On my 240d, Om616 i keep having issues with my alternater belt. I tighten it and about 40 miles later it comes loose, i can tighten again if i catch it in time, but most of the time it just snaps. Very tired of replacing the belts, all the tensioner bolts are very tight.

Have you checked the pulley alignment, and belt tension? Mis-alignment causes side wear, high temps, and subsequent failure (either it stretches & flips off, or shreds). Look at the sides of a belt in use before failure (engine off). Glazing is a good indication of high side loads w/slippage. Alignment can be checked w/straight edge against pulley edges (engine off).

Tension can be checked w/belt tension gauge, or by checking the deflection between pulley's (check the FSM for spec's).

Good luck!

what year 240d? do you have the star bolt tensioner or the L-bolt?

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1981 NA 300D 310k miles

X2 for Charlie.

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'84 190D 2.2 5MT (Red/Palomino) Current car. Love it!
'85 190D 2.2 Auto *Cali* (Blue/Blue) *sold*
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its a 74. its got the star shaped looking tensioner which doesn't really work.

Do you have the correct washers and nuts on all the mounting bolts and the adjustment bolt? Are the "teeth" of the adjustment bolt damaged? Are you using a belt size that pouts it ne'er the middle of the adjustment range?

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Current fleet
2006 E320 CDI
1992 300D - 5speed manual swapped

former members
1984 300D "Blues Mobile"

1978 300CD "El Toro"

Belt snaps if looseness is not dealt with fast after 40 miles? This seems a little strange to me.

I have had belt issues with these cars but usually written it off as too much rust established in the pulley grooves from the cars sitting around too long in our harsh climate. I suspect it chews up the belts.

Many new v belts seem to stretch a little initially and so I check them after a little use to see if the tension is still adequate.

Usually I tension them just past the point where I can no longer turn the alternator fan by hand. As I visualize it in my case the rust acted like sandpaper against the sides of the V belt when it is entering and leaving the pulley.

The ridding surface of the pulleys should be nice and clean and smooth.

A couple of thoughts - do you think the adjustment is changing, or is the belt stretching?

If the alternator is moving, something is loose.

If streched - alignment & tension are the issue.

I can't remember, does your 616 have one belt or two for the alternator?

If two, they need to be a matched set.

(A word on belts) Over the years I've stocked/used Gates, Goodyear & Dayco on all sorts of vehicles with good results. Unfortunately I've learned the hard way that Benzes are different and seem to prefer Continentals. I don't know why, just that I don't have comebacks w/Contis. Unfortunately the private label belts are made by the big three or China Inc. They just don't stand up to MBs. The last time I looked, The Contis were about $2 more than Gates for the same size. Its worth it. Its possible that the Contis have a different width or angle and fit the pulley better - I don't know.

As for rusty pitted pulleys, try a paste of scouring powder (think Bon Ami, Dutch Cleanser, etc) on the old belt before swapping it out. Run it for a few minutes, then clean the pulleys & swap the belt(s).

Good luck & keep us posted!