Miata Mailing List: November 1995, Message #229
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   From: Philip Logan 
Subject: Re: tachometer question
   Date: Fri, 3 Nov 1995 16:44:52 -0500



On Fri, 3 Nov 1995, Robert V. Meushaw wrote:

> I am trying to measure the engine rpm using the diagnostic port on the 
> Miata.  There is a terminal called IG- which supposedly is related to 
> rpm.  I have a digital multimeter which has a frequency measuring 
> capability.  I hooked it to the IG- terminal and got a reading in 
> cycles/sec, but it was not the same as the tach reading in the car.  
> Obviously, the tach is measuring revolutions per MINUTE and the frequency 
> meter is measuring pulses per SECOND, so there is a conversion factor 
> there, but there is not a factor of 60 between the two readings.  Does 
> anyone know the exact correlation between the IG- signal and the engine rpm?

The tacho in the car is not particularly accurate. I don't know how 
exactly the Miata tacho works, but there's usually a tachometer filter 
that's sometimes used just for cleaning up the pulses and sometimes to 
convert the pulses to a steady voltage. In any case, adding a load to the 
IG- terminal will affect the operation of the tacho filter and therefore 
the tachometer. When I connect my analogue tacho, the internal tacho 
reads zero. The digital meter has a higher impedance, so it doesn't 
affect the tacho filter so much.

So, don't worry about it on either count. Your meter is the one to 
believe.

Phil Logan