If you have a black rotary knob up on the left side firewall (mine is near the brake fluid reservoir) that has 7 numerical settings, try setting it up a few numbers. Each number corresponds to a 20 rpm increase or decrease. Default position on a 1986 300SDL is "4" but I don't know what other cars would use.

It may not even exist on your model. But that's where it would be and how it would work IF you have one. Knob lifts straight up to unlock, rotates, and presses back down to activate.

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~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL