Wrap around plug wire tachometer

[Spike0180]

Hey guys, I got a tachometer for father's day. It's a cheapo one with only 5ft of wire length, and I didn't notice when I asked for it that it's signal comes from wrapping a wire around the spark plug wire. Interesting. From what I've read (minimal), these things work when the electrical current in the spark plug wire creates a small voltage (.5V) from the wire wrapped around it. It uses this as its signal from the coil to tell it frequency. Is this all making sense? Is this an accurate understanding?

If this is all accurate, am I able to remove the hard wire that is used to wrap around the plug wire, and find where it is sending it's signal (softer automotive style wire) and just attach that to the negative side of my coil? Or will the full 12V likely destroy the Tachometer?

Another thing to note about the instructions on this teach. It suggests to wrap the signal wire around the plug wire 5 times, then an additioanl wrap until you get an accurate reading. Is this teach likely sensitive to voltage? This seems unlikely. Thoughts?

Thanks for the input everyone. I'll be experimenting with it one way or another. Getting a baseline reading, then trying to extend the wiring do I can have a tachometer at my gauge cluster, not my engine bay.


EDIT: for reference, this is the specific tachometer in question. http://www.searon-tech.com/product_view.asp?id=81
_________________
Brutis Patches Izabich: 1970 VW Transporter - 1776cc DP
Current State: Projects never truly end...
Location: Grosse Pointe, Michigan
Other cars: 2003 F150, 2003 Jetta GLI vr6-6sp

Sambastic: adj; the quality of being nit picky, elitist, expecting everyone to do things the way they believe is best with no regard to situation, "sambastic"

[Wildthings]

If you were to hook it up to the negative side of the coil it would need to handle somewhere around 300 volts, doubt that its circuitry is designed for that.

There should be very little current, so you may be able to extend the wires without problem. You might find that the signal degrades over a long distance though so maybe running coax instead of regular wire would help.

[telford dorr]

I think the wire wrap around the spark plug wire is a capacitive coupling (think: old ignition 'scopes used a version of this technique), and am afraid the capacitance of a long run of coax might attenuate the signal to nothing...
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[furgo]

[Spike0180]

Fist off, thank you guys.

Ah. Well 300v I agree is way out of the realm of possibility, I was thinking it would be 12v on the negative side of the coil. Can you guys think of any place I could get that signal from that would be a lower voltage? Or could I possibly regulate down the voltage?

As for lengthening the wire, if understand this correctly the concern is losing the signal on the way to the gauge. Bus I can increase the signal with additional loops around the plug wire? ... Maybe I can get enough loops to have a strong enough signal to still display correctly?

Hmm... on other tachometer "hacking" has anyone opened up a VDO tach? I don't see how to get into them without cutting through the plastic...
_________________
Brutis Patches Izabich: 1970 VW Transporter - 1776cc DP
Current State: Projects never truly end...
Location: Grosse Pointe, Michigan
Other cars: 2003 F150, 2003 Jetta GLI vr6-6sp

Sambastic: adj; the quality of being nit picky, elitist, expecting everyone to do things the way they believe is best with no regard to situation, "sambastic"

[skills@eurocarsplus]

dude....

just buy the right thing. telford is right, the signal degradation isn't worth the effort to make it work.

toss that on your lawn mower instead. even if you did get it to work, it would be a real chore to isolate outside noise...like being too close to other wires. think of that alternator wine when people run the amp + wire right next to the RCA's....this is a total case of the ends don't justify the means
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[Tram]

[telford dorr]

[Spike0180]

[sodbuster]

Notice that product you have is also an hour meter. We use these where I work primarily for it's hour meter function on some of our smaller pieces of equipment. Stuff like "Billy Goat" vacuums, mobile compressors and generators. Most anything powered by a small engine. The tach function as I recall does not work like a standard automotive tach.

Not the same as in the display does not sweep the RPM range as the motor revs go up and down. It more or less processes the signal and displays an average in a rather sluggish fashion. Been a while sine I looked at one. I'll go take a look and report back.

[Spike0180]

[sodbuster]

this meter works a little different than yours as it does not have the button to toggle between functions. it shows hours run while engine is off and RPM while running.

the tach gives a reading every few seconds while the engine is running. the changes in RPM raise of drop 10 to 20 RPM per "sweep" of the display change. kind of jerky by no means a smooth sweep of the RPM range. *

*pay no mind to the grey box in the pic. it's a separate thing from the meter.