Correct wiring for this Tachometer

[eavega]

This may seem rather obvious, but I only want to wire this tach up once. I have purchased a tach, image is below. Its to connect to my 1972 Evinrude 50 HP. The wiring diagram in the manual shows the gray wire coming out of the control is the tach wire. I have located this wire. The tach has three terminals; +, - and PL. I just want to verify that I am wiring this right; +12V to the + terminal, ground to - terminal, and gray wire to the PL terminal.

A further question is, why does it need power connected to it? This tach has no light. Does the supplied power hold the needle in place between pulses? Just curious about that.

Here is the bach of the Tach for reference.

Any help is appreciated. This is a "discount" tach that I purchased, which was sold as a universal 12-pole tachometer, which is supposedly what my motor uses.

Thanks

-Eric

 

[Silvertip]

Re: Correct wiring for this Tachometer

Why does a fish finder need power connected? Why does a radio need power? Your tach needs power because it is an electronic device. Electronic devices operate from a power source.

 

[seabob4]

Re: Correct wiring for this Tachometer

Gray to "pulse", purple to "positive", black to "negative", you should be good to go...

 

[eavega]

Re: Correct wiring for this Tachometer

Why does a fish finder need power connected? Why does a radio need power? Your tach needs power because it is an electronic device. Electronic devices operate from a power source.

not to seem naive, but isn't the pulse an electric pulse? I can understand needing ground to complete the circuit, but I didn't realize that the tach needed a separate power source in order to measure a power pulse.

-E

 

[seabob4]

Re: Correct wiring for this Tachometer

Eric,
Your tachometer, as Silvertip stated, is an electrical device, therefore it needs ELECTRICITY! In the form of 12 Volts DC. The purple lead supplies it's power, the black lead (ground) supplies the return path of the 12VDC back to the battery, and the gray wire supplies the "pulses" that tell the needle what to show you on the gauge...

One thing I try to do when I ask people who have knowledge of things I don't know about. I shut my mouth and do what they say. If it doesn't work, then I come back with more questions. But I do what they tell me first...

 

[eavega]

Re: Correct wiring for this Tachometer

seabob4, you seem to misunderstand. Its not that I wasn't going to do what you all told me to do, I was just curious as to how a tach worked and why it required power. That having been said, I did my best to ignore the sarcastic tone of Silvertip's response (really, a fishfinder needs power? Ya don't say!) to try to further understand how this device works. No harm, no foul. I wired it up last night per your instructions. I will hopefully get to test it today. Thanks for the responses, the help and the information

-Eric

 

[Silvertip]

Re: Correct wiring for this Tachometer

I used a simple analogy to answer your question and suddenly I'm the bad guy. Why is it you understand that a fish finder needs power but not the tach. Doesn't a fish finder send a pulse to the transducer and the transducer receives reflection of that pulse that must be interpreted and results displayed graphically? Think about that (dang I did it again).

Apparently trying to help people "think" as they learn has gone by the wayside and folks now simply want a question answered so they can move on only to be back with another along the same lines later (oh -- wait, I did provide a simple answer did I not). To show that my heart really is in the right place let's try again.

The manufacturer put the three terminals on the guage because they are engineers and that many terminals are needed! The pulse from the engine must be "interpreted" because it is a low voltage "signal" not a power source. It takes an electronic circuit powered by 12 volts DC to count the pulses coming from the engine and convert that number into an analog reading on the gauge face. Look at it like a hand held calculator. It has batteries in it to make it work. When you push a key that sends pulses (that's the third connection) to the electronics which "interprets" that keystroke and provides a readout. Your computer works the same way. Power is needed for nearly everything connected to it. It is the "signals" that pass between the PC and peripherals that are being "interpreted". Interpreted meaning counted, calculated, displayed and/or processed.

As you are installing the tach, if you have a temp gauge, fuel gauge, oil pressure gauge, or trim gauge, check the connections on the back. By golly all of those have three connections as well as well as one for the light. All for the same reason as just explained.

