Honda civic tach waveform

The tach on a 1993 honda civic VX is consistently low by about 700 RPM. This started as an intermittent problem, but is now consistent. To aid in diagnosis I would like to know what the waveform should be like. Does the tach respond to the duty cycle, or does it in effect measure the frequency of the pulses? If this error is in the signal coming from the ignition system, then the computer is getting erroneous information wrt engine speed. (the same signal is fed to the instrument cluster, the computer, and the external tach connector.)

What if the needle on the gauge slipped just enough to throw in out of calibration? Then all you need to do is pull the dash unit out, pull off the needle, and reset it.

The tach should be getting an impulse signal from the ignition system and converting it to a voltage to operate the gauge. That’s how most tachs work.

At the begining the problem was intermittent. Sometimes it would be 700 RPM low, other times it would be OK. This most likely excludes movement of the needle on its shaft.
I would have guessed that the ignition system provides pulses to the tach and the computer, where they are converted to a usable form. My question is whether the tach is in effect a frequency meter, or does it simply average the signal to get a DC current for the gauge. Knowing this will help in figuring out what is wrong, and whether the computer is getting bad data.

It’s most likely a bad gauge unit. Maybe corrosion on the cluster connection or a meter coil with a defect. I don’t think the signal to the tach is fed through the computer on this model.

If you have a U-Pull-It near you, a gauge cluster from a junker may be the cheapest diagnostic tool. I did this with my 2000 Ford explorer, and wound up using the junkyard gauge cluster as a replacement after swapping the speedo/odo combo. I was having tach issues and fuel gauge issues.

The wiring diagram shows the Vehicle Speed Sensor (VSS) feeding the cruise control, engine computer, and the instrument cluster. What does your repair manual say about testing the VSS? can tell you more about the VSS.

And not only did the tach work better, the car now had only 36,000 miles on it!

His speedometer works fine…(we assume)

The original odometer was swapped into the junkyard cluster as mentioned above. No change in registered miles.

The signal TO the tach and the computer are the same. (not THROUGH the computer).

True. The speedometer is fine.

The signal to the tachometer is from the ICM (Ignition Control Module) on the blue wire. You have no problem with the engine rpm, just the rpm INDICATION, right? So, there is no problem with the signal from the PCM TO the ICM. Just from the ICM tach output to the tach, and including the tach.
What are the voltages and waveform from the ICM output to the tachometer? Perhaps, the FSM (Factory Service Manual) has this information. You could check the wiring and connectors from the ICM to the tach, and check the ohms.

The RPM indication is the issue. I do not understand your conclusion that “there is no problem with the signal from the PCM TO the ICM” What is the PCM?

My manual shows that the signal from the ICM goes three places: The tach, the computer(ECU), and a connector for use with an external (service) tach. My manual does not show this waveform. I was hoping someone here would have that information. The vehicle is not here at present, so I cannot check it.

I agree that the problem is most likely in the tach, but want to make sure.

Thanks, Denis