator or fan if you already determined the pulleys are good.

Is the shroud bolted in place snugly? If you grab it on either side and twist does it move? Perhaps the fan shroud is loose?

I can't really tell from the video, but did you already check the generator strap? The bolts on the strap could be loose.. if they are at all loose the torque of the motor will twist the generator and could cause that noise.

Trevor

[mowog63]

Hi Brett,

I just read through the posts and watched/listened to the video that you posted...

Why did the original fan belt "tear off"?

That is the place to start.

Fan belts usually shred up because the pulley halves have issues that ruin the belt.

I think the issue you are having is with the pulleys.

Good luck,
erik.

[Brett Voigt]

Hi Erik. So after the first belt tore up I replaced the inner and outer pulleys as well as the pulley shaft. Since my last post i loosened the belt and greased the generator bearing. The knock sound almost vanished. That in mind I toool it to my fiancés dad who is a mechanic and has worked on vw buses in the past. He seemed to think everything was ok aside from a misfiring cylinder (but compression is good and we think electric ignition and some new wires/plugs should fix that). On the ride home I tore up another belt. Maybe did a total of 80 miles on it... after each 10-15 mile trup I checked tension and made sure it wasnt too tight/loose. I have no idea why it keeps tearing or what I’m doing wrong. Of note, each day its happened, the weather has been quite hot. Maybe 85+

[oldbusboy]

I wonder if there isn't a misalignment between the gen pulley and the crank pulley?

Trevor

[Brett Voigt]

I finally had some time to get back to working on the 912. The two pulleys are aligned. After messing around with it for a while longer, I noticed that when I rev the engine up, the belt sways quite a bit and the outer surface rubs on one of the shroud pieces. I looked closer at the belt that shredded before and the outer layer of the belt was essentially shaved off. Safe to say that the belt is too loose, right? But when I take one of the shims out, the generator makes that knocking sound again. I think I'm just going to replace the generator and be done with it. It is essentially almost 50 years old, lol.

[Bgravolet]

Did you ever find out the problem? My 912 is doing exactly the same thing.

[Brett Voigt]

Yea, I found the answer at 912BBS.org, Basically I had the front and rear nuts flipped, thus no matter how tight the alternator/fan bolts were, they didn't give adequate pressure to stabilize it. I flipped the bolts, and the knocking went away.