Finding a noisy pulley bearing - serpentine belt

[rich.g.williams]

The serpentine belt on a typical Mercedes car can drive as many as eight pulleys. Alternator, Water Pump, Crank Shaft, Power Steering Pump, AC Compressor, Kompressor, Idler Pulley Wheel and Tensioner Pulley Wheel. At best finding which Pulley (which Bearing) is creating noise is no easy task. Adding to this, modern Alternators have a freewheeling pulley called an Overrun Alternator Pulley and some AC Compressors have a freewheeling pulley that locks when the AC Compressor is in use.

Faced with something very noisy going on I followed the following procedure with varying degrees of success:-

1. As always remember that the serpentine belt when an engine is running is a serious hazard so whatever you do work carefully.

2. I removed the serpentine belt completely and ran the engine for a short time - the noise went away completely proving that the cause was not elsewhere.

3. I recorded sound from nearby pulleys using my mobile phone - didn't help much.

4. I used a long steel bar to listen to bearings - didn't give conclusive results for my noisy pulley problem.

5. I made a temporary belt out of a Bungy Strap - cut the end hooks off and superglued the the Bungy ends together, then threaded it from the crankshaft onto each pulley (including idler and adjuster wheels) in turn, (left the Power Steering pump till last knowing that it would wear my Bungy out quickly) - didn't produce conclusive results for me but would do for several problems.

After all of this I decided to replace the Power Steering Pump, that made no difference at all. Suspecting the Alternator next I came across a YouTube video about the Overrun Alternator Pulley, that made sense as the likely cause and sure enough the Overrun Alternator Pulley was seized up and rusted solid. Removing the Alternator and then removing the seized up Overrun Alternator Pulley were far from easy tasks but the new Overrun Alternator Pulley cured the problem and the noise went away.

How do professionals deal with this?

Photo of the "Bungy Belt" turning the Alternator and the Water Pump at the same time, shown in use below:-

 

[mattkh]

Well done.
Please throw some more light on what an alternator overrun pulley is.
I have heard of turning each of the many pulleys one at a time by hand to find the faulty one.

 

[LostKiwi]

As above remove the belt and turn by hand feeling for roughness or sloppyness.
Most commonly it's the belt idler or tensioner that gets noisy. Water pumps will often leak if the bearing is bad.

 

[rifiki]

If original change idler and tension pullies, they can let go without any warning.
Check alternator and others for rough/noisy running.
This happened on my CLK and caused more problems.
Should be a service item on time/mileage.

 

[rich.g.williams]

n wh

The serpentine belt on a typical Mercedes car can drive as many as eight pulleys. Alternator, Water Pump, Crank Shaft, Power Steering Pump, AC Compressor, Kompressor, Idler Pulley Wheel and Tensioner Pulley Wheel. At best finding which Pulley (which Bearing) is creating noise is no easy task. Adding to this, modern Alternators have a freewheeling pulley called an Overrun Alternator Pulley and some AC Compressors have a freewheeling pulley that locks when the AC Compressor is in use.

Faced with something very noisy going on I followed the following procedure with varying degrees of success:-

1. As always remember that the serpentine belt when an engine is running is a serious hazard so whatever you do work carefully.

2. I removed the serpentine belt completely and ran the engine for a short time - the noise went away completely proving that the cause was not elsewhere.

3. I recorded sound from nearby pulleys using my mobile phone - didn't help much.

4. I used a long steel bar to listen to bearings - didn't give conclusive results for my noisy pulley problem.

5. I made a temporary belt out of a Bungy Strap - cut the end hooks off and superglued the the Bungy ends together, then threaded it from the crankshaft onto each pulley (including idler and adjuster wheels) in turn, (left the Power Steering pump till last knowing that it would wear my Bungy out quickly) - didn't produce conclusive results for me but would do for several problems.

After all of this I decided to replace the Power Steering Pump, that made no difference at all. Suspecting the Alternator next I came across a YouTube video about the Overrun Alternator Pulley, that made sense as the likely cause and sure enough the Overrun Alternator Pulley was seized up and rusted solid. Removing the Alternator and then removing the seized up Overrun Alternator Pulley were far from easy tasks but the new Overrun Alternator Pulley cured the problem and the noise went away.

