When the tachometer runs, it seems to display correct values.
This leads me to hope that the tachometer is not defective but that the speed signal does not reach it due to an intermittent contact between the tachometer and the device that generates the signal.
The tachometer is fed 12 volts from a line that also powers the temperature and fuel gauges; the latter work flawlessly. The tachometer is also grounded.
I suspect that the third wire connecting to the tachometer is the culprit. I think that this wire carries a number of pulses per minute proportional to the engine RPM: I have not used an oscilloscope to look at it yet. According to the Hyundai electrical diagram, the signal comes from a power transistor and is filtered by a "Noise Filter". Most likely, this power transistor is "tickled" by the Engine Control Unit, the device in the know!
Could anyone tell me where the Engine Control Unit is and what is the path of the signal from the Engine Control Unit to the tachometer.
Thank you very much in advance for any help.