There are 2 wires that go through the big subharness connector and then on to the cluster that come directly from the ECM. The RPM data is certainly coming through on one of those. Y/Bl and B/G. I think you also have one of two possibilities, either the signal isn't getting to the cluster, or the circuit in the cluster which interprets it and moves the RPM stepper is bad. Unfortunately, that's about the extent of the data that the service manual has on the matter. If you can eliminate that there's a bad connection at the subharness connection, the next thing would be to connect your cluster to an otherwise good bike and see if it works or not.