Re: 350sdl serpentine belt tensionerr

Does anyone have experience in finding the causes of tensioner failure?
My tensioner failed after 20,000 miles. The water pump had a little play, so I replaced it. The alternator pulley was 2 mm out of line so
I bought the latest pulley and placed a spacer behind it to bring it in line. Both of these devices have a rotational error of .1mm. and no play.
The power steering pump shaft has an error of appx. .1mm and tends to bind a bit after 3or 4 turns, then it becomes free again. The pump was rebuilt 20,000mi ago. Bearing and all seals were replaced This is a two stage hydraulic pump which serves both the steering system and the hydraulic blocking (which is not active).
The a/c clutch was rebuilt 20,000 miles ago and does not have any rotational error but does have a slight amount of play. I have not measure it yet.
Incidentally I am using Hylomar blue as the sealant on the water pump. You can seperate the unit without destroying the seal. It does howerver cost $28.50 per 3.5 oz.. It is OE in the rebuilding process of MB.
I chose to buy a new (non rebuilt) water pump instead of MB's rebuilt units.
Thanks in advance for your comments.
Constantine
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"Incidentally I am using Hylomar blue as the sealant on the water pump. It is OE in the rebuilding process of MB."
Huh? I have NEVER seen blue sealant in/on any factory reman anything from MB. And I have been working at the same MB only dealership for 28 years.

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The motor was rebuilt at the MB plant in Berlin. The water pump came with the motor. You can see the crate engine at www.scaffolding.com/mb

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http://www.scaffolding.com/mb/603.970%20front%20angle.jpg
shows the water pump, but I do not see blue. I see the off-white of the sealant used on the timing cover.

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You can't see the sealant because such a small amount and is sandwiched between the pump and the motor. It is also light blue. I had contacted both Loctite and Permatex and both indicated that the only manufacturer of such a sealant, which is easily removable is Hylomar, a UK manufacturer.
Do you have any suggestions for actual troubleshooting? I am presently viewing the power steering pump as a strong candidate for the guilty part.

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There are I am sure other reasons why a tensioner on this car [I am thinking that it is a 1991 or that series?] would go bad, but the only one that I know of is improper installation of the tensioner. If the tensioner is rotated during installation it can cause it to die prematurally. Sorry, that I can't be of more help. Best of luck.
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This is a 91 350SDL. Actually the tensioner was factory installed. The tensioner rotates on ball bearings, how can there be any improper installation? Can you go into more detail? Thanks.

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I think that we are talking at cross purposes. When I think of the tensioner, I am thinking of the spring loaded rubber tensioner that holds the pulley up under tension. If you are talking about the pulley itself, then I have never seen one go bad. I would assume that the bearings are shot and that you need a new one.
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I am writting about the aluminum device that the black rotating pulley is attached.. The spring is attached on one side and the shock on the other end . It houses two seals (?) and the the bearings. Under extreme vibration, heat is generated through the shock and that heat ultimately leads to failed bearing housing, the loss of the high temperature grease which is inside the bearing housing and a seized bearing shaft with the bolt.
I am of the belief that external forces bring the tensioner to its failing point, which is probably not very substantial.
I now have brought all pulleys (power steering,A/C, alternator and water pump) into line and within .1mm of roundness. I am usng a Starrett dial indicator with a .2mm range, mounted on a magnetic base and attached to extension rods.
It will be interesting to see how long this pulley lasts.

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