WHY would a hydraulic belt tensioner be any quieter than a mechanical belt tensioner?

Today I came across the attached BMW technical service bulletin (SIB 110403) which intimates, under certain circumstances, an hydraulic alternator-belt tensioner may actually run quieter than the original mechanical alternator-belt tensioner on the M54 engine.

Why?

Specifically, since the most common noise complaint for BMW belt tensioners is due to the pulley bearings, why would a hydraulic tensioner be any quieter than a mechanical tensioner?



What kind of "noise" are they talking about in this BMW SIB?
- BMW_July_2005_E39_SIB_110403_mechanical_to_hydraul ic_tensioners.pdf

I saw the same bulletin a while ago. i just replaced my tensioner a couple weeks ago and previously to that a year and change before. stuck with the mechanical vs the supposed upgrade each time and a lot cheaper. think i spent way less than the hydraulic kit each time for the tensioner, idler pully, and new belts.

Bluebee, the noise issue makes no sense to me either, since very little movement actually occurs at the tensioner its self. I cant tell by the diagram if the pulley gets replaced (different part number) as well. A better bearing would make sense.

It could have to do with the angle at which the hydraulic tensioner applies pressure to the V belt. The hydraulic tensioner may act as a "shock" lack of a better term relieving some pressure on the bearings thus preventing premature wear (noise). I wish I had the means to make a sketch at the moment.
These belts apply a significant amount of resistance to the tensioners and maybe the mechanical ones apply pressure at a different angle leading to uneven stress on the bearings.

Originally Posted by Flybot

I cant tell by the diagram if the pulley gets replaced (different part number) as well. A better bearing would make sense.

It's my fault I didn't include the rest of the service bulletin.

Here is the PDF:
- BMW_July_2005_E39_SIB_110403_mechanical_to_hydraul ic_tensioners.pdf

And below are the other two screenshots.

I'm pretty sure the entire mechanical tensioner is being replaced.

But that still doesn't tell us why a hydraulic alterator-belt tensioner could be quieter than a mechanical alternator-belt tensioner.

What's different about the alternator belt anyway (from the air conditioning compressor belt)?

Originally Posted by bmw_jeff_325i

It could have to do with the angle at which the hydraulic tensioner applies pressure to the V belt.

That might explain why it's only the alternator belt (and not the AC compressor belt) that has this noise problem!

Originally Posted by bmw_jeff_325i

These belts apply a significant amount of resistance to the tensioners and maybe the mechanical ones apply pressure at a different angle leading to uneven stress on the bearings.

Interesting. Very interesting.

Here is what GP Burdell had to say over in this thread:
- How to tell if we have hydraulic or mechanical belt tensioners

Originally Posted by G. P. Burdell

The Gates technical bulletin posted by cn90 states that hydraulic tensioners are "mainly used on applications with high loads and/or angular vibrations, where a mechanical automatic tensioner can not provide sufficient damping or tensioner movement." It may be that the mechanical tensioner, while less expensive for BMW to install at the factory, wears its pulley bearings out faster and becomes noisy because it's less capable of handling the loads of the belt drive system compared to the hydraulic tensioner.

So, it seems, as far as we can surmise, the alternator tensioner pulley bearings may be the same ... however ... the force on the bearings might be different due to the way hydraulic tensioners work versus mechanical tensioners.

The key obvious questions unanswered in this BMW SIB are:
Q1: Is the bearing loading any different between hydraulic & mechanical tensioners?
Q2: Are the bearings themselves any different between hydraulic & mechanical tensioners?
Q3: Is the problem only related to the alternator tensioner or does it also apply to the AC belt tensioner?

GP Burdell and bmw_jeff_325i provided a decent hypothesis to answer question #1 ...

To help with question #2, I just posted over here the following reference, which suggests the bearings are EXACTLY the same, at least for the idler roller and for the AC mechanical belt tensioner.

However, this reference doesn't attempt to compare the bearings to the hydraulic tensioner bearings per se (so it's just one data point) ... but it does say the bearings are the same as in the idler roller (which would be common to both systems):
- E46Fanatics > Tuning & Tech > DIY: Do It Yourself > a/c belt tensioner pulley

Note that this E39 reference also says you can replace "just" the pulley in the mechanical tensioner:
- Squeak

Today, someone struggled with the wrong bolt attempting to remove a hydraulic tensioner:
- Need help changing my belt - problem with hydraulic tensioner

Luckily, folks set him straight with nicely edited pictures, which I reproduce below for the edification of those who need to know.

