I've been playing amateur mechanic the last couple days trying to replace the belt tensioner on my '87 300e (103 engine 124 chassis). It's most definitely shot. Squeally in the morning, squeally after work. Something needed to be done.

I first tried reasoning with it, but that didn't work.

So then I got some ramps to get up under the front end, loosened the shroud and moved it aside (a one piece variety of shroud, I'm still considering taking a hacksaw to it to get it out of my way), removed the big 19mm bolt through the tensioner, and had to recruit a friend to get the 11mm bolt (in other posts I've read this is listed as a 13mm bolt, but the head on mine was most definitely 11mm) that went through the tensioner's swivel hole. The nut on it was totally loose, and the washer behind it is what I hope was making the worrisome rattling noise up front. I could see, looking up from under the car, that the threads on the 11mm bolt were roughed up, and in fact were what kept the nut from falling off altogether. It looked like the classic case of someone not loosening the 19mm bolt before loosening the 11mm bolt before changing the belt (it does have a newer belt).

We managed to get that bolt out of there, and even with those two bolts removed the belt was still tight in there. I could bend it some, but wasn't able to remove it until after I beat on the old tensioner a little bit hammer and chisel style to loosen it up a bit.

Now I have to remove the plate in front of the tensioner, but to get to one of the bolts securing it I have to remove the power steering pump pulley wheel. It appears there are 3 bolts securing it to a flywheel on the power steering pump.

My main question is this, what direction do I use to twist those 3 bolts off of there? CW or CCW? Usually CCW to loosen, CW to tighten, but I was told sometimes pulleys are different in that regards. Also, I've read that I may need a special pulley tool. Is that true? I have a strap wrench to secure the wheel while I tried the bolts both ways, but I didn't get a budge. I was afraid to give it my all (which is what it's taken to get other bolts loosened on this car, along with an accompanying flesh wound) without being sure which direction to twist. I know I've read here that you can remove those bolts and lightly knock the pulley wheel from the power steering pump, but I want to make sure I'm barking up the right tree.

I'm also going to need a new 11mm bolt. I can use the hole on my new tensioner as a thread pitch determining guide, but I'm wondering if there's a special bolt I need to order.

I've also read that I need to apply RTV sealant somewhere when putting on the new tensioner, but where exactly do I apply it? I'm guessing on the 19mm bolt, because it's the only thing I've taken off so far that's long enough to get to an oil passage, but would I apply that at the end or under the head of the bolt?

The new tensioner is getting impatient, but I was able to reason with it, and it's now sitting quietly awaiting my efforts.

Thanks

Oak:

Though different models, your MB and mine share the same motor.

I've had my PS pulley off. The 3 bolts come off by loosening counter-clockwise.

I'd replace the "11mm" setup with the real bolt that uses a 13mm head. You can look up that bolt(photo included) at this sites parts site. I seem to recall it's about $10-$15.

Here's a pointer to a site with online MB repair information:

http://mb.braingears.com/

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

Thanks, Mike. I tried locating the 13mm bolt in question on this site (the new tensioner was purchased through this site), but couldn't seem to locate it. Does anyone know what exactly they call it on the parts search function?

Also, does anyone have any insight as to what kind of RTV sealant to use, and where to apply it?

Thanks,
Oak

Oak,

Locate the stamped bracket that resides in front of the tensioner, the one with three holes. One of the bolts goes into the lower timing chain cover. This is the one that gets the sealant. Apply it on both sides of the bracket and bolt it up. As to which sealant to use, I used the same that I used to seal the upper timing cover.

Don't know it you've got the PS pulley bolts off yet, but try some penetrating oil before turning them counterclockwise.

Mark

Excellent, thank you Mark. However, I was able to remove the tensioner without removing that bolt. I was able to remove the the other two bolts securing that plate, detached the power steering pump and leaned it aside, and then removed the water pump pulley which allowed me to rotate that plate up and down a bit enough to slide the tensioner assembly out of there.

Behind the tensioner (opposite side of the wheel) there's a silver circle with 2 flat sides through which the big 19mm bolt slides, and around that is a tight fitting connected with a loose screw (so it can move freely) to a shaft with threads at the top, and that shaft shoots through the accessory belt tensioner bracket. The Accessory Belt Idler Nut (using the terms of the shop here) is screwed on the threads at the top of the shaft, and that is actually what is going through the tensioner bracket. The 19mm bolt also goes through this plate when on the car, just to note.

On the old tensioner, the threads inside the smaller hole, through which the 13mm bolt (formerly known as the 11mm bolt) ran, were totally stripped. You can only see traces of threads, just a couple fibers. The 13mm bolt ran through the Accessory Belt Tension Damper, which was suspended on the other end upwards onto something else (can't remember, too lazy to run down to car). That damper is basically just a shock, and it didn't look like it was connected to anything else there, so I don't quite get how it plays much of a part in keeping the belt tight. I thought that bolt would go through a bracket with a smile line, and you would loosen the nut and move it up or down along the groove of that smile line to loosen or tighten, but I didn't find such a thing.

I still need to order a replacement 13mm bolt. I wasn't able to find it on this sites part, so I may put in a special order request. The project is certainly taking a teeny bit longer than anticipated.

A couple questions:

Are there any torque specifications regarding retightening the pulleys on the power steering pump and water pump pulleys?

Does anyone know of where I can access some diagrams, or pictures of the tensioner assembly? I did try the link Mike posted, and it looked like the same thing as the repair CDs I have, which I've found cumbersome to use, and I still haven't found much information on them regarding this tensioner replacement job. If anyone else has those CDs or is familiar with the site Mike posted, and knows where to locate the gem I'm looking for on there, please lemme know.

Oak,

Just finished my project of replacing the tensioner tonight. Wow, what a learning curve. It took me way too many days and 2 different parts orders. I could now go back out to the driveway and do the project in under 2 hours. I used the MB manuals for the the 124, the MB WIS discs and researched several discussion groups. By far the best analysis was provided by DUKE2.6 in this forum back in July 2005. I originally found his posts by typing in TENSIONER inthe search field. That search will also provide you with several pictures that aided my project. The first link gives a good overall perspective and the second is a step-by-step to replace:

Drive belt tensioner failure analysis


Drive belt tensioner


The thing that gave me the most fits was that I was trying to use a tensioner that was busted. All my issues went away after I purchased all new parts (tensioner, adj rod, dampner shock). I would recommend replacing if your car has miles on it. I would also recommend removing the Y bracket entirely and sealing that bolt that goes in the lower timing chain cover. The last thing you want is an oil leak down there.

Good Luck,

Mark

I just finished project tensioner tonight, and Mark, you're right, took too long, and I certainly didn't know what I was getting myself into. The only thing I ordered was a new tensioner. I used the old plate and the old adjusting rod and nut and shock. So far so good. By the way, the links you posted were very helpful, and likely would be for anyone attempting this job. I'm not very experienced as a mechanic, and this was probably the hardest bit of car fixin' I've ever endured. At least three different trips to the hardware store involved, confusion on my part, and a good dose of uncertanty along the way made this kinda tough on me.

Just to clear up an embarrasing bit of confusion on my part, the 11mm bolt that I was yammering about that connects to the shock is indeed 11mm, and not 13mm. The 13mm bolt, I later realized, is the Accessory Belt Idler Nut, which connects to metal shaft that's threaded at the top, and protrudes through a hole at the top of the Acc. Belt plate. When researching this job, I kept hearing about the 19mm and the 13mm bolts, but there's an 11mm one, too, and it connects only to the Acc. Belt Tension Damper. Doh.

I never did remove the Y bracket that goes on the outside of the tensioner assembly. I simply didn't have a tool to get that one bolt out that was located behind the engines main pulley, the nut with the oil passage. Ratchets were too big, and I tried an open ended wrench, but couldn't get a good angle, and ended up putting a nick in the bolt. I was able to remove and install the part through the passage created by removing the power steering pump. I will keep an eye on that area, though. The car already has a bit of a seal leak, which I understand isn't considered too bad if it can make it to the next oil change without losing more than a quart or so.

Anyway, I got everything in there, and still had a bit of a time getting the belt on and tight. The 13mm adjuster nut, I thought, would tighten the belt if the nut was raised to where the bottom was pressing up on the bottom of the Acc. Belt plate, but that loosened it on my car. I had to lower the adjuster nut to get it to tighten up, and did a little pry bar/push & pull by hand action to get it to tighten up satisfactorially. I got it where I needed it, though, and clamped down on the 19mm locking bolt. It seemed tight enough, so I put the fan and shroud (which is damn lucky to still be in one piece) back into place, started it up, and no squeal! I still don't know if I fully understand how the device works, but it seems to be doing fine so far.
Took it for a spin and not a squeal at all.

If I develop any problems, I'll update the post. It just seemed like such an over my head experience that I'm almost in disbelief that I finally did it.

Anyway, thanks again, and I'm now going to invent a special kind of glove that is lightweight, but tough, and only protects the knuckles, which will allow for full sensation in the fingers, where you need it, but not on the knuckles, where sensation just ends up sucking sometimes. I'll call 'em, knoves.