Miata Mailing List: November 1995, Message #204
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   From: "Robert V. Meushaw" 
Subject: tachometer question
   Date: Fri, 3 Nov 1995 11:02:26 -0500

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?