Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help



Go Back   PeachParts Mercedes-Benz Forum > Mercedes-Benz Tech Information and Support > Tech Help

Reply
 
LinkBack Thread Tools Display Modes
  #1  
Old 07-28-2005, 02:44 AM
Registered User
 
Join Date: Mar 2003
Location: Southern California
Posts: 2,272
Drive belt tensioner failure analysis

All references to the tensioner are from the perspective of a person standing in front of the engine compartment, looking at the tensioner. Thus, right/left and CW/CCW are opposite to the perspective of someone siting in the driver's seat.

The subject serpantine belt tensioner is from a 1992 300E, which only has 35,000 miles.

The tensioner consists of an aluminum housing, which has an arm for the pulley and an aluminum inner bushing whose axial movement is limited by snap rings, The front face of this bushing has a steel insert, which contains the "rachet" for the pointer and is the load bearing surface for the lock bolt load against the mounting strut (the "crowfoot bracket" or "Y-bracket") Looking at both the front and rear faces of the tensioner, from the inner aluminum bushing to the housing you observe the inner bushing, a steel ring, a rubber ring, another steel ring, and finally the housing. The steel ring/rubber ring/steel ring visible on both ends are sealed ball bearings that keep the inner bushing firmly fixed, radially, relative to the housing. Viewed another way, since the 19 mm hex lock bolt fixes the inner bushing to the engine structure, the sealed ball bearings fix the housing radially and the snap rings fix it axially, but allow it to rotate relative to the inner bushing, which swings the tensioner pulley in an arc of about 2.75" radius.

Job number 13-3452 describes a tensioner functional test. It impllies that in the unloaded state, imaginary lines from the two flats on the back side of the inner bushing should pass equidistant on each side of the pulley axis. (Confirmed by KermitF observing a new tensioner - thanks!) This should be the unloaded state of a good used or new tensioner. To test, mount the tensioner in a vice with soft (hard wood or equivalent) jaw inserts clamping the flats. Then, using the pulley as a hand hold, rotate the housing about 30 degrees off line in each direction, which should take a good deal of force. (This is the approximate angle the pointer moves through during tensioning, which indicates the relative angular displacement between the inner aluminum bushing and housing). If the pulley does not return to its original position after being displaced this angular amount in each direction, it is not serviceable. DO NOT USE IT!

On the car, tensioner failure is indicated if the pointer does not move back to the bottom of the ramp as belt tension is released - assuming that the pointer started out at the top of the ramp or just beyond. Such a tensioner is suspect, and it should be removed for the above test and be prepared to buy a new one. Using a failed tensioner will just cause you more heartache and may damage other parts if you attempt to use it to tension the belt.

Since this tensioner had failed - the center bushing could be rotated to any position without springing back - I decided to disassemble it and figured that this could not be done non-destructively.

After removing the snap rings I used a hammer and wood block to knock out the front bearing by pounding on the back side of the inner aluminum bushing. Then I hammered on the front side and knocked out the rear bearing and inner aluminum bushing as a unit.

What is contained inside the housing - between the inner bushing and housing and the two sealed ball bearings is the "tensioning element" - a big rubber bushing. Of course this was destroyed by my disassembly, but it was already completely broken internally before I started.

It works like a suspension bushing. As the 13 hex mm tensioner nut is rotated CW to the point of taking all the slack out of the belt, continued CW rotation of the nut continues to push the tension rod down (remember, they are LH threads), which rotates the inner bushing CCW while the housing rotates very little due to belt stiffness holding it in place. All of the relative rotational displacement between the inner aluminum bushing and housing is absorbed by bushing deflection or what I will call "strain" - just like what happens when you twist or bend a rubber eraser - and the greater the torsional strain of this rubber bushing, the more torsional resistance it offers, but it's probably highly non-linear.

Suspension bushings work the same way, HOWEVER, the bushing bolts should only be torqued at normal ride height so that the bushing is not strained in torsion when the car is sitting still. Bushing torsional strain only occurs with suspension movement while the car is moving. This is also the reason why you should not store a car on jackstands as the bushings will be strained in torsion, which can damage them. Permanent full time strain also damages the tensioner rubber bushing. In fact, failure is virtually guaranteed!

