new belt wear problem
 

Put a new belt on my E300D. The back on the old belt was shiny and otherwise clean but the ribs were all cracked. I checked the tensioner pully wheel and it was smooth and spun easily and was not loose. Put a new Continental belt of the right size (1240 mm) and all went well. Now I note there is some apparent abrasion like effect on the back of the belt. The only contact it makes is on the tensioner pulley wheel since that is the only wheel that the back of the belt is in contact with. There is no looseness of the belt and the pulleys all spin true. Whaaa?
My first thought was that this was just a thin coat of rubber comming off of the belt back due to the many contortions these belts take. Is that it or do I have trouble? Have driven several hundred miles since but still have this stuff on the back that I can rub off with my fingers. Another thought was that the belt back was rubbing on itself as the space between the loop is small as it comes off of the tensioner pulley and the backs going in opposite directions are quite close to each other. But no vibration or contact seen. I am puzzled. The shock and spring appeared and seem to be healthy. Help.

The belt must be put on a certain direction if this is a serpentine belt.

Was not aware the belt was directional. Could you explain how to orient this?