Work your way back through the wiring connections to the harness adapter on the engine, and then the actual connection from ECM circuits. When it suddenly starts working, I assume all is nominal? Can you relate this to heat (engine finally warms up) or vibration (you get up and go, and then it starts working)? Those will be clues.
Hello 370: I am pretty sure it is related to heat as only time seems to fix it no matter what I do with the boat. Pretty sure it is in the ECM modules. I am assuming the tach signal goes from the coil to the ECM and then out to the tach?
Does the 2003 have smartcraft? I think 2004 was the first with smartcraft. But, I had intermittent issues with my gauges. The oil pressure, speedometer and trim indicators were working when they wanted to. The gauges were fine. The communication just wasn't correct all the time. They still go off and on when they want. But, it has been as bad now that I am using the boat a couple days a week instead of it sitting most of it's life.
Hello All and Thanks
The boat has been re powered with a 383 MAG MPI, 350 horse, but does not use Smartcraft. Gauge is standard pulse signal with switch for different cylinder numbers. I tried a brand new gauge but same results. After motor warms up, all works fine. I am pretty sure signal comes from ECM module, mounted vertically on starboard side with two multi pin connectors, rectangular.