I have to retighten my alternator belt every 30 miles. The tensioner wont hold tension! Why? And yes, I did search.

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1981 300SD 512k OM603

havent seen this on these engines yet, but here are some ideas from elsewhere.


Try to source your vibration.

loosen all the bolts on the alt, tension it, tighten all the bolts ensuring that all the washers have retained their shape. The tension adj bolt will not hold the alt in place by itself with a considerable vibration in effect.

The other issue is the fastening bolt on the sliding adjuster. If the bracket is out of shape, that fastener can bottom out before it actually provides a good sorurce of friction.

Or... you have spilled oil on the belt and it is degrading, oil will destroy belts very quickly.

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Not sure what setup you have on your car, but my 82 300sd's alternator has an adjusting screw to set tension, and then a couple of bolts to lock it down so the tension can't change.

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Thank You!
Fred
2009 ML350
2004 SL600
2004 SL500
1996 SL600
2002 SLK32
2005 CLK320 cabrio
2003 ML350
1997 C280 Sport

Do you have the threaded bolt on top to adjust tension, or the geared teeth on the bottom?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

There can be two issues. One, as jt20 noted, is the belt is actually degrading and stretching. This won't go on much longer if that is the case. You will hear a bang! and the water temperature will rise rapidly. The belt can only get so big before it breaks.

The other cause is you are not tightening the bracket correctly after adjusting. The net result is either the bracket "unadjusts" itself, or, more likely, there is something loose and the alternator support is shifting. These alternator brackets with the integral adjustment feature have evolved over time and there are several versions so the discussion below may be dating me....

The way the adjuster works is you have to loosen the support bolts at the block end of the bracket, adjust the position with the adjuster bolt, and then tighten the support bolts. I have seen one or the other of the support bolts against the block vibrate free and go missing which wreaks havoc. Also, one of the bolts has a sleeve over it between the alternator and the bracket. The sleeve is what the bolt clamps the alternator against the bracket when the nut is tightened there. No sleeve, and the metal on the bracket and the ear on the alternator yield a bit - and the alternator is no longer tight. The rest of the bracket is not robust enough to hold the alternator and the bracket will begin to distort. Not good. Pulleys get out of alignment, belt gets extra wear.

So, check out exactly what kind of bracket you have and be aware of the other fasteners on the bracket, as well as the sleeve. Good luck, Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

I dont think its the belt rapidly stretching. This problem started after I change the tensioner for a good used. My original one snapped because the PO bent it loosening the belt. It is the threaded slide type, not the gear type. I want the gear type though! Seems like a much better design (my friend has one on his 420sel). It seems like the 13mm bolt is just vibrating loose. Maybe im not tightening it enough? I tighten it till the belts are under tension but not "strung" like a guitar or anything. Can i just drop some threadlocker down the shaft of the 13mm adjustment nut? That could fix it but Its still not "right".

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1981 300SD 512k OM603

It sounds as if you are attempting to use the tension adjusting screw as the hold down as well.

The alternator is adjusted with the slide, but once adjusted you have to lock down at least two more bolts to keep the alternator in place. Likewise, you have to loosen those two or more bolts to adjust the alternator with the slide screw. If you do not do so, I'm sure you'll break the adjuster slide mechanism.

__________________
Thank You!
Fred
2009 ML350
2004 SL600
2004 SL500
1996 SL600
2002 SLK32
2005 CLK320 cabrio
2003 ML350
1997 C280 Sport

I slipped in my repair CD to see exactly what it says.

There are 3 bolts, not counting the tensioning screw, all on the backside of the alternator / bracket assembly.

The manual says loosen the three bolts/screws, adjust with the tensioning screw, tighten down the three bolts/screws, and finally, tighten the adjusting screw another 1/4 to 1/2 turn "(to tight seat)".

I can't figure out how to pull the pix off the cd to illustrate.

__________________
Thank You!
Fred
2009 ML350
2004 SL600
2004 SL500
1996 SL600
2002 SLK32
2005 CLK320 cabrio
2003 ML350
1997 C280 Sport