Is the 300SDL reading 3 when warmed and idling, or even when the engine is off?
If so, it couldn't hurt to rap the sender a few times with a screwdriver handle.

If it's just running that high when you're driving, it's normal. Heck, mine doesn't stop at 3, it pegs off scale on the top end when the engine is above 1000 rpm!

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...