Rubber is subject to "creep" - strain it long enough, and it will not return to its original shape when the load is removed. Also, rubber is subject to degradation from thermal effects, and this degradation increases exponentially with temperature. Tensioners that see a lot of low speed drive time in hot weather, which usually creates the highest underhood temperatures are not going to last as long as one that sees primarily highway speed driving, and this car spent a lot of time in Palm Springs.

The bottom line is that this design belt tensioner is a recipe for failure. The individual parts are not cheap or shoody. In fact they appear to be of very high quality. The problem is that using a torsionally strained rubber bushing to maintain belt tension is just a piss-poor design architecture, and I'm utterly dumbfounded that Mercedes used a rubber bushing instead of a mechanical spring tensioner, which, if properly designed, should last virtually forever.

It's also too bad for us that the aftermarket has not come up with a substitute mechanical spring design.

Failure of this rubber bushing - as indicated by the pointer not moving down the ramp as belt tension is released can lead to consequential damage when you install a new belt and attempt to tension it. (The tensioner can also fail in service - either slowly due to creep or thermal degradation or suddenly if the rubber breaks.)

When you begin tensioning the new belt, the inner bushing starts out rotated farther CCW than if the tensioner was good and the inner aluminum bushing returned to its original unloaded angle relative to the housing. As a result, attempting to tension can cause the tensioning rod to interfere with the inner aluminum bushing as you continue to rotate the bushing further CCW, which will bend the rod. In extreme cases you could even run the nut off the top of the adjuster rod thread. (Remember, it's a LH thread so as you turn the adusting nut CW it pushes the rod down, which rotates the inner aluminum bushing CCW.)

So the bottom line is observe that pointer as you remove belt tension. If it's not between the top of the ramp and the thick solid line, set it there, then watch to see that the pointer slides down the ramp as you release belt tension. If it doesn't, replace the tensioner. DON'T attempt to reuse it!

To be sure, set it up in a vice and run the above mentioned test. Observe the radial position of the inner aluminum bushing and housing by drawing those imaginary parallel lines from the flats toward the pulley. Are they equidistant from each side of the pulley axis? If not you've probably got a paperweight. Run the angular deflection test anyway, and I'm sure you will find that it fails.

Buy and install new tensioner.

Duke


Last edited by Duke2.6; 07-29-2005 at 12:13 PM.
Reply With Quote
  #2  
Old 07-28-2005, 03:56 AM
Registered User
 
Join Date: Jan 2003
Location: Sydney, Australia
Posts: 627
Quote:
Originally Posted by Duke2.6
The problem is that using a torsionally strained rubber bushing to maintain belt tension is just a piss-poor design architecture, and I'm utterly dumbfounded that Mercedes used a rubber bushing instead of a mechanical spring tensioner, which, if properly designed, should last virtually forever.

Duke
Duke,

You summed it up beautifully! How they get a few things like this so wrong on what are otherwise well designed cars amazes me. I have learned to replace these tensioners as a matter of course whenever I have removed them for other work (eg. water pump replacement). Even the simple action of removing and replacing the belt can be enough to cause an ageing tensioner to fail. Whilst they are not cheap to buy, it saves a lot of hassles in the long run to simply replace them when you are already working in that area.

Greg
__________________
107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.
Reply With Quote
  #3  
Old 07-28-2005, 08:50 AM
I told you so!
 
Join Date: Jun 2002
Location: Motor City, MI
Posts: 2,853
What surprises me is that the replacement part is the same lousy design. Often the replacement part of a known problem is an improved design.
__________________
95 E320 Cabriolet, 159K
Reply With Quote
  #4  
Old 07-28-2005, 09:00 AM
LarryBible
Guest
 
Posts: n/a
Yes, the action of replacing a belt will sometimes be the demise of the tensioner. That's why my 300E has only the third belt in it's 280,000 mile life. I inspect the belt closely and often. It has yet to leave me beside the road. Luckily the belt is a much higher quality item than the tensioner.

