Xref: utzoo rec.autos:40300 sci.electronics:15267
Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!mit-eddie!uw-beaver!milton!whit
From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: rec.autos,sci.electronics
Subject: Re: Digital Tachometer.
Summary: DON'T treat the primary as simple 12V signal!
Message-ID: <10216@milton.u.washington.edu>
Date: 30 Oct 90 05:46:38 GMT
References: <Mb9oF2q00VpP4Bd0VG@andrew.cmu.edu> <36367@rphroy.UUCP> <90300.143408JXS118@psuvm.psu.edu>
Distribution: usa
Organization: University of Washington, Seattle
Lines: 20

In article <90300.143408JXS118@psuvm.psu.edu> JXS118@psuvm.psu.edu (Jeff Siegel) writes:
>I had looked into building a digital tach for my 1980 model Chevette.
>On that car there is is pickup for a standard(? 8-) ) analog tach on the
>primary side of the HEI coil. This signal is more or less a 12 volt pulse
>each time one of the cylinders fires. I used a simple resistor divider and
>a zener diode

	The primary side on MY car connects to a CD ignition system; it
feeds either nothing or 350V to that coil.  On oldfashioned points-type
ignitions, the voltmeter will read 4V to 12V (depending on internal
or external ballast resistor) when the plug is NOT firing, but goes
to 300V when it DOES fire.
	The use of a zener diode and resistors is the ONLY way to
connect to this signal.  Big resistors are preferred (the coil
might be heating the resistor instead of firing the spark plugs if
you use a resistor of less than half a megohm value).  It might be
prudent not to touch this terminal when the ignition is operating, too.

		John Whitmore
		whit@milton.u.washington.edu