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  #1   IP: 24.19.226.68
Old 05-14-2022, 11:25 PM
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Angry How to "wire" a new tachometer to the alternator

Greetings,

I purchased this tachometer and have it mounted..( looks awesome!).

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

In the instruction sheet, they show one wire going to the "W" terminal
on your alternator.. This method I now hear is not as good as one which
connects to the coil. So be it...I want to get this one operable and
"programmed " to read the correct RPM.

I am at a loss as to where this "W" connection is made on (or in?) my
alternator....any help is most appreciated.

Cheers!
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Old 05-14-2022, 11:50 PM
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It connects to an AC tap in the alternator. It's my sincere recommendation that a good alternator shop install the AC tap for you with an eye on maintaining your ignition proof integrity. After that is done, the alternator reinstalled and the tach wire attached to the AC tap, you'll need a digital photo sensor tachometer tool (here's the one I have ) to calibrate your new tach.
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Old 05-15-2022, 12:35 AM
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It is way cheaper and less effort to just buy a tach intended to connect to the coil
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Old 05-15-2022, 02:05 AM
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85mm tach with sensor wire going to COIL?

Thanks for the comment on why not just get a tach that uses
COIL sensing.....NOT the alternator w wire.

Suggestions as to a tach that has that feature ....and is 85mm appreciated..thanks
....
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Old 05-15-2022, 08:53 AM
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Joe is correct that a tachometer designed for gas engines that gets its signal from the coil is cheaper and simpler (doesn't need an alternator AC tap installed and an optical tool for calibration) but alternator tachs are in pretty common use on most diesels and some gas engines, mine for example. Your opening post made it clear the decision to keep your tach had already been made.

Of course Moyer Marine, the host of this forum, offers coil triggered tachometers in two different sizes and two different finishes. It's one stop shopping.
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Old 05-15-2022, 09:23 AM
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The Moyer big tach (https://moyermarine.com/product/tach...ce-gaug_08_75/) is 85mm or close enough, 3-3/8" is 85.7mm, so I am pretty sure "85 mm" tachs are the same thing.
That IS the proven solution, if not the cheapest.
You can add an AC tap, that is how most diesels are set up.
You can try the tach anyway, it might work if you can find the right calibration number. AC tap, speed sensor, or coil, in the end it is all pulse-counting. Many of these come with both AC tap and coil settings.
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Old 05-15-2022, 09:26 AM
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https://www.moyermarineforum.com/for...ght=tachometer

Here you go, you can read about my adventures with a cheap Chinese tach. I did end up making it work.
I'll repeat to be fair to our host - The Moyer tach will just work, so it is up to you whether time or money is your most precious commodity.
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Old 05-15-2022, 10:11 AM
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To put it in perspective, both financial-and hassel-wise

My first AC tap modification cost me $20 at a respected marine alternator shop* and the optical tach tool I referenced earlier is $40 so for an additional $60 you can keep your tach if that is your preference. Looking ahead, a future alternator replacement will also require an AC tap and tach recalibration** but by this time you'll already own the tool to do it.

However - and it's exceedingly rare - a coil connected tach introduces an additional engine failure point. It has happened before. I found a mention of it here but gave up looking for the actual thread for greater detail.


*I have since learned how to install an AC tap myself.
**Alternator pulley size is the reason. There's no guaranty the pulley on a replacement alternator will be exactly the same.
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Old 05-15-2022, 12:57 PM
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I can't imagine any shop around here even looking at an alternator and telling me what color it is for $20, so you got a deal on that.
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Old 05-15-2022, 01:14 PM
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Quote:
Originally Posted by joe_db View Post
I can't imagine any shop around here even looking at an alternator and telling me what color it is for $20, so you got a deal on that.
They were a great shop with a long history with the Newport Beach boating community and Southern California boating industry. They retired several years ago and closed up shop.

Full disclosure: I was having other work done on the alternator (internal regulator replacement) so the alternator case was open anyway for the AC tap work.
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Old 05-15-2022, 04:22 PM
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Exclamation Latest thoughts on my cheapskate tach "issue"

Ok..thanks for all the comments. Latest thoughts:

1) Why do I need a tach anyway??? (I needed it because I removed an ancient
mechanical non-working knotmeter from my 29 foot 1965 Columbiia
and was tired of seeing the ugly hole. Did I never want for a tach ?...hmmm
I can hear the speed close enough really. So..did I install a nice looking
hole plu? MAYBE!

2) Reading all the customer comments on the "cheeptac" I see some responses
from the company saying that really...you should BUY andl install a
30$ sensor. Yeah... maybe hack an install on the access plate to the
starter/flywheel interface. Yuck.

3) In the ads for these "cheeptacs"" they say that they need is a reliable
pulse stream (duhh). I have an Indigo electronic ignition. So...there
must be a way to interface with the signal from the opto sensor in the
distributor I would think. Interesting...

Again, being retired I am able to expend huge amounts of time fiddling
with unimportant things on my Moonduster .......which probably saves me maybe.....about $13 a year.
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Old 05-15-2022, 05:05 PM
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This is my tach:
https://www.ebay.com/itm/85mm-Marine...8AAOSwX0VdQ98H
$29.95

Last edited by joe_db; 05-16-2022 at 07:16 AM.
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Old 05-15-2022, 05:35 PM
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I will try the 5 k resistor on my coil...why not?

