Do you have an oscilliscope? If they did not change the design from the earlier versions. There is just a pulse amplifier in the computor. You should see the pulse output with a scope. If there and reasonably clean I would suspect the tach unit in the dash itself. It has to incorporate the counter device. I am racking my brain to think of a common item to generate the pulses to check the dash unit but it does not come to mind. Perhaps someone else has a better ideal.