Craig Jennings Posted June 30, 2014 Report Share Posted June 30, 2014 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. So I dragged out my tacho You need to see what is + 12V, what is Ground and what is 'switched ground' or whatever that is, signal I suppose, from coil. 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 thin film 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. Now using Ohms law 200V / 42000 = Â ~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 amperage12 / 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! Quote Link to comment Share on other sites More sharing options...
Craig Jennings Posted June 30, 2014 Author Report Share Posted June 30, 2014 And for those with fine taste in Kiwi metal will be able to identify the backing track Quote Link to comment Share on other sites More sharing options...
Adamw Posted July 1, 2014 Report Share Posted July 1, 2014 You can also usually make these high level tacho's work using a relay coil like this: http://www.haltech.com/wp-content/uploads/2013/07/Relay-Wiring-for-Tacho-Output1.pdf Quote Link to comment Share on other sites More sharing options...
Craig Jennings Posted July 1, 2014 Author Report Share Posted July 1, 2014 I thought about that one, would be handy if you didn't want to delve into the dash.Nice addition Adam, now we have it covered hopefully! Quote Link to comment Share on other sites More sharing options...
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