On a recent trip the tachometer quit working. I took the coach into my mechanic to get it fixed and they can't figure out what is wrong. I have a rudimentary understanding of how this all works. Here is what they are telling me.

First, it has a 159 tooth flywheel and there is a magnet on the flywheel which a sensor reads and sends a signal to the tachometer which reads out the RPM based on revolutions the flywheel makes each time the magnet pass the sensor.

They have replaced the sensor, the wiring and tried 3 different tachometers. There is a strong signal at the tachometer with a 1/4 Ohm drop in signal strength which is normal. They have set the dip switches as recommend for a 159 tooth fly wheel and no matter what they do nothing seems to work.
Does anyone have any suggestions for what is wrong???

Mike