Elantra 92: the tachometer runs intermittently

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.
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Jean Castonguay
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Your ignition coil has a tachometer interface on it. The signal goes from there to the noise filter you describe. The noise filter then sends the signal to the tachometer.
The noise filter bolts onto the surge tank. Check the wires to the noise filter. Often, the insulation comes off the wires and they short together.
Since the signal originates from the ignition coil, the ECM is not a potential cause unless your car is not running properly.
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On Fri, 24 Aug 2007 16:22:06 UTC, "hyundaitech"

I found the noise filter bolted on the left end of the air intake manifold, about 15 cm from the fuel pressure regulator. The resistance between pins 1 and 2 of the connector is infinity, so the resistor between these two pins is open.
Could you tell me where the surge tank is located?
Thanks again.
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Jean Castonguay
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