Just went through this with my W201. Got a used tensioner pulley and it was worse that the OEM one, which was not my problem afterall. Went ahead and bought new from MB.

Removed the tensioner and inspected it. I've replaced it before, and the OEM one really was bad at the time.

This time, the chirping was from the alternator. I spun the shaft and it growled and gave resistance. A box store wanted $199 for a rebuilt unit. I bought new bearings for my unit, and a regulator, for $44, and did it myself. The hardest part was pressing the bearings on and off of the rotor. Had it back together in 30 minutes, installed on the car in an hour, and no chirping anymore. Still have my OEM unit after 300K. Proud of it.

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My first Mercedes Benz. 4/6/07.

1984 190E (201 024) 2.3L four.