Hello Chan,
The tensioner is mainly hydraulic. Oil friom the engine keeps the tensioner pumped up and taunt agaist the chain. An internal sping does a bit of the work but it is mainly hydraulic. These tensioners are very durable and usually last the life of the engine. There are no soft seals, just a check valve and a metal seal at the cap of the tensioner. The tensioner takes up slack in the chain until it is fully extended. At this point the timing chain should be replaced. During chain replacement, the tensioner, chain guides and sprockets should be examined also.
Knowing when the chain should be replaced is a little tricky. You will need to have the valve cover off. Make sure that there is no slack on the side of the chain going down on the left side (injection pump side). Turn the engine by hand a bit if you need to remove any slack in the chain. Next examine the chain on the right side. You should be able to determine if the tensioner is taking up all the slack. If the tensioner is extended all the way and you still have slack, replace the chain. Automatically replacing the chain at 100,000 miles is a good rule of thumb. As Dan indicated, the cahin may need replacement sooner, but this is unusual.
The side plates on a lot of the original early chains were shaped like the figure "8". Later chains were shaped like an oval. If yours is the figure "8" style it is more likely to be the original chain.
"jwis" is one of the original equipment timing chains. It has 134 links and the manufacturer's number is D67-134. Do not use off-brand timing chains. I have seen some "Regina Extrada" brand timing chains which are obviously lighter duty and not original quality. You can get the original manufacturer timing chains from Mercedes or other aftermarket sources.
Chain stretch is actually a small amount of wear at each of the 134 pins in the timing chain. When you add all this pin wear together the chain becomes longer and weaker. When a cylinder head is milled you actually increase slack in the chain by twice the amount the head was milled.
Catastrophic chain failure occures when the timing chain becomes so long from wear that it can actually jump out of time allowing the engine valves and pistons to crash into each other. At this point the chain can actually break, the camshaft can break into pieces, the valves can bend and pistons can be damaged. If the chain tensioner is not functioning, or is at the end of its range the chaince of catastrophic chain failure are increased. Badly worn chain sprockets and chain guides can also increase chances of failure or increase chain wear. As with most engine moving parts, clean oil reduces chain wear !
Chain sprockets, chain guides and tensioners are usually replaced during major engine repairs, and only if needed at chain replacement. Original chain rails were made of aluminum and rubber, it these rails wear out they made some noise, but did not break. Replacement chain rails are plastic. When they get old and brittle they break and fall into chain gallery and cause more problems or catastrophies. If you have the plastic chain rails you will need to be a little more diligent about your chain and rails as they get older.
Hope this helps!
Take care,
