Here is what I did today. I removed the serpentine belt and the spring for the tensioner and then checked all pulleys for the squeak. The tensioner pulley spins freely with no noise. The alternator has no noise. The water pump pulley has a little in and out movement, but no squeak. Power steering pulley same thing, no squeak. What did squeak was the tensioner shock absorber. It still had good resistance, but I feel it is probably leaking. I sprayed some white lithium grease on the bottom mount of the shock for the tensioner and on the tensioner spring. Put it all back together and it is very quiet. We will see how many days it will go before it starts squeaking again. In the meantime, I guess it is time to start looking for some parts to rebuild the tensioner assembly or maybe just replace that shock absorber. I need to look and see if it is even available.
I have been ordering parts for my son's 2000 BMW 328i from Pelican and I am extremely pleased with their service and the quality of their parts and the ease of ordering from their website. I have used other places for my Mercedes parts, specifically the Classic Center in Irvine CA. and have been very pleased with them. I am going to try Pelican for Mercedes parts now.

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John
1986 300sdl
1985 380se