Or check the little red fuse on top of the relay.

OP - the fact your tach worked before the swap means something changed. The fact that others have done the same swap and maintain tach function is confusing.

The tach signal on the diagnostic port is for the sensor at the harmonic balancer up front. Kind of a different animal with a different frequency...I think. Correct me if I’m wrong.

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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