As far as WHY they use a rubber bushing, I'm sure their thinking was for vibration and noise suppression. I'm not defending their design, I'm only stating why they chose this approach.

Have a great day,
Reply With Quote
  #5  
Old 07-28-2005, 10:36 AM
Registered User
 
Join Date: Mar 2003
Location: Southern California
Posts: 2,272
It's interesting that we rarely seem to heard of tensioner failure in service, but it did happen to a friend's '88 190E 2.6 back when the car was fairly new. He drove about 20K miles a year and offed the car when it had 90K. I can't remember the mileage it failed, but the belt broke, took out a power steering hose, which made a mess in the engine compartment and the combination of the broken belt and oil spray took out the hood pad. The culprit was the belt tensioner, but the dealer did the repair, so I never saw anything.

I had my belt replaced apriori when the dealer changed the water pump under warranty at 45K miles. That was circa 1992. As you say, the belt itself appears to be very high quality and long lived - maybe good for up to 100K miles and inspecting it is fairly easy. On the subject 35K mile 300E the belt had numerous transverse cracks on the inside ribs, and I attribute this to lots of time at low speed in hot weather (Palm Springs). The belt on my car is now about 13 years old, but only has about 30K miles, and it looks like new.

Job number 13-342 in the old 103 engine manual under step 11 has the following note:

"As from November 1986 the alternator is attached at the bottom to the bracket with a 13 mm collar bolt. If this collar bolt is removed, the V-belt can be slackened by swiveling the alternator without slackening the bolt (16) [which is the tensioner lock bolt in the photo]."

I would suggest that, especially in a pinch, replace the belt by using this method as I think most old tensioners may not be reuseable, however I have to add that this "trick" is NOT mentioned in job number 13-3420 (belt replacement) in the CD, which was written later in time.

The rubber bushing does provide torsional damping, especially when new, but this probably decreases as the rubber bushing ages. Also, there is a job in the CD "Retrofiting Tensioner Damping Device" or similar verbiage that implies the external damper was not part of the original design, and early 103 engines may not have had it.

In any event, I'm sure many of us could come up with a better design architecture, and it's too bad for us that the aftermarket has not done this.

I just placed the 190 in summer storage and brought out the '91 MR2, so I have six months to think about it.

Duke
Reply With Quote
  #6  
Old 07-28-2005, 10:42 AM
LarryBible
Guest
 
Posts: n/a
Quote:
Originally Posted by Duke2.6
It's interesting that we rarely seem to heard of tensioner failure in service, but it did happen to a friend's '88 190E 2.6 back when the car was fairly new. He drove about 20K miles a year and offed the car when it had 90K. I can't remember the mileage it failed, but the belt broke, took out a power steering hose, which made a mess in the engine compartment and the combination of the broken belt and oil spray took out the hood pad. The culprit was the belt tensioner, but the dealer did the repair, so I never saw anything.

I had my belt replaced apriori when the dealer changed the water pump under warranty at 45K miles. That was circa 1992. As you say, the belt itself appears to be very high quality and long lived - maybe good for up to 100K miles and inspecting it is fairly easy. On the subject 35K mile 300E the belt had numerous transverse cracks on the inside ribs, and I attribute this to lots of time at low speed in hot weather (Palm Springs). The belt on my car is now about 13 years old, but only has about 30K miles, and it looks like new.

Job number 13-342 in the old 103 engine manual under step 11 has the following note:

"As from November 1986 the alternator is attached at the bottom to the bracket with a 13 mm collar bolt. If this collar bolt is removed, the V-belt can be slackened by swiveling the alternator without slackening the bolt (16) [which is the tensioner lock bolt in the photo]."

I would suggest that, especially in a pinch, replace the belt by using this method as I think most old tensioners may not be reuseable, however I have to add that this "trick" is NOT mentioned in job number 13-3420 (belt replacement) in the CD, which was written later in time.