How do professionals deal with this?

Photo of the "Bungy Belt" turning the Alternator and the Water Pump at the same time, shown in use below:-


View attachment 83753

When the Serpentine Belt was removed I did try turning each pulley by hand (before running the engine without belt item 2. above). Turning pulleys by hand made me think that the Idler Pulley Wheel may have seen better days so I replaced it first. The Alternator, Water Pump, Crank Shaft, Power Steering Pump, AC Compressor, Kompressor and Tensioner Pulley Wheel pulley all felt fine (I didn't know about Overrun Alternator Pulleys then).

All the pulleys passed my Bungy Strap Belt test without making the problematic bad noise. However I realised that the Bungy Strap Belt test was a really good way to test all Serpentine Belt pulley bearings for noise - generally for Mercedes and any other make of car, hence my photo and post.

I changed the Power Steering Pump because I thought there was something wrong with it internally. The Alternator seemed fine not noisy (that was before I knew about Overrun Alternator Pulley).

After finding out about Overrun Alternator Pulleys I realized that my Overrun Alternator Pulley was seized up and rusted solid. I removed the Alternator and then removed the seized up Overrun Alternator Pulley, fitted a new Overrun Alternator Pulley, fitted the Alternator back into the car, ran the engine and the noise had gone away.

 

[rich.g.williams]

I realised that the Bungy Strap Belt test was a really good way to test Serpentine Belt pulley bearings for noise - generally for Mercedes and any other make of car, hence my photo as follows:-

 

[Blobcat]

If original change idler and tension pullies, they can let go without any warning.
Check alternator and others for rough/noisy running.
This happened on my CLK and caused more problems.
Should be a service item on time/mileage.

Agreed, I was lucky when my idler disintegrated as I'd just arrived home and the belt was still just on

M272 Serpentine Belt Idler and Tensioner failure and replacement

Sat waiting for garage door to open tonight and thought “that’s a strange noise the door is making”. Then realised I could smell plastic burning... :( Engine off, bonnet open, front engine cover off and bits of belt strewn around :( Top idler wheel completely gone... You can see bits of...

forums.mercedesclub.org.uk

 

[rich.g.williams]

Well done.
Please throw some more light on what an alternator overrun pulley is.
I have heard of turning each of the many pulleys one at a time by hand to find the faulty one.

I realised that the Bungy Strap Belt test was a really good way to test Serpentine Belt pulley bearings for noise where there was ambiguity and it is applicable generally for Mercedes and most other make of car. I did try turning each pulley by hand (before running the engine without a belt - item 2. above). Turning pulleys by hand made me think that the Idler Pulley Wheel may have seen better days so I replaced it first. The Alternator, Water Pump, Crank Shaft, Power Steering Pump, AC Compressor, Kompressor and Tensioner Pulley Wheel pulley all felt fine (I didn't know about Overrun Alternator Pulleys then).

So here is my understanding of Overrun Alternator Pulleys (they are also called Freewheel Pulleys and there are other types). When an engine is idling say at 750 RPM an Alternator cannot generate much power unless the Alternator pulley is a lot smaller than the Crankshaft pulley. Typically the diameter ratio between Crankshaft pulley and Alternator pulley is around 3:1 so with engine RPM at 750 Alternator RPM is 2,250 great - plenty of power available from the Alternator. However if the engine RPM is 6,000 the Alternator RPM is 18,000. The mechanical energy stored by the Alternator rotor increases proportional to the square of its RPM and at 18,000 RPM that's a lot of mechanical energy. That's ok until you want the engine RPM to drop quickly. With a fixed pulley the the Alternator is driving the Serpentine Belt as the engine slows down quickly and this wastes energy and causes the Serpentine Belt to be unstable.

Therefore modern Alternators are fitted with a "freewheeling pulley" and in the case of my car called an Overrun Alternator Pulley. See diagram:-

1. Inner race
2. Outer race
3. Freewheel unit
4. Support bearings

For my car the problem noise was caused by seized up Overrun Alternator Pulley clattering slightly back and forth.

 

[rich.g.williams]

Agreed, I was lucky when my idler disintegrated as I'd just arrived home and the belt was still just on

M272 Serpentine Belt Idler and Tensioner failure and replacement

Sat waiting for garage door to open tonight and thought “that’s a strange noise the door is making”. Then realised I could smell plastic burning... :( Engine off, bonnet open, front engine cover off and bits of belt strewn around :( Top idler wheel completely gone... You can see bits of...

forums.mercedesclub.org.uk

That surely is an extreme case!

 

[Blobcat]

That surely is an extreme case! View attachment 83799

Once bearings start to go they can let go or seize within a very short space of time.

 

[curious]

Once bearings start to go they can let go or seize within a very short space of time.

Don't say that.

After successfully identifying a very annoying suddenly squeaky idler on my Vito by squirting some GT40 at them all until I found the right one, it's been quiet ever since.
(Six months ago and now fingers crossed after your comment.)

 

[rich.g.williams]

Once bearings start to go they can let go or seize within a very short space of time.

Yes I accept that. When I turned the pulleys by hand I felt that the Idler Pulley Wheel may have seen better days so I replaced it. The Tensioner Pulley Wheel felt good so I left that alone. Also agree that replacing Idler Pulley Wheel and Tensioner Pulley Wheel could be a service item. Also heard suggested that it should be done when the Serpentine Belt is replaced.

 

[rich.g.williams]

For anyone planning to replace the Overrun Alternator Pulley on their Alternator.

In my case the Overrun Alternator Pulley looked well rusted up so rather than buy the "special removal tool" I decided to cut the Overrun Alternator Pulley off with an Angle Grinder using a thin (1mm) metal cutting disk and plenty of water to keep the metal cool. After cutting off part of the outer race its possible to pull out all the bearing parts and then the outer race comes right off. I then cut flats into the inner race to fit a large 22mm open ended spanner. Using a Torx bit to hold the Alternator shaft and hammering the other end of the spanner with a Copper mallet, impact got the inner race loose (inner race is a clockwise thread onto the Alternator shaft so comes off anticlockwise).

 

[rifiki]

Agreed, I was lucky when my idler disintegrated as I'd just arrived home and the belt was still just on

M272 Serpentine Belt Idler and Tensioner failure and replacement

Sat waiting for garage door to open tonight and thought “that’s a strange noise the door is making”. Then realised I could smell plastic burning... :( Engine off, bonnet open, front engine cover off and bits of belt strewn around :( Top idler wheel completely gone... You can see bits of...

forums.mercedesclub.org.uk

I was about a 100yds from home when it let go. That steering gets heavy.

 

[rich.g.williams]

So far great! Overrun Alternator Pulley, Power Steering Pump, and Idler Pulley Wheel all replaced and running quietly and sweetly. I love Mercedes engineering, such a lot of failsafe built in.

 

[M80]

Wot, no new harmonic balancer?

Couldn't resist.

 

[rich.g.williams]

what's a harmonic balancer?

you got me worried now, should I have fitted a new one lol ?

 

[EmilysDad]

what's a harmonic balancer?

It's when you have a harmonica in each hand ....

 

[M80]

what's a harmonic balancer?

you got me worried now, should I have fitted a new one lol ?

Can cause vibes, but you say yours is smooth now, so you shouldn't worry.

Btw I wasn't aware of the overrun pulley. So valuable education there thanks.
Now I'll have to remove the belt to check it, although I didn't need to before I did know.

 

[rich.g.williams]

Can cause vibes, but you say yours is smooth now, so you shouldn't worry.

Btw I wasn't aware of the overrun pulley. So valuable education there thanks.
Now I'll have to remove the belt to check it, although I didn't need to before I did know.

You will know all about it if the Overrun Alternator Pulley has seized up! It can make a real sharp clacking sound and/or a real wheezy sound in Neutral or low gear when engine speed is quickly dropping right down. When I first started hearing it - thought there was a problem with the automatic transmission in low gears.

Still puzzled as to why mine was seized, a 2003 car that only done 45K, the Alternator was fine but the pulley badly rusted inside. There's a plastic cap on the front of the pulley that comes off. Tempting to put a dob of grease in there, or a shot of WD40.

By the way its easy to check the Overrun Alternator Pulley without removing the Serpentine Belt, just carefully lever the Alternator front cooling fan around with a screwdriver, you can get it to turn clockwise but not anticlockwise. Be aware of the Copper coils inside.