| ------------------------------
|
| Date: Tue, 10 Nov 1992 16:30:51 CST
| From: "Andrew C. Green" <acg@hermes.dlogics.com>
| Subject: Re: Electronic Tachs
|
| Dave Van Horn (DaveVH@microsoft.COM) writes:
| > Would someone please explain to me how electric tachometers are
| > supposed to work?
|
Andrew responds:....................
| I have seen two different types of "electronic" analog tachometers. The
| first, and older type, was a General Motors unit of 1970 vintage that I
| installed once. It was powered solely by a single wire from the ignition
| coil. When the points opened, this apparently rerouted the voltage up
| the wire to the tach where it supplied enough juice to move the needle.
| The unit was ink-stamped "8 CYL" on the back, implying presumably that
| they had a specially chosen resistor inside with enough electronic
| cholesterol to position the needle at the right spot for the given
| on/off cycles of current voltage and amperage produced by a V8 engine.
|
Andrew, I suspect your description of this unit leaves out a step. What you
have described seems to be a guage which reads ON-time vs OFF-time rather
then rate. Your description seems to fit a DWELL meter. It you add a stage
containing something like a 555 timer chip which would trigger on the point
pulses and be ON FOR A CONSTANT LENGTH OF TIME independent of the length of
time the points are on it would work. Then the pulse modulated voltage would
be proportional to the number of pulses rather then their duration.
I use an electric tach from a Spitfire on my GT6 in place of the original
mechanical tach. ( i'm running a distibutor with no tach drive.) The
resistor you describe is there and by changing the value I was able to
make it run on 6 cyl instead of 4 cyl.
/DickN
|