[Tomnavigator]

I recently received a private message from a forum member asking me about the benefit/need for a clutched alternator pulley. I think other forum members will benefit from this information. If one person benefits, then this is worth my time. If you are impatient, don't read this. To everyone else, this post will be long and I hope educational.

First, a little about me. I have had diesel vehicles for over 20 years. I currently have 4. 2001 2500hd duramax truck, 2004 VW Touareg V-10, 2006 VW bug, 2010 BMW335D. I have the range of diesels from including 4 cylinder, 6 cylinder, 8 cylinder, and 10 cylinder diesels. I once worked for General Motors for 18 years as an engineer. My last 8 years were at the Corvette plant in Bowling Green, KY. I am now retired.

I will use a 6 cylinder engine in my examples since we own a 6 cylinder engine. A 6 cylinder engine fires 3 cylinders every time the engine turns over one complete revolution. As the piston goes up on the compression stroke, the engine slows down slightly because of then energy expended compressing the gasses. Then the cylinder fires and the engine speeds up slightly. This happens 3 times for every revolution of the engine. All engines speed up and slow down as they turn whether gasoline or diesel. This is more pronounced in diesel engines since diesel engines have more compression and more torque.

The speeding up and slowing down of the engine creates a jerking motion or studdering motion as the engine turns. This motion is what is called a harmonic vibration. Almost everyone has ridden on a bus, train, electric tram,or subway, when it speeds up you are thrown back when it slows down you are thrown forward. The harmonic vibrations of the engine have the same effect on the accessories of the engine. The harmonic vibration is hard on the components driven directly by the engine. Therefore, manufacturers install a harmonic dampener on the engine. This is a pulley on the crankshaft of the engine with a rubber donut in the middle and the accessory drive pulley on the outside. The purpose of the harmonic dampener is to dampen out the studdering of the engine for the accessories. As the engine slows down the engine accessories try to keep turning. The rubber donut flexes a little one way like a rubber band to allow the accessories to keep turning. As the engine speeds up the donut flexes the other way. This dampens out the harmonic vibration of the engine as seen by the accessories.

The harmonic vibration is greater on diesel engines because diesel engines have more compression (slowing down the engine) and more torque (speeding up the engine). Harmonic vibration is more pronounced at idle speeds. Harmonic vibration is also more pronounced on engines with less cylinders. Vehicles that spend more of their life idling have more harmonic vibration issues (failures listed below) than vehicles that spend more of their life at engine speeds greater than idling. Less harmonic vibration equals less stress.

Harmonic dampeners help with but do not totally eliminate harmonic vibrations. You can see the harmonic vibration left in you engine when you look at the drive belt when you engine is idling. The whipping (flexing of the belt) is caused by the remaining harmonic vibrations as the engine speeds up and slows down.

Many manufacturers find the remaining harmonic vibrations unacceptable because of one or more of the following reasons; 1) Vibrations can be felt by the driver when the engine is idling, 2) The vibrations create a noise, 3) The flexing of the drive belt is excessive, 4) The drive belt life is unacceptably shortened, 5) The life of the harmonic dampener is shortened, 6) Accessory component life is shortened. The solution to these problems is installing a clutched pulley on the alternator.

A clutched pulley acts like a ratchet wrench preventing the pulley from slowing down the alternator when the engine slows down. But, turning the alternator when the engine turns. This allows the alternator to continue to turn when the harmonic vibrations are slowing down the engine. Allowing the alternator to continue turning eliminates most of the stresses on the drive belt, accessory drive belt tensioner, and harmonic dampener when the engine slows down due to the harmonic vibrations. This reduction in stress makes these components (harmonic dampener) last much longer.
Now for the bottom line.

BMW chose to save a little money by installing a fixed pulley on the alternator instead of a clutched pulley. This is why the harmonic dampeners tend to fail on the BMW 335D more frequently than some other cars. It does not mean the factory harmonic dampener is bad. It just means the harmonic vibrations end up fatiguing the rubber causing failure.

