The more you talk, the less I think my amplifier will work.
I suspect that the tachometer has a high impedance, fast amplifier on the end of its sense wire but your meter doesn't.
I believe your meter shows 2.5 mv, but that's because it isn't fast enough to catch the peak reading.
If you use my amplifier and throw 0.1 volt at the sensor wire, I doubt it will work.

 

Is the tach casing made of metal?

 

After going to the Enmco site and looking at all of the tach/hourmeter data sheets they all show the signal wire as two wires, one from the sparkplug and one ground. They also show the lead wires as only 5 1/2 feet long. Maybe they are limited to that distance from the engine? Was your unit new? Maybe the wires were messed with if it was a used unit.

 

.................
In respect of the shielding, I cannot connect shield at tach end. As I said there is no shield or shield connection on my tach unit. It just has a single wire dangling from the resin sealant at the back.

Like the impedance test, the only way to get a 0v at the tach is to get to the tach battery -ve. Maybe this as a shield connection?

But does it make sense to connect both engine -ve and tach battery -ve to either end of shield? Would that not create ground noise at the tach?.......................

So your tach is not like the picture that shows a shielded wire?
If it only has one wire then it must be using capacitive coupling to the vehicles chassis (ground) to complete the circuit.

-ve could be used as a return if the case is plastic.

Connecting both ends of shield will not create ground noise, that only can occur if there's a ground loop (more than one ground return connection).
As I said you need two connections to complete a circuit.

 

OK trying to answer all questions:

It is a new unit, no shield, single wire.

As I said originally its similar to the link I posted but no shield arrangement.

Case is plastic.

No connection to vehicle ground.

As I say, within about 6ft works perfectly but longer than that, it starts to fail.

Would it help to put it on a scope and take a pic?

 

If there is only a single wire, then the return must be capacitive coupling between the unit and the vehicle metal chassis ground.
But in a small boat there's no metal chassis, so it doesn't have a good return.
Try running a wire from the boat circuit ground to a metal plate the size of the tach or larger, and place it next to the back of the tach to see if that works.

 

If there is only a single wire, then the return must be capacitive coupling between the unit and the vehicle metal chassis ground.
But in a small boat there's no metal chassis, so it doesn't have a good return.
Try running a wire from the boat circuit ground to a metal plate the size of the tach or larger, and place it next to the back of the tach to see if that works.

I think I have the answer to that one already
The tach is mounted on the dash/helm which is a 4mm thick aluminium plate. This plate is grounded via a 4mm cable to the negative bus bar in the fusebox behind the helm. In turn, the busbar is directly connected by a 4mm cable to the battery at the back of the boat.

For clarity, this is same battery that is connected to the outboard engine ground via a 10mm starter cable.

However, there is no direct hardwired or direct contact with the tach to ground.

Hope that makes sense.

 

I'll ask a dumb question, why do you even need a tach? A tach is only a diagnostic tool, they added them in cars to make the owners think they are Rickie Racer. The hour meter is an OK thing but it could be mounted near the engine, no need in a dash.

 

I too am a new member.You have too more options.
Since your signal is good at about half the length of the cable you do not have a problem with shielding.
Increase the guage of your cable to minimize resistance.
Use a 120/240 volt transformer to increase your voltage x 2.
Good luck.

 

Try using smaller diameter bare wire to wrap around the plug wire and use more turns.
The bare wire will be closer to the plug wire and that, along with more turns, will give you a higher voltage.
Solder the bare wire to the insulated wire running to the tach, or use a wire-nut or other connector.

 

I'll ask a dumb question, why do you even need a tach? A tach is only a diagnostic tool, they added them in cars to make the owners think they are Rickie Racer. The hour meter is an OK thing but it could be mounted near the engine, no need in a dash.

I'll assume that was just a flame and you're not serious.

Try using smaller diameter bare wire to wrap around the plug wire and use more turns.
The bare wire will be closer to the plug wire and that, along with more turns, will give you a higher voltage.
Solder the bare wire to the insulated wire running to the tach, or use a wire-nut or other connector.

That sounds a sensible option with minimal effort to try. I'll have a go over the weekend. I've got some 0.5sq.mm solid core copper wire in the garage so I'll wrap that around and also try running a 2.0mm cable to the helm (as per sugestion by merts)

Failing that, time to buy a better quality tach I reckon!

 

I'll ask a dumb question, why do you even need a tach? A tach is only a diagnostic tool, they added them in cars to make the owners think they are Rickie Racer................

Gasp.
That was spoken like a true non-engineer, since most engineers want to know everything about what the engine is doing, whether necessary or not.
They also want gauges for oil pressure, temperature, fuel consumption, battery voltage, etc., not idiot lights.
I remember as a kid, thinking how neat it was that the foreign sports cars had tachs to show what the engine speed was, just to know that info, not that it was "needed".
When the first American cars came out with tachs, I was elated.
I can't believe you don't understand that.
We may have to demote you from engineer to management.

 

My latest car has a tachometer, and I find it useful because I can't hear the engine most of the time and I haven't learned the car's vibrations yet. The tach tells me if it ever went into 5th gear. It tells me how hard I'm romping the engine when I have to accelerate out of the economy range. Some day soon, I hope to find out what the, "red line" is...then not go there.

 

I'll assume that was just a flame and you're not serious.

To all that disagreed with my question. A tach is just a doodad that isn't a necessary part of driving. How did people get along without them for so many years? It is just a "cool" factor in a dashboard. Sure race cars have them, but if you have time to watch the tach in a race, your not going fast enough. Automatic transmissions sure don't need them, and a stick shift car your better of shifting by the "seat of the pants" feeling instead of watching the tach.

Oil pressure, volt/amp meters, and water temperature gauges tell you what you NEED to know about the health of the engine operating system, tachs are just fluff.

 

Now to get back to the problem. @bobtooke, you have a wire wrapped around a spark lead, one end is going to the tach, what is the other end of that 'wrap' wire going to? What type of engine is this? Inboard or out board? Two or four stroke? The device your using, that Enmoc is really a hour meter that has a tach built into it. You said it is "like" the one in the link, why not show the actual link so we can see how it is supposed to be wired? Why not use a real tach that is linked to the ignition module like a real tach is?

 

The ratio of engine rpm and indicated relative velocity, can tell you many things about the marine environment you are on or in.
Unlike in a car, the sea 'road" is moving in it's own direction.
RPM and shaft count is very important for navigation.
Even more so in autonomous applications.

 

.......................
Oil pressure, volt/amp meters, and water temperature gauges tell you what you NEED to know about the health of the engine operating system, tachs are just fluff.

One's man's fluff is an engineer's necessity.
I use the tach on my automatic transmission car to determine when it has achieved lock-up.
Sometimes when going up a grade it downshifts and drops out of lock. I will then back off the throttle slightly so that it more readily goes back into lock, since that's a more efficient mode for the transmission.
I can tell it's in lock when the RPM's decrease slightly and then a small increase in throttle doesn't change the engine RPM.

 

To all that disagreed with my question. A tach is just a doodad that isn't a necessary part of driving. How did people get along without them for so many years? It is just a "cool" factor in a dashboard. Sure race cars have them, but if you have time to watch the tach in a race, your not going fast enough. Automatic transmissions sure don't need them, and a stick shift car your better of shifting by the "seat of the pants" feeling instead of watching the tach.

Oil pressure, volt/amp meters, and water temperature gauges tell you what you NEED to know about the health of the engine operating system, tachs are just fluff.

Clearly that was a flame then and you're just trying to provoke a reaction. No-one with any basic engineering knowledge would actually believe that.

 

If you not trolling and really do want to contribute, I would be grateful so will answer the questions.

Now to get back to the problem. @bobtooke, you have a wire wrapped around a spark lead, one end is going to the tach, what is the other end of that 'wrap' wire going to?

Nothing, blank ended as per how it was supplied and instructed.

What type of engine is this? Inboard or out board? Two or four stroke?

Honda outboard, four stroke, electronic fuel injection & ignition.

The device your using, that Enmoc is really a hour meter that has a tach built into it. You said it is "like" the one in the link, why not show the actual link so we can see how it is supposed to be wired?

Because I couldn't find a link on the web to show instructions for the actual unit. The Enmoc looks identical but has a shield wire whereas my version does not.
You can find them for sale on Ebay though: http://tinyurl.com/hs96j9b

Why not use a real tach that is linked to the ignition module like a real tach is?

The original Honda tach is 85mm diameter face and about 150mm deep. There's just not sufficient space in the helm panel to fit it. All I need is a basic digital read out to determine economy, performance and prop slip on the fly. If this unit would work at the helm it would be ideal.

 

No not trolling or flaming. Have been around racing car since ~14yrs old. Every thing from go karts, 3/4 midget cars, motorcycles, off road/hill climbing Jeeps, dune buggies, pulling tractors and drag racing. Don't know of one driver in all the years(I'm 68 now) that relied on a tachometer when driving. The type of thing your using are mainly hour meters, they just added a rudimentary tach into the chip.

I'd look and see if there is a smaller 'real' tach that would connect to the ignition module if doing this for myself.