The serpentine drivebelt system is a very maintainance free system. The inspection methods given by a number of manufacturers give a criteria of number of cracks per inch. I have never seen one of these belts brake from age (I have seen many burned-up over frozen bearings though). We have replaced belts with many segments missing from the groves. These belts will eventually trigger the A/C engine/belt speed comparitor and they usually get replaced in Florida.
The problem with the tentioner is that its efforts are dampened by a rubber hub that works on the same concept as a suspension control arm bushing. The center of the bushing is turned by the adjustment and the tentioning arm is loaded by the tention transmitted through the rubber hub. There is no need to replace unless the hub is sheared or obviously failing. Experience can detect a weak hub.
By guide rails I'm not sure what you mean. The belt is guided by the pulleys and the grooves. Another common failure point in this system is the bearing bracket. This is what the fan rides on (not the water pump as in most motors). The single bearing doesn't handle the load of the fan/fan clutch and belt system as well as the other bearings (water pump, P/S, A/C clutch, Alternator).
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician
[This message has been edited by stevebfl (edited 03-17-2000).]