hey,i need to replace my stock alternator on a 1989 300ce, the voltage regulator didnt help .im not familiar with the german serpentine belt tensioners,what do i need to do to loosen up the tensioner in order to get the serpentine belt off the alternator pulley,any info would be great,thanks.marc

Loosen but don't remove the center bolt on the tensioner, it will be a 19mm or a 10mm allen key head bolt.
Back off tensioner rod. Pull on belt until you have enough movement to remove the belt.

An image for you.

Location of 19mm bolt.
Loosen this a few turns.

Loosen the adjuster with a 13mm tube spanner or socket with an extension.
You don't need to remove the shocker as indicated in the image.

thank you guys ,so let me quess the 19mm bolt loosens up to move left to right and the tensioner rod keeps it all tight from moving? am i even close.

mlozzi, the vertical rod adjusts the position of the tensioner while the horozintal center bolt "locks" it in place. Turning the vertical adjuster rod before loosening the center bolt will destroy the tensioner. In fact, don't be surprised if after installing the new belt the tensioner will not retighten. In that case you need to buy a new tensioner, and installation is quite involved due to the power steering pump limiting access.

As I recall removing the power steering pump pulley may provide enough clearance to remove/reinstall the tensioner. If you decide on that approach loosen the power steering pump pulley bolts before removing the belt because the belt will keep the pulley from turning.

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Fred Hoelzle

Ensure you follow the service manual for this procedure. The tensioner's internal rubber degrades over the years and it WILL fail if you incorrectly remove it and it is the original tensioner, I promise you. Follow the manual in the correct order (it is not difficult at all) and you'll be A-OK.

http://www.w124-zone.com/downloads/MB%20CD/W124/w124CD1/Program/Engine/103/13-3450.pdf

You do not need to do anything with the power steering pump to remove the tensioner, although I cannot remember if a bolt goes through them both, but you get what I mean. I did it plus water pump removal without touching it

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

thanks so much for all the pics and diagrhams,by the way your mercedes looks hot,i think what i will do is follow the advice of loosening the 19mm bolt on the center of the tensioner,then loosen the 12 mm threaded stud,i will then try to wiggle it left or right,then pull down on the belt to loosen enough slack to remove the alternator,i need to reinstal a reman bosch alt 70 amp,the voltage regulator didnt work and the brand new battery is now dead,im hoping when i install the new alternator it putting out about 13.9 volts instead of 10 volts and dropping.thanks guys.marc

my son was able to replace his alternator on a 300e simply by loosening and removing the alternator itself.... he never touched the belt tensioner.

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1989 300ce 129k
( facelifted front,updated tail lights, lowered suspension,bilstein sports, lorinser front spoiler, MOMO steering wheel, remus exhaust,stainless steel brake lines). (Gone)

1997 s320 154k (what a ride). Sold with 179k miles. Replaced with Hyundai Equus

1994 e320 Cabriolet 108k



1972 280se 4.5 153k Owned for 12 yrs, sorry I sold it


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im in the procces of replacing my alternator,i have managed to loosen up the 12mm threaded rod that runs down to the belt tensioner,but im also aware there is a bolt that runs thru the center of the tensioner,someone told me it was 20mm,i found that 15mm fit it ok,im having a hard time trying to loosen the belt,see there is a protective metal half sleeve that covers the outer edge of the bolt,and i have large fingers,is there a special tool i can get to loosen up the tensioner,its icy cold and ive been working in the snow on the cold ground,id like to speed things up a bit if possible,thanks guys.marc

well i tell you what with this bitter cold snowy weather i just may try that,i only just began work on it,i have the top 13 mm bolt out of the akternator nothe bottom is giving me a hard time,i dont know why they put them half circle protective metal sleeved around the bolts,but i need to but a new 6 point socket and try it,then maybe i will get it on with out messing with that pain in the ass tensioner,oh i love general motors tensioners so much better,lol.thanks guys.marc