There is only 2 sensor that could control the tach. crank sensor (if equipted) or the cam sensor. There is also a lead that goes from the ECM to the PCM for tach signals too. There is also a error code for that lead if there is a problem.
But maybe the cluster is screwy... Here is how to test it... Cluster Testing
A forum community dedicated to Dodge Cummins Diesel truck owners and enthusiasts. Come join the discussion about reviews, drivetrain swaps, turbos, modifications, classifieds, builds, troubleshooting, maintenance, and more!