You have four choices to handling the harmonic vibration issues of your car.
1) Do nothing until your harmonic dampener fails then install a new harmonic damper. Depending on your idling time you may have not ever have a harmonic dampener fail.
2) Install a clutched alternator pulley now. This will at least double the remaining life of your harmonic dampener and reduce the chance failure of accessory drive belts, drive belt spring tensioner, and alternator.
3) Wait until the harmonic dampener fails and install a clutched alternator then.
3) Install fluidampener harmonic dampener which does not flex as much as a rubber dampener. This is why it is required to install a clutched alternator pulley with a fluidampener.

Now, my opinion. Keep in mind an opinion is just like an asshole, it may stink. But, everyone has one and my opinion is worth just what you are paying for to get it. Nothing.

A driving style that includes a lot of city driving (stop and go) will include a relatively larger percent of the time idling thus increasing the stress on the harmonic dampener. A driving style where you spend more time on the highway reduces idling time and creates less stress on the harmonic dampener because the percentage of the time idling will be reduced. I think that people that have early harmonic dampener failures have a higher percentage of city driving than people that spend most of their time on the highway.

I think the most cost effective way to make your harmonic dampener last the life of your vehicle is to install a clutched alternator pulley on your vehicle now no matter how many miles you have on your harmonic dampener. This will reduce future failures of your harmonic dampener. If and/or when you have a harmonic dampener failure, replace the harmonic dampener with a rubber one and the new harmonic dampener will probably last more than twice as long as the factory one.

A piece of trivia for anyone who has lasted this long on my posting. There is much confusion over harmonic dampeners vs harmonic balancers. I have explained harmonic dampeners above. A harmonic balancer is just an accessory drive pulley that is mounted to the crankshaft that is solid. It is usually installed on race engines where vibration is not important. The name "harmonic balancer" comes from marketing. It allows the customer a way to ask for the component they want when going to purchase it and it allows the manufacturer of the hydraulic balancer to ask more money for a simpler pulley. It may or may not be heavier than the factory hydraulic dampener. No car comes from a manufacturer with a hydraulic balancer. They all have hydraulic dampeners. We have hydraulic dampeners on our BMW 335Ds.

[lnxguy]

Where did you get your data that states using a clutched alternator pulley would double the life of a stock HB? Or is this just an assumption?

[Tomnavigator]

lnxguy,
I confess, the life being double is my estimate based on several things. One, you never hear of vehicles with clutched pulleys having chronic hydraulic dampener failures and there are failures of our harmonic dampeners in less than 70,000 miles. Second, just watching the difference between the whipping of the accessory belt with and without a clutched dampener, you can see there is much less stresses on the belts, etc. Third, totally eliminating the jerking of the belt, alternator, etc. when the engine slows down, allows everything to continue turning which intuitively means longer service life. Fourth, my experience with my past diesel vehicles shows that failure of clutched alternator belts significantly increases engine vibrations and belt whipping.
I doubt that anyone other than a manufacturer has any empirical data on service life of components with and without clutched pulleys and they are not going to share this with anyone.
Notice that I put the extended life being more than double in my posting in the section of "In my opinion" and I tried to clarify that it would "probably" make life more than twice as long. If I had actual data, I would have given it. I have over 50 years of working with and on vehicles, a little education, and experience with diesels with clutched pulleys. I have owned 6 diesel vehicles with over 892,000 miles on them. Four of them I still own. The cumulative age of my current 4 diesel vehicles is 75 years old (average age 18.75 years). My 335D is my newest diesel with over 100,000 miles on it.

Again, this is my opinion and worth every penny you are paying for it. I do back my opinion with the fact that I bought a clutched pulley for my 335D. Right or wrong, this is my opinion.

Another clarification on harmonic balancer. There are actually harmonic balancers that have a balance weight on them to help eliminate unbalanced forces in some engines. They are just not frequently found and seem to have gone out of favor over 60 years or so ago.