how merc tachs work on 00 90hp ELPTO

[wn6ngp]

Does anyone have a general idea as to how the tachs work on the 2 stroke 90hp elpto? Are they just frequency meters counting the number of pulses/sec coming out of alternator? Reason I am asking is that I'd like to design a small digital tach. Like a series of LEDs in a row. Very simple. I can always just get out an oscilloscope and run the motor on a water hose etc but if someone here knows some of the details already I could get a lot done without dragging out all that equipment. Based on some generic google answers its either measuring the frequency or the voltage output of the alternator.

 

[sam am I]

Merc. picks off the stator's charge winding mag pulses via the rec/reg's gray wire on that/most engines. The rec/reg doesn't do squat anything with the charge winding pulses for the tach other than provide a convenient wire (gray) to hook the tach to but, they do charge ppl $400 for said rec/reg with this "magical" tach output.....Geeesh!!

Hence, go back in the day a bit, Merc used(still can if you want to) to just tied the tach wire to either yellow charge winding wires directly at the rectifier studs......Anyway,

Tach's (99.9%) use circuity to convert freq to voltage (F~>V converter, see pdf link below), either via analog or digital means. Digital conversion such as in a embedded controller for example.

As motor speeds up, freq of pulses (pulses at a given amplitude, say anywhere from 10V to 200V, the tach doesn't really care about amplitude due to the design) speed up, the F~>V circuity creates a proportionate (hopefully mostly linear) varying DC voltage that is then fed to a meter movement and/or LED bar graph and/or numerical read out via a A~>D (analog to digital converter).

You can of course buy what you looking for already built, but if you're anything like me, you like to learn under to spirit are cheapness.

http://www.ti.com/lit/ds/symlink/lm2907-n.pdf

 

[wn6ngp]

Thanks for all the detail
OK, well this falls into the category of "If you can think of it, then it probably already has been done"

I looked at the stator and it looks like 8 coils so I would assume that means 8 pules/rev? The tach has a switch for 1, 4, 3. So that would reference a standard 4 stroke tach of 2 cyl, 8 cyl, 6 cyl.. It says set it to 4. In a typical v8 engine you get 4 ignition pulses per rev.so maybe you only get 4 pulses/rev on the outboard due to the full wave bridge diodes only outputing half the pulses coming in. So at 1000rpm = 4000 pulse/m or 66hz. Does that sound right?

 

[sam am I]

Thanks for all the detail
OK, well this falls into the category of "If you can think of it, then it probably already has been done"

Yeah, fraid so BUT, you could always make one Better? Brighter? Faster? Auto Dimming? What about like a star burst like design where there are concentric circle that expand out, each circle represents like 500 RPM, so there's 12 circles, the last circle is 6K RPM. Skies the limit....

I looked at the stator and it looks like 8 coils so I would assume that means 8 pules/rev?

No,

8 pole stator = 4 Pulse/Rev

12 pole stator = 6 Pulse/Rev

The tach has a switch for 1, 4, 3. So that would reference a standard 4 stroke tach of 2 cyl, 8 cyl, 6 cyl.. It says set it to 4.

Yes, for 4 stroke..........

2 cyl = 1 Pulse/Rev

6 cyl = 3 Pulse/Rev

8 cyl = 4 Pulse/Rev

In a typical v8 engine you get 4 ignition pulses per rev.so maybe you only get 4 pulses/rev on the outboard due to the full wave bridge diodes only outputting half the pulses coming in.

Typically, tapping the tach's sig off the output of the full-wave bridge (actually a frequency doubler) isn't done (that's DC due to that's connected to the battery and it is acting a filter), they simply just pick off the tach feed sig via the charge stator (AC, yellow wire side), you'll by default get 4 pulses/rev on your 2 stroke 90HP OB, that is, if your stator is truly 8 poles.......You certainly could be right, I'm not sure

Usually however a Merc in that/your 90HP size, they're typically 12 poles. As stated above, 12 poles = 6 pulses/rev, better double check yours again and/or perhaps a OB mech will chime in here and set the # of poles record straight here on that/your OB's stator.

BUT, If it turns out you really do have a 8 pole stator, set your 4 stroke tach to 4 pulses/rev (V8 4 stroke), tap the AC (either yellow wire) charge winding's on your OB and you're golden.............Winner winner chicken dinner eh?

So at 1000rpm = 4000 pulse/m or 66hz. Does that sound right?

Ummmm, 4 p/r * 1000 r/m (r's cancel) = 4000 p/m = 66.6 p/s = 66.6 c/s = 66.6 hz........yes

 

[wn6ngp]

I was thinking that there was a separate set of diode(s) that didn't connect to a battery so you got just + pulses for the tach lead. . I havent' looked at the motor but according to the repair manual diagram there seem to be 8 coils in series under the flywheel.

Maybe we could do something clever with an arduino based tach, a use for christmas tree lights in the off season. I hate to leave all my squireled away electronics junk go to waste.

 

[sam am I]

I was thinking that there was a separate set of diode(s) that didn't connect to a battery so you got just + pulses for the tach lead. .

I'm not aware of any additional diodes being used/required for the tach signals, on the motors anyway.

I havent' looked at the motor but according to the repair manual diagram there seem to be 8 coils in series under the flywheel.

Count the magnets in the flywheel.....6 magnets = 12 poles, 2 poles/mag * 6 mags = 12 poles.

The stator charge winding coils (bobbins) can vary 8 or 10(plus a few ig/trigger coil windings) I've seen depending on the OB and year but, the coils are wound, positioned and rotational timing of the poles relative to coils are such that it is the number of magnet poles in the flywheel we use to determine pulses/rev. Hence "12 poles" = 6 pulses/rev as in "12 magnetic poles"

Maybe we could do something clever with an arduino based tach, a use for christmas tree lights in the off season. I hate to leave all my squireled away electronics junk go to waste.

I would be doing it embedded for sure these days, just too easy to code it up...

 

[merc850]

Why not buy an old tach from a marina, take it apart and replace the meter with your type of display. The "counting" work circuitry is already there.

 

[sam am I]

I'm not aware of any additional diodes being used/required for the tach signals, on the motors anyway.

Now I am........Was reading manuals, because...yes, I'm mad (See Carpalope). Saw this, go figure? Embedded diode in a terminal block!! Hmmm, well figured for future tense, this needed to be added.

Not sure it's necessary (esp nowwa days), perhaps for some older specific merc made tach or something was overly susceptible to a bit of stator noise (high freq switching transients or the like) ...Most filtering and such is done in the front end of tachs these days.