The rubber bushing does provide torsional damping, especially when new, but this probably decreases as the rubber bushing ages. Also, there is a job in the CD "Retrofiting Tensioner Damping Device" or similar verbiage that implies the external damper was not part of the original design, and early 103 engines may not have had it.

In any event, I'm sure many of us could come up with a better design architecture, and it's too bad for use that the aftermarket has not done this.

I just placed the 190 in summer storage and brought out the MR2, so I have six months to think about it.

Duke
What a GOLDEN piece of information! I will use this method henceforth. It makes me want to drop what I'm doing and go do a belt inspection, although I will look in my log book first. I think I replaced it only about 10,000 miles ago.

Thanks for your studiousness (is that a word?)

Have a great day,
Reply With Quote
  #7  
Old 07-28-2005, 03:33 PM
Registered User
 
Join Date: Apr 2002
Location: westchester, ny
Posts: 747
Duke, Thanks for all that analysis & information. It will be very helpful.
Rich
__________________
87 300e (white/black; amg body kit)
88 300ce (red/cream; amg body kit)
93 300ce cabrio (white/blue/blue top)
93 300ce cabrio (black/grey/black top)
98 ml 320 (totaled @ 137,000 miles)
99 clk320 (black/grey/black top)
Reply With Quote
  #8  
Old 07-28-2005, 04:27 PM
Registered User
 
Join Date: Sep 2004
Location: DFW / Collin County Texas
Posts: 1,882
Excellent piece of info! I know the tensioner on my 300E is fairly new (PO receipt), but for some reason when I replaced the water pump back in February I could never get it quite as tight as it used to be. As a result, if I really gun the car I can cause the A/C compressor to slip which kills it. I wonder if I need a new tensioner now?

I wonder if the swiveling of the alternator trick will work on the M104? I have an A/C compressor to do this weekend on a friend's car, and I hate to kill his tensioner in the process. As far as I know, it's the original tensioner at 145K.
__________________
08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280
Reply With Quote
  #9  
Old 07-28-2005, 09:08 PM
Registered User
 
Join Date: Mar 2003
Location: Southern California
Posts: 2,272
It sounds like the 300E tensioner is "bad". The only way you'll know for sure is to remove and inspect/test it per the CD procedure I outlined in the top post.

Don't know about the 104. I found the note about removing the alternator bolt in the pdf of the original printed 103 engine manual that's part of the 201 tech data on the at http://mb.braingears.com site. The later written belt change procedure that's part of the 124 and 126 CDs does not contain this note.

Duke
Reply With Quote
  #10  
Old 07-28-2005, 11:07 PM
Registered User
 
Join Date: Mar 2004
Location: Atlanta
Posts: 131
belt tensioner

Duke,

Having just installed my new tensioner and have studied the parts and compared them against various posts here, I can confirm that, in the unloaded state, the imaginary lines from the two flats on the back side of the inner bushing do pass equidistant on each side of the pulley axis.

(My old tensioner looked to be @ 40 degrees off from this - for comparison purposes.)

KermitF
Reply With Quote
  #11  
Old 07-29-2005, 12:55 AM
radunegru's Avatar
Registered User
 
Join Date: Jul 2004
Location: New York
Posts: 334
What is so great about it???!!!
I just changed my belt few hundred miles ago and I did not loose any tensioner or alternator(M104 engine). I just manege to sneak the belt only by hand. It took me about 20 min but I 've done it. Is nice and tight now, a bit tighter beacuse the new belt comparing with the old one is 1.5" shorter.
__________________
New Addition to the family
2013 Passat TDI 5Miles
2004 E500 4MATIC Wagon 95k
1996 E300 Diesel 173k Sold
1993 300E 2.8 230k Sold
1994 E320 White Wagon 373K Sold
Still dreaming about ....500E


Radu Negru
Reply With Quote
  #12  
Old 07-29-2005, 01:27 AM
Registered User
 
Join Date: Mar 2003
Location: Southern California
Posts: 2,272
Quote:
Originally Posted by KermitF
Duke,

Having just installed my new tensioner and have studied the parts and compared them against various posts here, I can confirm that, in the unloaded state, the imaginary lines from the two flats on the back side of the inner bushing do pass equidistant on each side of the pulley axis.

