My 80' 300D alternator belt keeps on getting loose. I tighten it up and then again in a few days its loose once more. Whats the problem? How do I find out what the engine number is or chassis? And does anyone have a vacuum diagram for a 80' 300D benz?

When I replaced mine I had to retighten it a couple of times. I think it stretched a little. The last time I did it I loosened all three bolts. snugged up the adjuster bolt. Tightened the two mounting bolts, then tightened the adjuster the rest of the way. That has held ever since.

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Sonny


86 190D N/A 2.5L Auto 265k "Ruby" -Sold-
79 300D 242k "Condi" my first -sold- RIP
2013 chevy sonic hatchback - had to for work

If your car has been sitting for a long time like years. Check the pulley grooves for rust. When rusty they seem to wear belts very fast. Some brands of belts also may just be too prone to keep stretching. Most good brands just stretch a little and stop. If pulley grooves are basically clean adjust the belt once or twice more. If it continues and your adjustment is locked down try another brand belt. Though this was brought up before so I might look in the archives under belts.