Your 81 had an 85 or 84 transmission put in, those are the only 2 model years with a RPM sensor in the bell housing. Prior to 84, the RPM sensor is located at the front of the engine near the harmonic balancer. You should not ground the sensor at the bell housing which is poor practice.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked