For 85 and up diesels you get the tach pick up off the ring gear. This has two wires. How does this translate to RPM? I am trying to read this with an arduino and it's not clear if it voltage or resistance or what....

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1995 E420 Schwarz
1995 E300 Weiss
#1987 300D Sturmmachine
#1991 300D Nearly Perfect
#1994 E320 Cabriolet
#1995 E320 Touring
#1985 300D Sedan
OBK #42