Drive belt tensioners are a sometimes overlooked maintenance item, and if your car has 100K-200K miles on the clock then it could need replacement. As the name suggests the job of the tensioner is to keep tension on the drive belts on the front of the engine that link components suggest as AC compressor, power steering pump or alternator. If tension is lost then of course the belt can slip, or in the worst case fly off and get tangled in the fan or oil pump pulley. Not good!
Diagnosis
The first stage in diagnosis is just to look and listen at the tensioner - does it look smooth? Any squeaking or rubbing noises? If you have a mechanics stethoscope place it on the metal behind the moving parts - any grinding or rumbling? If you suspect a problem, it's time to remove the belt itself to examine further. Before removing anything take a picture of the routing of the belt, and perhaps also mark the direction of travel so you can be sure to reinstall exactly the same way. Mercedes cars and newer BMW's tend to use a tensioner with a strong internal spring. You just attach a ratchet and push down against the spring and the belt is released. Older BMW's tend to use a slightly different design with an external spring that is clamped by a bolt. Here you need to release the bolt to remove, but re-assembly is more complex - you'll have to pull on the spring with a ratchet to get tension, and re-tighten the bolt with a wrench at the same time. On some models it can get quite tight with relatively little space between the belt area and the radiator in front!
With the belt removed, just spin the tensioner pulleys by hand - do they feel rough? Any play back and forward? Any problem at all is sign they need replaced, as any play will be amplified greatly at engine speed. On both BMW and Mercedes models like the BMW 540i or the Mercedes C240 the tensioner is just held onto the front of the engine by a few bolts, so just remove and replace. Some Mercedes cars including the W220 chassis have a little detail in that one of the bolts is behind the sprung part - so you have to remove tension to access it. To save you needing three hands there is a small hole you can insert a locking pin into to clamp the spring for removal and installation.
If there are any cracks or frays in the belt, just replace that too for piece of mind, they are usually cheap.
Mercedes style - push down on the captive bolt to release tension:
Mercedes Belt Tensioner - Febi 1122000970
BMW old style - the top of the tension spring circled in green, but it's the red circled nut that locks the spring in place:
BMW Drive Belt Tensioner Assembly - Genuine BMW 11287515865