(My old tensioner looked to be @ 40 degrees off from this - for comparison purposes.)

KermitF
Thanks for that confirmation. Your old tensioner was definitely shot. If you had tried to tension the belt with it, you probably would have ended up bending the tensioning rod.

Can you run the vice test? (You don't need to worry about the jaws damaging the flats.) Or just put a 10" cresent on the flats, turn the inner bushing a bit and see if it rotates back. If not, the rubber bushing is broken, which I suspect is the case as was the case with the subject 300E tensioner.

Duke
Reply With Quote
  #13  
Old 07-29-2005, 08:59 AM
LarryBible
Guest
 
Posts: n/a
Quote:
Originally Posted by radunegru
What is so great about it???!!!
I just changed my belt few hundred miles ago and I did not loose any tensioner or alternator(M104 engine). I just manege to sneak the belt only by hand. It took me about 20 min but I 've done it. Is nice and tight now, a bit tighter beacuse the new belt comparing with the old one is 1.5" shorter.
Let us know how long the belt lasts, won't you?

Have a great day,
Reply With Quote
  #14  
Old 07-29-2005, 11:14 AM
nglitz's Avatar
Registered User
 
Join Date: Aug 2004
Location: Hamilton Square NJ, near Trenton
Posts: 391
My 260E has an idler pulley above the alternator. As an alternative to removing the lower alternator bolt, I removed that idler pulley. It looks to be identical to the puller on the tensioner. There's a plastic cover over the front of the idler pulley and then an internal hex bolt to remove (Allen wrench). After a new belt, the idler pulley can be re-installed by catching the tip of its bolt in the hole and then using the Allen wrench to pull it into position before starting to turn the bolt. Obviously care needs to be taken to avoid stripping the idler's bolt in the process of re-inserting it.

When I first did this in the process of installing a new belt (that probably wasn't really needed) I could watch the tensioner needle swing right up the ramp. A week later, the tensioner died on me and needed replacement. I ran it for a week with a dead tensioner (maybe 200 miles total) and not a squeek. The new belt and the way it wraps around the significant power users (alternator, water pump, A/C compressor) probably help.

OTOH, my daughter's '94 Ford Taurus has the world's simplest serp. belt tensioner. One spring and the tensioner arm has a square hole for a wrench handle. Pull the arm back, swap belts and release the wrench handle. Hard to imagine anything simpler.
__________________
Norm in NJ
Next oil change at 230,000miles
Reply With Quote
  #15  
Old 07-30-2005, 12:20 AM
wbain5280's Avatar
Registered User
 
Join Date: May 2001
Location: Northern Va.
Posts: 3,386
Thanks for this very informative post on the tensioner. I will check this on my 300SE.

The two Volvos I had, a 6 cyl 960 and a 5 cyl V70, both have a spring loaded serp. belt tensioner. It's a very simple design. MB should use this. IMHO.

__________________
Regards

Warren

Currently 1965 220Sb, 2002 FORD Crown Vic Police Interceptor

Had 1965 220SEb, 1967 230S, 280SE 4.5, 300SE (W126), 420SEL

ENTER > = (HP RPN)

Not part of the in-crowd since 1952.
Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
w201 belt tensioner xp190 Tech Help 2 04-15-2005 02:22 PM
Belt Tensioner!!!! 190D22 Diesel Discussion 3 04-11-2005 11:04 PM
Help Roadside break down: 190e belt tensioner questions haasman Tech Help 4 11-25-2003 11:04 PM
Need help: Belt tensioner 300E M104.980 Speedswede Tech Help 0 11-15-2001 08:30 AM
190e 2.6 and Squeaky Belt Drive 2benzman Tech Help 5 07-08-2001 09:29 PM



All times are GMT -4. The time now is 06:20 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2018 Pelican Parts, LLC - Posts may be archived for display on the Peach Parts or Pelican Parts Website -    DMCA Registered Agent Contact Page