You know what’s funny, I went through the same thing on my 1982 240d and I recall there was some kind of pickup there. It was funny because the car didn’t have a tach...just a big nice clock. But I recall there was something down there. Maybe it was a stud on the balancer? Memory failing me. I feel like there was a cable with a diagnostic plug at the firewall...but I may be mixing up my cars.

Good luck. Probably best to pull everything you can from the 83 300d for the transplant.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles