This engine is found in most Merc S class classics - late 80s to 1995. I am not sure if the same arrangement is found on earlier or later engines.
I have now had to be recovered in my S700 4 times in 23 years, and 3 of those were for broken fanbelts. 1 was random, 2 a coincidence but 3 is definitely a pattern. It is probably a service item and should be changed every 50k or so - I don't know, I don't have that info. But the reason it rarely gets done is that it is an absolute pig of a job. I know this to be true because I have been witness to the grunting and swearing that has emanated from the garages I have used, plus the posts I have seen on other groups.
So if you have a Mercedes, take note of this post, which will be in the index, for future reference.
All the difficulty stems from releasing the tensioning roller in order to slip on the new belt - at first glance there seems to be no way of doing it.
The first thing I discovered is that most of the online information, and in particular the Russek manual, is either wrong, or at the best, ambiguous. The diagram in the Russek manual is accurate in the general layout, but lacking in detail. After the event, I managed to find much more accurate diags - see below.
The procedure is as follows. Identify where is the tensioning spring, which runs parallel with the hydraulic damping cylinder. Here you find a nut (3 - yellow arrow) which is located on a bolt, which appears to be attaching the top loop of the spring to the bracket. This nut has to be taken off - but if you try, the whole thing just spins. What you can't see, and what isn't explained clearly is that there is a hex bolt head at the other end of the bolt (6 - red arrow) - but where is it? It isn't obvious. The bolt is actually a 3 inch long bolt that goes into a hole in the top corner of the engine block - the hole is about 3 inches deep, and the head of the bolt is back there - you have to reach your fingers in to feel it, and you can just get a spanner on it. One you do, then the front nut removes easily.
Then you have to insert a bar/lever into the recess in the top of the spring bracket - this is depicted in the diag with arrows showing the direction to push. This bar has to be provided by you. By moving this bar to the left or right you can move the spring bracket, release the spring tension, and then using a drift (anything handy) you can then push the bolt back through the bracket and into its hole - you can also help it out with your fingers round the back - but don't take it out completely - just enough so that it no longer fixes the top spring bracket in place. Once this is released, the spring will lose tension, and the tension pulley will fall down a couple of inches, finally allowing enough slack to get the belt around all the various pulleys. It is still a tough job, but it will go on.
The blue arrow indicates the actual pivot point of the bracket - for info only, it has no function in the job.
It is also worth mentioning that there are 2 ways to approach the job - access is really tight, and that is half the problem - you need a good set of tools to get at that nut and bolt. But if you take the radiator off, the job is a whole lot easier. Taking the rad off is a 20 minute job. If you are doing this job yourself, and are not pushed for time, and maybe doing other work, consider removing the rad.
Once it is on, you have to re-tension the spring and then lock it back in place, under tension. To do this, you have to push your bar, which is still in its hole, well over to the left, which will pivot the top spring bracket anticlockwise, until the point where the holes in the bracket and the engine block align, and you can push the bolt forward, through the holes, and the spring is then tensioned and locked in place. Replace the nut, and that's it, job done. When you know what you are doing, and you have just the right sized bar, it's a 5 minute job. If you don't, then it's a frustrating episode with much swearing and skinned muscles.
The confusion arises from the fact that removing the nut is counter intuitive because you think that is what is holding the spring in place. It is and it isn't - the spring is held in place by a collar in the bracket, and the bolt slides through the middle to lock it in place. the actual pivot point of the racket is further down.
I just had to pay a French garage 2 hours labour because they had no clue, despite me presenting them with photos from my collection, and the Russek manual. In the end what saved us was a telephone call to my own mechanic friend in N Wales.
None of this will make the slightest sense to anybody! It doesn't apply to Fiat family owners, and it is of no interest to Merc owners, until it happens to you - at which point, this detailed description should be useful to you or your garage.
I am also now reasonably convinced that in motorhome use, Merc fanbelts have roughly a 5 to 8 year lifespan - probably nearer 5, especially if you have modified electrics, like I do - ie big batteries. In motorhome use, the alternator works harder and hotter and so does the belt. The belt can get damaged slightly if the engine is turned off when the alternator is hot, and the belt can get slightly "cooked" at that point. The alternator pulley is the smallest on the belt, and therefore is working hardest. My intention now is to make sure I have a new belt either every time other work is carried out at the front, or every 4 or 5 years.
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