The car is my good old '86 190E 2.3.
I drove the car back home from a short errand. As I turned into my
driveway, the belt tensioner failed causing a loud screeching noise. I
lifted the hood and found the serpentine belt totally loose but intact.
I tried to adjust the tension but the adjusting bolt had already
reached its maximum range.
I bought the genuine MB replacement part and a new belt and began to
remove the broken tensioner. Succeeded in loosening and removing the
large 22 mm bolt and then... I couldn't figure out how to proceed.
The Haynes Repair Manual describes the procedure to replace/install the
serpentine belt, but there are no instructions related to the
replacement of the tensioner or the damper.
Please help me with the steps required to replace the tensioner. I sure
don't want to remove more bolts and nuts than it's absolutely necessary.
Check the tensioner wheel... if it rattles as you spin, you should just buy
the wheel and replace it... very easy.
The tensioner has two part... one is at the tensioner... in front that holds
it in position...
The other part is up on top of engine... brass hollow nut thing to tighten
up the tensioner... if this part is loose... you might have broken tensioner
rod... you can tell if you can just pull it up.
In any case, you need to loosen up the upper tensionser adjustment nut...
then fish the new belt in... and tighten the upper tensioner and finally the
lower tensioner bolt to lock it in.
Thanks, Tiger, for your quick response.
The tensioner itself is held by the lower bolt, as you stated. After
removing the old belt, I tried to move the tensioner by hand and it did
not offer any resistance to my effort. Then I assumed that the internal
spring was broken and bought a complete assembly that includes the
tensioner wheel, lower tensioner bolt, adjusting rod, the hollow nut, a
plastic position indicator and a bracket. The new tensioner's spring is
firm and robust as it should.
So, I decided to remove the old wheel assembly and replace it with the
new one. This is where I stand right now in my DIY project: Stuck.
I have not figured out how to REMOVE the bad assembly without taking off
the AC compressor pulley or who knows what else.
If I ever get the new assembly in place, the belt adjustment procedure
is a no-brainer.
I replaced this tensioner in my 1986 190e 2.3.
very simple really. Remove the big bolt (22mm?) that goed through the
front of the tensioner. Unclip the little shock at the bottom of the
tensioner. Release the tension adjustment from the big tall nut at the
top of the timing case...
That's it. then it all comes out, although it does take a bit of
patient wiggling to extract it.
PS No need to remove any AC or anything else.
If you only had the center bolt holding it on then one of two things has
1) you have never had it replaced
2) you never brought the car back [in the USA] to the dealer for all the free
upgrades for the A/C
in the late 80's, one of which was a modified belt tensioner.
The updated and now replacement tensioner has a support bracket over the front
of it. The center
bolt goes thru it. 3 more bolts hold it on and you have to remove the power
steering pump pulley to
get to one of them.
99 out of 100 102 engine tensioner failures is the center hub rubber shifting.
You will run out of
adjustment threads long before the tensioner gets the belt even snug much less
tight. The center hub
actually moves 90 degrees over time. The center hub is the piece with two flats
on it that the
adjuster arm sits on. Compare a new one with a bad one side-by-side and you will
see the difference
Thanks Karl and Marty for your thoughts.
Just like President Reagan used to say "here we go again..." Thanks for
the additional info on the replacement of the tensioner.
1) I bought the car new so I know that this is the original tensioner.
It was never "upgraded."
2) To the best of my knowledge the MB dealer did not upgrade for the AC
and tensioner assembly.
3) The tensioner replacement kit does have a NEW bracket, a threaded
rod, a plastic position pointer, a new center bolt with a 22 mm head
(the old one has a 17 mm head).
4) As a matter of fact, I pointed the difference in head sizes to the MB
Parts associate and he told me that MB had upgraded the tensioner
assembly. Now that Karl explained the configuration of the new
tensioner, I understand why there is a NEW bracket.
5) Karl mentions the upgrade for the AC issued in the late 80's. If I
replace the tensioner with the new one without also upgrading whatever
was supposed to have been replaced in the AC, will it work? Or the
upgrade for the AC is a prerequisite?
6) The description of the failure caused by the center hub rubber
shifting matches exactly the failure mode of the original tensioner. I
ran out of adjustment threads trying to adjust the belt tension.
I apologize for the lengthy details but I feel that we all have to have
the complete picture before I attempt to finish the job.
The A/C upgrades included: A new 2 piece manifold hose instead of the one piece,
replacing the green
pressure switch on the drier with a red one [this turned the fan in front of the
condenser on at 275
lbs instead of 325], adding seals around the condenser so all the air went thru
the condenser and
not around it, changing a relay in the fusebox and adding a wire to the 3 legged
temp sensor so the
engine fan will lock on when the aux fan comes on. These upgrades made the
system work very well.
Thanks again, Karl, for your help and the amazing knowledge that you
shared with me.
I just came down with pneumonia and won't be able to work on the car for
a little while.
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