I am trying to track down a rhythmic grating sound from the belt system on my 1991 300CE, 137m. Using the screwdriver as a stethoscope, all is smooth at the alternator, air pump, compressor, water pump, p/s pump and either end of the fan pulley bracket. I located and eliminated a bad tensioner shock. I do feel a little roughness when the screwdriver is on the belt tensioner. There still seems be enough belt tension because it still took plenty of muscle to take the belt on and off when I replaced the shock. The history I have shows belt tensioner replaced at 87m, 9/1998.

Anywhere else to check?

glenmore
1991 300CE
2000 C280
1990 LS400

I'd look closer at the air pump. It only runs the first couple of minutes on a cold engine and can become Quite noisy when engaged.

remove 1st belt, run engine. remove second belt, run engine. remove......

Huh? There is only one belt!

Steve

Thanks! You nailed it! I had been doing the stethoscope thing at random times not particularly doing it at startup. After searching further under smog pump, I tried listening right after startup. Sure enough the noise was there and after a minute or so, the pump clicked off and away went the noise.

Now the trick is to see if I can rebuild it with minimum $$.

Thanks,

glenmore
1991 300CE
2000 C280
1990 LS400