Tacho Sensor - Alt v Injector Pipe; Which is Better?

[Wotname]

What are the pros and cons of using the alternator W terminal sensor style of tacho compared to using a injector pipe pulse style (Tiny Tach type).

Leaving aside the initial calibration and the analogue versus digital display, which is more reliable?

I have never used either, only ever had magnetic pickup or cable driven tachos in the past.

I have read that the alternator sensing is unreliable when the alternator is not loaded. Is this true? If so, what sort of amps is needed from the alternator before the sensing is stable?

In the proposed installation, the alternator is only being used to charge the start battery and drive the engine instrumentation so I am assuming the alternator will be lightly loaded most of the time.

Has anyone has problems with the Tiny Tach style of injector pipe pulse sensing?

Are there any other issues I have not yet considered?

[sailah]

I have a Balmar alternator and Westerbeke gauge cluster. Mine is driven from alternator. I was getting a very unstable reading for years, and it seemed most prevalent at low rpms. If you started engine it wouldn't read, only when you rev'd it. Then it would suddenly start working and stay accurate, except when you throttled down. I thought it had something to do with the belt slipping or the Balmar belt saver function on my regulator.

Turned out it was just a bad connection on the back of the alternator. Mine has a plastic plug with a white/blue (? I think that's what they are, forget) wires. After fiddling with it, tach jumped to life. Then I set about fixing that connector and it's been 100% since then. Can't really blame that reliability on anything but myself.

[SVHarmonie]

You are confusing a "lightly loaded" alternator with one that is "unloaded." The system you describe is lightly loaded, and will work just fine

A common example of a system where the alternaotr "unloads" is a boat with a solar system where the solar panels drive the system voltage above what the alternator is set for. The field current can then go to zero. When that happens the alternator stops outputting the tach signal. This never happens on a system without another charging source. And would never happen on a system with a sophisticated external alternator regulator which understands the problem and has its own tach output.

Alternator outputs are by far and away the most common system for reading engine RPM. They are reliable and robust. There might be a good reason for spending lots of boat bucks on alternative, but reliability is not it.

[Jim Cate]

wottie, the alternator driven tacho has several error sources but I've never had an issue with needing a load on the alt to make it work. Don't see how that would occur, either!
But belt slippage is a common cause of inaccuracy and accurate calibration (which you didn't wish to address!) can be difficult.

The pulse counting type, whether triggered by a magnet passing by or pressure waves in a fuel pipe (damn clever idea, that!) are, I believe, more stable and less easily confused... and offer direct calibration opportunities.

Many, many years ago when I was a young hot rodder type, a friend started selling one of the first electronic tachos for cars. I asked him how he calibrated them... his reply:"calibrate??? Hell, no one cares if it is accurate, they just want a needle that goes up and down, and preferably up to 8 grand". I suspect this is true of many yotties as well, only with smaller values.

Jim

On edit, I see that Harmonie has pointed out that having an additional charging source can shut down the alt output, and he is quite right about that... we get that at times, and I had neglected to mention the issue, as it didn't seem to apply to your setup.

[HankOnthewater]

I have only experience with alternator sensors, currently both on Perkins engines, one in the van, one on the boat.
No, never had a problem with the loading or lack of it. It just works. But a slipping belt WILL give incorrect readings. And a broken belt... yes, an instant return to zero RPM.
In case of V-belt, fitting a slightly narrower belt, a worn belt, or have a worn out groove on the pulley(s) will give incorrect readings. Obviously a serpentine belt does not have these issues.

When buying a new-to you boat, how does one know if the correct pulleys are fitted, meaning both the crankshaft pulley and the pulley on alternator? Other than standard pulleys will show incorrect readings, although some (?all) tachometers allow calibration. Then one of those laser pointing handheld tachos is needed. I got one and checked quite a few rev counter's on other boats..... and found that often the readings are out by plus or minus 10%, some boats worse.... assuming that the handheld laser thingy is accurate.....

The tacho on the van has worked for me for 9 years (van is 23 yo), tacho on the boat stopped working, I think it is a wiring issue; anyway I am planning to remove the alternator next week, giving it a birthday, and if the tachometer itself is broken.... I will replace with aftermarket one, with magnetic pickup. Where one pulse is one revolution: less calibration issues, less V-belt issues.

I have seen the tiny techs, I would think super reliable, but the readout is far too small for (my) everyday use

[conachair]

Interesting topic, this I've yet to test and is part of a circuit board made by jlcpcb and designed just off not too much time on google for an esp32 to hopefully measure RPM from an alternator W terminal - think it will work driving a 3.3v esp input pin? The ESP can detect rising edge or falling edge. RPM flag is attached to an ESP pin. ta

[Wotname]

Originally Posted by SVHarmonie

You are confusing a "lightly loaded" alternator with one that is "unloaded." The system you describe is lightly loaded, and will work just fine

..........

You are correct! I was confusing lightly loaded with unloaded .

Thank you for lifting the veil of confusion from my eyes - everything I have read about alternator sensing now makes sense (no pun intended).

Now that is clear, I can think more about the calibration aspects. The plan was always to use a hand held tacho (at least for fine tuning) rather than relying on the number of alternator poles and pulley ratios etc.

I am assuming belt slippage is only an issue when the alternator is loaded up. In my application, the alternator will be trickle charging the start battery (and engine instrumentation) for the vast majority of the time so I am hoping belt slippage inaccuracy will be at the most, a minor inconvenience.

Thanks to ALL who have replied and shared their knowledge - it has been very helpful to me.

[Wotname]

Originally Posted by HankOnthewater

............

I have seen the tiny techs, I would think super reliable, but the readout is far too small for (my) everyday use

This is my biggest concern about the Tiny Tach but otherwise I like the concept for all the reasons you and others have posted.

[Compass790]

Just a thought that maybe it's quite hard on a tiny tach sensor being attached to the injection line. Not just the standard engine vibration but the injection pulse as well.
Hope someone with experience can tell you about longevity.

[Mike Banks]

Neither. Small magnet in flywheel, or on crankshaft fan pulley, with Hall effect sensor bought from a wreckers (Ford uses one on the fan pulley) is a better option.

Stalin's Revenge, the wonderful underrated Lada had one on the flywheel.

[model 10]

I installed a Tiny Tach. First the memory failed and the factory was no help. I have little idea how many hours I have on the engine. The select button cover on the screen failed (UV?) so now the unit is not water proof. If the sender on the fuel line get's wet, it doesn't work. The resolution is to 10 rpm, so it jumps up an down all the time. If it had worked and the screen had not failed I would have been happy enough. Oh, there is no dimmer on the light.

[Wotname]

^^
I understand any item can fail but your experience does not inspire confidence in the Tiny Tach. Thanks for posting your empirical observations, Ecos.

Perhaps the quality (or otherwise...) is reflected in the price.

[jacktheflyer]

Two issues I've had with the alternator Tacho. The first relates to belt slippage caused by premature wearing of the belts. This is well documented elsewhere for Yanmar 3YM30 engines.

The other experience was where one engine used significantly more fuel than the other to the extent I ran out of fuel on that engine a couple of times. eventually found the pulley on the stbd engine was larger than the port engine and so the engine was running slightly faster and taking the lions share of the load.

[RogerLloyd]

I have a Tiny Tach. It is very difficult to see, being so small. However, no problems with reliability over many years. It also counts engine hours.

[a64pilot]

You don’t need a magnet, a Hall effect sensor will work astonishingly well counting the teeth on the starter ring on a flywheel, nearly certain thats how mine works.
Using an alternator is just cheap, manufacturers should be shot for that.