What kind of signal is the tachometer using?

[killerpirate]

hello,

I was wondering, what kind of signal does a mercury 2 stroke produce for the tachometer? (so the grey wire coming from the regulator/sender unit). I assume this is a square wave of which the frequency is going up as you hit the throttle?
I want to know the specs of this signal.
-What is the duty cycle (50/50? or does this change if the revs go up)
-I assume the signal is 12V pulse (going from 0V to 12)
-What are the frequencies of the signal when idling, and on full throttle?
-Is the frequency linear with the revs of the engine? So for instance, if the engine was turning 1000 rpm, would the frequency be 1000Hz?

[H2OPERF]

1 hz = 60 rpm, so divide your rpm by 60 to get hz then multiply this value times pulses per rev of the stator on your engine, V6 =12 pole alternators =6pulse per rev. sine wave output im not positive on newer stuff but most only use one diode in the rectifier so around +0-9v on each pulse... 1000rpm=100hz/6000=600hz

[killerpirate]

Thanks. I have a 115 merc 2 stroke 4cyl. So does this one have 8 pole alternator? I assume that the signal going to the tachometer is dc (12V pulse). Where is the 9 v coming from?

[H2OPERF]

Should still be 12 pole and 6 pulse per revolution, look at the tachometer charts online for your engine year see what they call for, honestly havent worked on one of those in years, most of the motors above 70hp use 12 pole, rude and merc... 6 pulse from a 12 pole alternator tells you they are only using 1/2 the diode bridge so you will get around 1/2 the dc voltage since you are using everyother +peak on the sine wave, pulsed dc lol.. max is around 18v at wot so u get 9ish depending on rpm idle will be much lower.. A square wave would be digital. Hope this helps... Dave

[H2Okie]

[H2OPERF]

Plus you loose about .6-.7 volt thru a diode. The newer electronic stuff like omc watercooled still use the same 6 pulse at around 9v not sure exactly whats goin on inside those to get the output..

[PanRonnie]

at 6 pulses per crank rotation times 1000 rotations per minute would be 6000 pulses per minute divided by 60 seconds is 100 pulses per second or 100 Hz but my math could be wrong

[H2Okie]

Great video for illustration and yes your math is correct . For me it's easier to keep straight by first taking the rpm's value 1000 and divide 60 to convert to revolutions per second RPS = 16.67 and then multiply by pulses per revolution ( 6)
16.67 x 6 = 99.99 or 100 PPS /Hz

But that's just me .