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.

 

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.

 

Thank you very much, 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.