Wow, this one is an epic saga.
Car 1998 E300TD Avtgd.
Had this vibration at idle from the front of the car for a while. Mechanic diagnosed as one of the shock absorber arms of the belt tensioner arrangment mounted to the front of the engine. Duly replaced the part and the vibration ceased.
50 miles later it starts again. Mechanic reckons that its the pivoting arm that the two shock absorbers attach to. So I change that too ...Vibration stops!
50 miles later its started again. I drive the car for a week, annoyed, sometimes it idles purring like a cat and at other times it makes this vibrating noise. It seems that the vibration occurs every time the belt makes one complete revolution so I am wondering why this should be. The belt runs around the pulleys of the water pump, air conditioning unit, power steering pump and alternator (i think), yet the power steering appears perfect, the alternator charges perfectly, the water pump was replaced by main dealer 2 months ago and the air condition is also perfect. All pulley wheels appear to be running straight and true.
Where was I? .... oh yeah, I drive the car for a week ... annoyed .... until pulling up at a junction when there is a strange twanging noise, a chinking of metal against metal and suddenly silence. 'has the noise gone away' I wonder hopefully .... 'beeep' ... yes you've guessed it .... 'Battery Not Charging' ..... ****!
The pivoting arm ( a cast ally unit) has snapped clean in two dropping half of its carcus underneath the car along with its attached pulley wheel. The belt too is is 'off'.
Car goes into garage and the bits are replaced ... again! and guess what ..... it still vibrates!!!
Its driving me crazy. Any ideas welcomed! What is causing it?
Thanks in advance
John

 

Hi,

Interesting.

I would suggest the key to this is the frequency of the vibration.
You have indicated once per rev of THE BELT.

In which case here is a shot from the hip.

A bad join in the belt? May not be visible. What sort of belt are we talking. Vee, polyvee, toothed? Is the tension automatically applied by the tensioner or is it just a follower and you have to set the belt tension? If so is the tension correct?

If the frequency is different what else is rotating at that frequency (or harmonic)?

You will have to determine the frequency by establishing and timing the rythm which is obviously quite low from your comment.

This frequency may determine that it is once per rev of one of the auxillaries and not the belt itself. e.g. A "tight" spot on the PAS pump.

Just needs a bit of math brought to the problem.

However, I seem to remember reading on this forum that there is a "design" problem with this tensioner mechanism. Can't be any more precise than that. You need to search on the database. I assume you fitted the "latest" design when replacing the parts in which case all should be well.

Final point. For the vibration to physically fail the tensioner mechanism and for you to feel it in the car the vibration must be quite severe. Start by removing the belt and ensuring that all pulley turn freely without high spots / play

Stuart.

 

Cheers for your comments Stuart.

When i say vibration, it is not something you can feel in the car, neither as a passenger nor through the pedals or wheel. Yet on opening the engine bay the pivoting arm and the shock absorber arms are clearly vibrating in time with the noise.

Ayou say, if the vibration is not exactly once per belt rev then it implies one of the auxilliaries. I will be checking it tommorrow morning so ill post more feedback then.

As for the design, now you come to mention it, there was a change of design in the tensioner arrangement. I assume all the correct parts were fitted, my merchanic was aware of the change as I had to take the car in so he could check out which parts were required.

I dont work on the car myself although I have worked on more simpler cars quite alot. (Built a kit car ten years ago based on simpler ford running gear) The Merc is not something i'd feel too comforatable about working on in all honesty but what I can do is try and work out what is going on, since anyone ive taken it to so far doesnt seem to know.

Anyway, cheers again.

 

Ok. So you can't feel it in the car...thank goodness for that.
However the vibration must be quite severe for it to fail the arm so something must be amiss.
Something still shouts "tension" to me.
First step is to check design level and that the correct belt is fitted.
Secondly ensure no highspots etc on pulley rotation as I said before.
Not sure how the belt is removed / tensioned on your model.
My petrol automatically applies the correct tension (overcentre cam) SUBJECT to the correct belt length.
Whats the history on the car. Could somebody have changed a pulley size. e.g. Alternator to improve alternator output = smaller pulley = wrong belt length?

Stuart.

 

Try using a mechanics stetescope
http://cgi.ebay.co.uk/BRAND-NEW-MECH...QQcmdZViewItem

I used
one to track down some noise I had in an old volvo. Might be worth a try for you as you have spent a bit of money on the problem so far

 

i got the same problem mate on a w210 e220 cdi did you manage to fix it???