Dave Van Horn (DaveVH@microsoft.COM) writes:
> Would someone please explain to me how electric tachometers are
> supposed to work?
Ahem. I haven't seen any replies to this, so I'll take the plunge. The
electronic tachs I've seen are generally pulse counters that count each
pulse from the coil firing a plug over time and display the result divided
by the number of cylinders. e.g. 6000 pulses per minute divided by 6 cyl-
inders should show 1000 rpm. Grossly oversimplified, but there you are.
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.
The second type of electronic tach, a more common type, is a true pulse
COUNTER. This counts the number of on/off cycles over a measured time
period (courtesy of an internal chip) and divides by the number of cyl-
inders in the engine, the latter being set by a slide switch or jumper
on its circuit board. The result is displayed on either an analog scale
or a digital readout, either of which are powered by an external 12-volt
supply, making for multiple wiring connections to hook it up.
When all is said and done, they're both self-contained designs that seem
to work well, in my experience. As you've already determined, malfunctions
often occur in the wiring, however. Since you have seen the symptoms on
two different units, you probably should investigate grubby connections at
the coil or distributor instead. Your "signal" input to the tach is most
likely just a plain old wire that's supposed to deliver 12 volts to the
tach when the points are open. Good luck!
Andrew C. Green
Datalogics, Inc. Internet: acg@dlogics.com
441 W. Huron UUCP: ..!uunet!dlogics!acg
Chicago, IL 60610 FAX: (312) 266-4473
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