My 85 300d does the same thing--seems to whir all the time. It can't be normal, but my CC doesn't work so I never really worried about it. Like you, I've planned to fix it but never have gotten around to it. Mine works intermittently. Strange thing is, it seems to "work" on one stretch of road that I travel fairly often. Most other times, it'll just put my go pedal to the floor. I've tried to let her go to see at what speed she might feel is appropriate, but well, let's just say she wasn't yet comfortable propelling me to 92 mph.

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1998 E300D, 287k, barely broken in.