Induction Type Tachs

[bill-mcnamara]

Wondering if any of you folks have tried to use this type of tach with twin motors?

I’m getting very strange readings from these little guys, even showing RPM on an engine that’s not running yet.
Or while at cruising speeds, they show values up in the 7-8 K range, almost like they’re being additive. Not so bad at low speeds, but a single motor still activates both gadgets.
I’ve played around with settings on the instruments (S1 & S2) but to no avail.
The translated instruction sheet isn’t much help to my feeble mind.

I’ve also re-routed each tach’s wiring up different sides of the boat hoping that would eliminate the apparent interference?

Any ideas what I might try next?…..

Bill.

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Attachments:

[nj-boatbuilder57]

The impulse travels up the cable, but the computation is done in circuitry within the meter module. Perhaps the interference is occurring because the modules are right next to each other. Maybe try separating the modules….

Just a thought.

[Pete]

Induction tachs are very sensitive to the magnetic signal in the plug wire. In my experience point condenser ignitions do not give a clean signal for these type ignitions on some engines.

Not known what engines and ignitions you have.

What is exactly the number of wraps you are doing on the spark plug wire.

Do these tachs have a ground cable ❓

What do the S1 and S2 buttons do ❓

Try isolating the signal from the wrapped wire from the other wire(s) by wrapping them in alum foil. Might work and worth a try.

Never could get this type tach to work on a 2 cylinder Merc or 4 cyl Merc 55 with point ignitions 😡

I have an older Tiny Tach model 1C with a wrap wire and a ground wire that works great on my Merc 25ss with CD ignition, but not on my Yamato Y80 with point condenser ignition. In my case on the 25ss I have to do 3 turns on one plug wire then in series to 3 turns on the other to get correct and not erratic rpm readings. I think Tiny Tack recommended the wrong tach when I asked them when purchased it was their suggestion to do the wrap I note above.

[bill-mcnamara]

NJ-57; thanks, for the idea, but have already tried separating the displays, same effect.

zul8tr; thanks also for your ideas. I’ll try to answer your questions.
1)Engines are the pair of ’56 Javelins as in earlier post, with standard OMC magneto ignitions.
2)3 wraps per wire as I recall, maybe 4?
3)No provision for ground wires on these units.
4)Not really sure what S1 & S2 buttons do, as supplied "translated" instructions were brief and confusing.

Are you suggesting wrapping the entire lead from engine to dash be wrapped in foil?

Sounds like you’re thinking these units may not work at all with point ignition.
May just have to chalk the idea up to a learning experience.

Bill.

[Pete]

quote Bill McNamara:

NJ-57; thanks, for the idea, but have already tried separating the displays, same effect.

zul8tr; thanks also for your ideas. I’ll try to answer your questions.
1)Engines are the pair of ’56 Javelins as in earlier post, with standard OMC magneto ignitions.
2)3 wraps per wire as I recall, maybe 4?
3)No provision for ground wires on these units.
4)Not really sure what S1 & S2 buttons do, as supplied “translated” instructions were brief and confusing.

Are you suggesting wrapping the entire lead from engine to dash be wrapped in foil?

Sounds like you’re thinking these units may not work at all with point ignition.
May just have to chalk the idea up to a learning experience.

Bill.

Bill

"Are you suggesting wrapping the entire lead from engine to dash be wrapped in foil?"

No just alum foil wrap the other spark plug wire in the engine. Not sure of the design in those engines but do both plug wires come very close as they enter the mag or do they go into the mag some distance apart ❓ If they are very close get the alum foil as close as practical at the mag junction. Once inside the mag the 2 plug wires still have intimate closeness to cause interference with the inductive tach, Regardless it is worth a try to foil up the plug wire not being used for the inductive mag pickup.

Like I said never could get them to work with point ignitions.

Found your earlier post, very nice rig.

Let us know your results

Pete

[bill-mcnamara]

Pete, thanks for the note.
The plug wires do exit the magneto in parallel,so that may be the source of the induction, although the pulses would be milli-seconds apart I would guess?
I think I’m just chasing my tail on this one, and will just fold up the idea until I come across a pair of original OMC tachs that require a sending unit on each engine.
No big investment on these two tachs, so no harm done.

Bill.

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