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  1. Hi, this is a thread I did over on the Link forums and it went down ok. I've been trying to get it up in tech articles but Gaz no longer runs them and I can't get another admin to let me post in there, it can be transferred if deemed worthy later. So I'll put it here with a bit more info for the layman as opposed to those on the Link forums. You can also drive it with a coil (like from a relay) to step up the voltage but I think that's hoary If we peer review this we can have a pretty good "how to" with a bit of luck. As below: Howdy, Seeing as there is weird info online and not much is clear in regards to driving a high level tacho with a low level signal I figure I'd take mine apart and apply Ohm's Law as I figure it's got to be simple enough surely! 97 Mazda B2200, link atom g4. Background: "High level" tachs are driven from the neg post of the regular coil which can see 200 - 400V which is where the "high level" comes from, it's back EMF from the collapsing primary coil. What we see from a computer and sometimes from an output from an igniter is 0-12V (more or less) and is known as "low level". Now something that takes high voltage to drive, will not drive with low voltage, thankfully they're really only dragging down the current so we can replace the resistor with a little bit of Ohms law to match the new feed voltage. What you will need: Soldering iron and solder. Multimeter. Screwdrivers. New resistor. Here's how I did mine. So I dragged out my tacho You need to see what is + 12V, what is Ground and what is signal from coil. Do this by tracing the ribbons or traces or wires that connect to your tacho. You'll see that they use the traces for +12V and earth for things like lights so you'll be able to figure out which is which there which leaves the other one to be your signal. Now that you've found your signal, check to see what size the resistor is (hopefully there is one and I've not wasted your time). Mine is a through hole resistor which is nice and simple, they may exist in surface mount too but those are easy also, don't be discouraged. You'll want a fine tip on your soldering iron though. So mine is a 42k resistor. That sausage with bands around it and wires out either end. (http://www.digikey.com/~/media/Images/Marketing/Resources/Calculators/resistor-color-chart.jpg?la=en-US if you don't have a resistance tool on your multimeter) Now using Ohms law 200V / 42000 = ~0.005 amps. (guessed at 200V, might be 400 but with such a large denominator it doesn't really matter) Because we have only 12V to play with we go backwards to find the resistance needed for that amperage 12 / 0.005 = 2400, so we need a 2k4 resistor. So I took out the 42k resistor, replaced with 2k4 resistor and put it back in. Video of tacho working: http://i801.photobucket.com/albums/yy295/Bangbug_bucket/video-2014-06-30-21-48-13.mp4 RPM sweep works nicely too, thanks link!
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