how exactly does the tach work?

[scuba_sean]

ok, does anyone here know exactly what makes the tach move? does the tach wire that goes into the ECU just vary resistance as the RPM change, and then the needle moves dependant on how much resistance there is? if not, then how does it work? i'm pretty handy with electronics, and I want to figure out how to make a sift light without spending a lot because i'm way to po' (something that all the regulars on here know )

[corn322]

hmm... I'm not sure how tach's work, but I know how you could make a shift light. get a 12v dc bulb, some wires, and a switch. actually, you might not need the switch. but anyhoo, set everything up where you want it. and as for triggering the system, stick wires through the back of your tach to the front, and kinda have them set up so that when the needle comes by, it will make the wires connect. yes, very ghetto. it might also set off the shift light over speed bumps.

[scuba_sean]

i dunno about that, that would look kinda ugly. and you'd have to run a wire down the tach needle

[Nismo_Freak]

Pull out the gauge cluster and get a wiring diagram

[scuba_sean]

well i just did some feild research nissan style. i stripped the tach wire going into the ecu and had a pal rev the engine slowly as i measured the voltage. it seems that at idle, it is aprox. 13.85V. for every 1500 rpm you go up, the voltge decreases by .1V. so i did a few calculations, and at 5500rpm, (which i think is the prime shift point), the wire is at 13.0V. so now i need to find some kind of relay that will turn a 12v circuit on if the voltage across the primary circuit is less than 13v, and turn it off if it is more than 13v. anyone heard of such a relay?

[7thGear]

just build a circuit that with a resistor that will pass current once you hit 13 volts... then just put a lighbulb/led/sound speaker on the other end.

or just use an led that needs 13 volts to turn on!

or if you know an led wont turn on until 3 volts hit it, put a restor that can handle up to 10 volts, that way there wont be a delay....

i think what i said made sense... its been awhile since electronics class

[scuba_sean]

first off, it's the other way around, it needs to go on under 13V and off over 13V. and second, electronics class?? i think you need to lok over your textbook again man. resistors work based on RESISTANCE not voltage. if you look at Voltage = Current x Resistance, the voltage would be the 13V, resistance would be the constant, and current would change. i have everything all built and ready to go, i just need some way to make it go on at the right time. somebody please help me out here, i need this before next race day!!!

[offtheline]

Im interested in this project. I dabble in electronics myself. Can you give me a detailed update as to what you have found so far?

[scuba_sean]

okay, the Y/R #3 wire in the ECU is the one for RPM. It reads 13.95V at 1500, and goes down .1V for every 500RPM or something. I know that is does hit 13.0V exactly at 5500RPM. I don't know if the amperage changes as the voltage does, because if it does, then i could probably use a sensor from like an alternator charge circuit or somethnig.