I have a 1990 240 and love the damn thing. Most reliable car I've ever driven, easiest to work on and holy crap is she fun in the snow. That's not the point though.

I'm having trouble with my idle speed after I drive on the highway for a short period of time after having fried the speedometer. I hooked up the dreaded tac wire after replacing something in the cluster and watched my speedo go from zero to 120.

I'll drive on the freeway though then when I pull back onto the neighborhood streets it's like my car still thinks I'm driving at freeway speeds and idles excessively high and can feel it even when the car is down shifting. It's most noticeable when I put the car in park. I should also mention that as soon as I restart the car returns to regular idle speed. I've since replaced the instrument cluster with another out of a 90 240 as well and the problem is persisting. I've checked the code and it reads out 3-1-1.

I read another post where the guy had this identical issue but I believe he drove a manual and the previous owner had installed the incorrect cluster or something along those lines. After he spent a pretty penny on a new cluster I think he said the problem stopped. Point being there's no way in hell in buying a brand new cluster when I know it's as simple as pulling a wire from somewhere or replacing a cruise control brain etc.

Please help! I'm simply dumbfounded as to how to fix this issue.

Thanks