 

[eavega]

Re: Correct wiring for this Tachometer

Silvertip

We seem to be having a grand case of not understanding each other. I appreciate the help. What I was looking for was a further explanation of how the tach worked. My simple interpretation of it was wrong, as what I believed was that the pulse itself provided the power to move the needle. This is something that is completely possible without having a separate power source (think electromagnet IF the pulse is a sufficiently strong electrical current, right?). Using a fishfinder (which has a digital display) or a radio (which has tuners, amplifiers, filters, etc), or even a computer (which requires voltage to actually work memory) was not comparison that I would be able to relate to since any of those devices are quite different to this instrument. I was simply curious and wanted a better understanding. I would assume that the good engineers that designed the instrument would not put power connections on it if they were unnecessary. I was curious as to WHY they were necessary.

I was, in fact, not SIMPLY searching for an answer to my original question of HOW to wire the tach. I think I stated that quite clearly in my original post when I wrote that I had an ADDITIONAL question. I am an engineer myself, and as such curious by nature.

Oh, and in reference to your statement about the different gauges requiring power, well, my boat has none of those. In fact, I didn't even have a tachometer previously installed on this boat so I would have no idea that any of those gauges required power (although I would probably assume that they did).

In any case, thank you for your further explanation. The bit you included about the pulse being a low voltage signal that needs to be interpreted by the electronics in the tach was the information I was asking about. Now I know that my previous assumption of how the tach works was wrong. Thanks for the simple, clear explanation.

Rgds

Eric

 

[Silvertip]

Re: Correct wiring for this Tachometer

What can I say except I'm surprised that it took this much discussion to explain what is a basic electrical concept to an engineer. Sorry for that but when I was in school a tachometer was one of the first circuits we were required to design and build from scratch since it was a simple concept and required processing a waveform and converting that waveform to engine rpm indication. But then I once worked in a non-electrical department that had a EE as manager. I asked him point blank one day why he wasn't using his degree. His reply really gave cause for concern about our education system. He said he was a good student, could learn anything from a book but when it came to design work he was clueless. He ended by saying he can't screw in a lightbulb and be sure it would come on when he hit the switch. That isn't learning -- its passing a test.

 

[eavega]

Re: Correct wiring for this Tachometer

Never said I was an EE

-E

 

[Silvertip]

Re: Correct wiring for this Tachometer

Quote: "I am an engineer myself, and as such curious by nature."

While you did not specifically state you were not a EE you made no effort in ths electrical discussion to point that out which would, by implication, indicate you were. Ever thought about politics?? Tag -- your it!

 

[eavega]

Re: Correct wiring for this Tachometer

I resign from this verbal volley. Thanks again for the explanation of how the durned thing works. I am enriched by the extension of my knowledge of all things nautical. If I can ever be of assistance to you, please don't hesitate to ask.

Rgds

Eric

 

[seabob4]

Re: Correct wiring for this Tachometer

Eric,
On a different subject, bet you didn't know that all the work on the American side of Honda Marine goes on right there in Alpharetta...

 

[eavega]

Re: Correct wiring for this Tachometer

I didn't know that. I do know that Honda has a huge facility out here. I took my Motorcycle Rider Safety course on their campus.

 

[Stratosman86]

Re: Correct wiring for this Tachometer

Very interesting thread, now I have a question that will add to this mayhem.

I am restoring an old Starcraft and tried to remain true to the oldies theme and have installed an old Aquameter tach. It only has TWO wires coming off of it, a black and grey. Now I know the grey is for the signal from the engine and the black is for the ground, how do I get 12 volt power to the gauge? I have the two wires on it hooked up correctly but alas the tach does not work. Do any of you know how this tach works?

Thanks,
Darren

 

[seabob4]

Re: Correct wiring for this Tachometer

Darren,
You got a pic of the backside of that tach?

 

[Stratosman86]

Re: Correct wiring for this Tachometer

Not handy, but I will get one tonight and post.

Thanks, for trying to help, others have told me to just get a newer tach, which I have plenty of, but I was trying to keep from having to cut big holes in this new cedar dash.

Darren

 

[Silvertip]

Re: Correct wiring for this Tachometer

It works just like the el-cheapo dwell/tachs that you can buy from JC Whitney for 15 bucks or so. They are not accurate and are self powered using the strength of the pulse. So you see -- there are some OLD tach designs that used the pulse to power them. The reasons modern tachs have three wires (one of which is +12 volts) is that the system is more sophisticated and therefore more accurate.

 

[eavega]

Re: Correct wiring for this Tachometer

AHA! See, my assumptions weren't wrong! They were just "Old School"

-E

PS. My tach works like a champ!

 

[Stratosman86]

Re: Correct wiring for this Tachometer

Seabob, here are the pics:

Darren