Belt tensioner strange position on M104 (picture inside)?

[StephFR]

On my 300CE24 - 91 (M104 engine) I've got an intermittent noise: like squealing 1/10 of a second every second, that I suspect to be originated from the accessory belt.

First guess was to check Belt tensioner position. I was surprised (see attached picture) to see that the pointer is in position that I called '0' on the picture: I was expecting it in position '1', the higher part of the triangle shape.

What this position should tell about the tensioner: bad setting or damaged one?
Should explain my noise?

Thanks,

[sixto]

Assuming it was set properly when it was installed I would say it's in its death throes. If you set the pointer to read properly and still hear the noise then the tensioner is definitely worn.

Sixto
87 300SDL

[StephFR]

A new belt was installed 1 year ago (previous owner) by a non Mercedes garage (the job done on the water pump didn't give me good impression also...) so I am having some doubt if it was done correctly.

Can this position change in 1 year by itself? I thought it should stay.

Is this position equivalent to trying to overtensioning the belt or trying to do it correctly on a worn tensioner?

If the tensioner is worn and need to be changed: is it recomended to also changed the damper and the tension adjuster also? anything else?

[haasman]

Is the belt properly tensioned. Mercedes and other German cars tend to run with a high belt tension. An old test (not sure it really applies here) is no more than a 1/2" deflection in a long belt span (between two pullies).

If the belt is tight Sixto is probably right: the tensioner has failed. The tensioner is made up of an outer housing of metal and an inner shaft that is attached to the outer by rubber. If the inner twists too far is breaks and you loose the tension.

It also simply be that the tech who changed your water pump didn't understand how to adjust the belt in the first place. BTW, the tensioner is locked in place by a bolt, 17mm I believe, but is tensioned by the long nut in your picture.

If you are not sure, have someone who has "been there and done that" look at it for you.

Haasman

[StephFR]

What I did:

- first I was surprised, before untightening the tension pulley to see that this one was touching the coolant pump pulley! May be confirming that the tensioning device was not set correctly...

- then I realised that the cursor on the tensioning device was loose: I could move it, so meaning no reference position any more,

- I tightened the tensioning device to a new position which I estimate based on old trick (twisting the belt to check tension); this resulted in shifting the tensioning pulley 1 inch away from the water pump one,

- after that, the tension was kind of OK and my noise disapeared! Cool!

Now, it's been 2 weeks, and I get this noise back and the tensioning pulley touching again the pump one...

Questions:

- seems the tensioning device dead?
I am surprised that when this one is dead, it allows the tensioning pulley to touch the pump one, resulting I thing in stress on the pump bearing!
Does it seem normal, or could I have a too short belt?
I am having a hard time to convert the belt ref into a length to check (need to contact the manufacturer, because it's not a comon brand name).
- when the tensioning device has to be changed, is it recomended to change the damper and the tigthtening rod also? Something else?
I'd like to order parts before dimounting...

Thanks,

[haasman]

Sounds like the tensioner unit has failed and is twisting inside the housing.

Haasman