My tach amp connection looks different, yet the sensors are in the same locations, crank and inner fender. Dash may have been swapped out? Hard to tell. Tach is in same face as clock. If you pull the cluster and find a plug that is not plugged in, maybe so. My '82 240 manual did not have a tach, or those connections, if I remember correctly. My '82 300DT automatic does have a tach, yet different amp set up. PM me for photos.

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83 SD

84 CD