See also:
- Belt Replacement Instructions (Incl. Fan Removal)

100K miles on vehicle and I replaced the cooling system which included the tensioners. I have mechanical, I replaced with mechanical. Never any noise and if they last 100K miles I'm happy.
Problem is that the mech tensioner is very hard to find now.

Originally Posted by gtxragtop

Problem is that the mech tensioner is very hard to find now.

I got mine easily from OemBimmerparts and it has been working fine, for me.

Another as yet unmentioned difference between the hydraulic and mechanical tensioners is that the hydraulic, apparently, has an adjustment arc!
- E39 (1997 - 2003) > AC tensioner installation

Originally Posted by bluebee

I got mine easily from OemBimmerparts and it has been working fine, for me.

Another as yet unmentioned difference between the hydraulic and mechanical tensioners is that the hydraulic, apparently, has an adjustment arc!

Hydraulic tensioners on the inline 6's do not have the adjustment arc you show above, only the v8 cars have those to allow for removal of the hydraulic shock.

Originally Posted by bimmerteck

only the v8 cars have those to allow for removal of the hydraulic shock.

Ah, that's GREAT to know (as I only know what I know and read).

How's this summary, to help others?

• I6 E39 mechanical tensioners (no adjustment necessary nor possible)
• I6 E39 hydraulic tensioners (no adjustment necessary but possible via the centerbolt)
• V8 E39 mechanical tensioners (no adjustment necessary nor possible)
• V8 E39 hydraulic tensioners (adjustment necessary via the bolted arc)

I had both mechanical tensioners on both belts. The noise I had was from the pullies - they were loose. I changed both with identical mechanical tensioners. This year. And my car was among the last crop of e39 build. To better judge, it would have been nice to have a sound clip of the "offensive" noise, but without a proof I am very much inclined to believe the sound was coming from a failed pulley bearing, which you will experience anyways, mechanical or "hydraulic" tensioner. BTW, the "hydraulic" tensioner is not hydraulic at all. It has a good size spring in an oil bath. To me hydraulic = a shock absorber of some sort (oil dampener with a valve)

Originally Posted by bluebee

Ah, that's GREAT to know (as I only know what I know and read).

How's this summary, to help others?

• I6 E39 mechanical tensioners (no adjustment necessary nor possible)
• I6 E39 hydraulic tensioners (no adjustment necessary nor possible)
• V8 E39 mechanical tensioners (no adjustment necessary nor possible)
• V8 E39 hydraulic tensioners (no adjustment necessary component removal facilitated via the bolted arc)

Fixed.

Originally Posted by doru

I had both mechanical tensioners on both belts. The noise I had was from the pullies - they were loose. I changed both with identical mechanical tensioners. This year. And my car was among the last crop of e39 build. To better judge, it would have been nice to have a sound clip of the "offensive" noise, but without a proof I am very much inclined to believe the sound was coming from a failed pulley bearing, which you will experience anyways, mechanical or "hydraulic" tensioner. BTW, the "hydraulic" tensioner is not hydraulic at all. It has a good size spring in an oil bath. To me hydraulic = a shock absorber of some sort (oil dampener with a valve)

Correct, the "hydraulic" in this case is essentially an oil damped spring pushing on the pitman arm which holds the tensioner pulley. Like many cases in TIS . . . a lot seems to be lost in the translation.


As to why they may be quieter, I believe the oil damped spring using the mechanical advantage offered by the lever that the tensioner pulley is mounted on affords a more consistent tension on the accessory belt throughout the tensioner's range of motion. Where the coiled spring "mechanical" style tensioners vary in tension throughout the range of motion.

Originally Posted by bimmerteck

Fixed.

• I6 E39 mechanical tensioners (no adjustment necessary nor possible)
• I6 E39 hydraulic tensioners (no adjustment necessary nor possible)
• V8 E39 mechanical tensioners (no adjustment necessary nor possible)
• V8 E39 hydraulic tensioners (no adjustment necessary component removal facilitated via the bolted arc)

Thanks! This is what I love about this place. Everyone pitches in where they can to help the tribe!

Originally Posted by doru

the sound was coming from a failed pulley bearing, which you will experience anyway, mechanical or "hydraulic"

I think, if I understand the situation, that the mechanics of the hydraulic tensioner (angles, loads, whatever), is such that there is less of a load on the pulley than on the mechanical tensioners.

It's not 'really' a function of hydraulic or mechanical ... it's more a function of one design versus another ... where the hydraulic design somehow offers a lower load on the pulley bearings.

Hence, the pulley bearings on the hydraulic tensioner, being under less load, last longer before failing. Hence no sound.

Or so I've been told ...

Originally Posted by bimmerteck

"hydraulic" in this case is essentially an oil damped spring

It's my understanding that "hydraulic" is a coiled spring in oil; mechanical is a circular spring, in air.

Originally Posted by bimmerteck

more consistent tension on the accessory belt throughout the tensioner's range of motion.

OK. I'll accept that. This means, in effect, the bearings won't make noise at any particular point in their range of motion.

BTW, there was a good DIY today, for rebuilding the hydraulic tensioners:
- How to tell if you have mechanical or hydraulic belt tensioners (1) & how to switch from mechanical to hydraulic (1) and what is the difference between the two types (1) (2) & how to rebuild your hydraulic tensioners (1)

The real reason for the hydraulic tensioner is because they relaized they screwed up the oil filter housing design.

The OFH is also aluminium and due to ONLY 2 mounting bolts for the mechanical tensioner (vs 3 for the hydraulic tensioner), the OFH will warp, then it will rattle (this rattle is the root cause of the BMW service bulletin), then it will crack.

How I know? Read this post over at Roadfly. Eurodavid figured it out, and it wasn't because BMW was kind to him. Go to reply #46 if you're not in the mood to read everything.

Doru, doin' some archive-ology!

Originally Posted by bobdmac

Doru, doin' some archive-ology!

Because I thought it's only fair to let people know what the real reason is from going from mechanical to hydraulic tensioner. The noise is not created because of the speculation of how the roller applies force on the belt, yadda, yadda, yadda. We, as a bunch of enthusiasts aknowledge issues with our car model, and try to explain why certain parts fail, why did BMW changed design etc.

Some good outcomes are BikeStillRule new DISA flapper design, Rajae's Vanos O-rings & anti-rattle kit, Zionsville integrated radiator (although mighty expensive), Chiefs conversion to NPG coolant (IMHO works for v8 better than for i6), some new cupholder designs (also expensive - TEC style), etc.

Having the real root cause of the issues exhibited, you could eliminate one by one as many problems as you can (in time), and hopefully end up with a bulletproof car. I followed quite a few things, and I am happy with the outcome. I work on my car in the summer (preventative), and drive it in the winter non-stop. BTW, for the last few days, the temperature was really low (today it was -28°C for example), but I still drive my 9 year old car as it were new. Does not skip a beat, knock on wood. Grace to this and other forums that help keep the e39-ers on the road.

Doru, I agree, and full props. I just read through the thread again because of the good info in it. I've found many of the old, resurrected threads have good info, and so for me archive-ology is a good thing.

I thought the noise was from the bearings.

I didn't realize it was from the ofh.

How does the ofh cause noise?

Originally Posted by Trebbia

I thought the noise was from the bearings.

I didn't realize it was from the ofh.

How does the ofh cause noise?

That question for real?

Thanks for the roadfly link doru it was an eye opener for sure. Sad thing is I just replaced the mechanical tensioner in my 530. Guess I'll be spending that money again soon on the hydraulic unit.

Originally Posted by doru

The real reason for the hydraulic tensioner is because they relaized they screwed up the oil filter housing design.

The OFH is also aluminium and due to ONLY 2 mounting bolts for the mechanical tensioner (vs 3 for the hydraulic tensioner), the OFH will warp, then it will rattle (this rattle is the root cause of the BMW service bulletin), then it will crack.

How I know? Read this post over at Roadfly. Eurodavid figured it out, and it wasn't because BMW was kind to him. Go to reply #46 if you're not in the mood to read everything.

My OFH required replacement last fall for this very reason. The tensioner was also replaced at that time.

Good look up D!

Originally Posted by Tuskfish

Thanks for the roadfly link doru it was an eye opener for sure. Sad thing is I just replaced the mechanical tensioner in my 530. Guess I'll be spending that money again soon on the hydraulic unit.

I know, I'm in the same boat. Did a total cooling overhaul this last spring, and replaced the mechanical tensioner with a similar unit...I didn't know at the time...