Thanks for the reply and the link to your older posts. I just ordered some resistors per your older message and have nothing to lose trying this test.

Clip wire test:

"Sense" tach wire to 5k resistor.....then to NEGATIVE coil
terminal.

Nomal 12 input connect.

Start engine and "play"
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Old 05-15-2022, 06:38 PM
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Indigo ignition----a good sourcem of tach pulses

I have this Indigo ignition system on my engine... and to solve
my issue of trying to get a good pulse train sent to my new tachometer,.
I am very interested in to the idea of "sharing" that opto sensor
data.

I am calling Indigo tomorrow to discuss....no reason it should not work.

Wondering about the "programming" if I go this route but the frequency
wouldbe tha same as sensing the coil ( which I hear is a good source yet
is prone to having messy pulse shaoe issues. The opto sensor on the
Indigo shouls put out a very clear pulse train.



Comments appreciated of course...
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Old 05-16-2022, 07:15 AM
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The original Indigo ignition works like points, the tach should work as normal with maybe a resistor inline.
If you have the new Indigo adaptive dwell ignition, I think it has a multi-spark feature that may confuse tachometers.
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Old 05-16-2022, 04:28 PM
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Options for attempting to use the Indigo for tach sensing

Great.....

Thank you. I bought my Indigo at least ten years ago. I
conclude it is the "old" design circuiotry.

So...I think I have four.....or more wiring choices.

1) Physically splice in to the "photo interrupter" SENSE wire in the Indigo
harness and connect that to the desired "W" input on the tach,

2) Same as above....yet put that sense wire thru a 5k 1/2 watt resistor.

3) None of the above.....just connect the coil negative terminal directly
to the "sense input" wire on the tach.

4) Same as #3...but splice in a 5k 1/2 watt resistor in the wire
going from the tach to the negative coil terminal.

Opinions welcomed of course!!

Thanks
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Old 05-16-2022, 07:04 PM
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Quote:
Originally Posted by bigoledave View Post
Great.....

Thank you. I bought my Indigo at least ten years ago. I
conclude it is the "old" design circuiotry.

So...I think I have four.....or more wiring choices.

1) Physically splice in to the "photo interrupter" SENSE wire in the Indigo
harness and connect that to the desired "W" input on the tach,

2) Same as above....yet put that sense wire thru a 5k 1/2 watt resistor.

3) None of the above.....just connect the coil negative terminal directly
to the "sense input" wire on the tach.

4) Same as #3...but splice in a 5k 1/2 watt resistor in the wire
going from the tach to the negative coil terminal.

Opinions welcomed of course!!

Thanks
I would pick #4.
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Old 05-19-2022, 05:13 PM
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Which exact sensor MAY work with the tach I have?

Ok.....

(I have the resistors now and will try a test this weekend to see if
I can just use the negative COIL terminal, with a 5k resistor.)

As a backup, I may want to purchase a "real" sensor to get RPM data
from my flywheel teeth ...i.e..sense teeth passing by this sensor which
MAY be easily mounted to the small inspection plate covering the
starter gear/flywheel interface. Before I buy one...Amazon wants
to know which EXACT sensor MODEL I want. Here are the options:

PNP NC
PNP NO
NPN NC
NPN NO


https://www.amazon.com/dp/B07NYD3CPK...NrPXRydWU&th=1

Comments on which one should work with this tach appreciated.

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
Thanks
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Old 05-19-2022, 06:34 PM
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Maybe you should ask Indigo?

Bill
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Old 05-19-2022, 08:12 PM
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I had an Indigo Ign and used a std Tach with a coil hookup. The pulse is still a pulse with most EI set ups.

One wire for tach power
one wire lite hook up how you wish
One wire for ground
And one wire to hook to the coil.

Just be sure you select a tach that will accommodate 4 cylinders, most will function with 4~6 or 8 cylinders via a switch on the back of the tach.

The hard part is the hole in the boat for the tach.

Dave Neptune
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Old 05-22-2022, 10:24 PM
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I had played around with different tachs. Even went and I had an alternator shop install a tap wire. In the end, the tach I ended up with that works great is the Isspro. I wanted the hour meter to keep track of my newly rebuilt engine and I did not want to install a separate gauge.

I spent a good amount of money on my Isspro (close to $200 I think) but it works great, is accurate, and it is reputable so should last me a long time (I hope).

It was easy to install and connects to coil like the one Moyer sells. Only difference between this one and the one sold here is the additional hour meter.
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Old 05-22-2022, 11:24 PM
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I like the look of my new tach...but..

Thinking of just giving up on getting my new "hole plug" to actually work..

Yep.....looks nice...but....I have had Moonduster since 1989 and never "needed"
a tach....see my old posts about replacing a cruddy old "speedometer".

I tried the idea of connecting the negative coil terminal to a 5k resistor
and then to the "tach sense" wire......and it did not work at all. I then connected it direct and still no change,

Sooooo, do I buy a "tooth sensor" and start hacking on my flywheel cover(s).
NO.

Do I buy a new alternator with a dedicated "w" wire?
NO

***Will someone post a thread on how to get an "W" tach sense output
from my 34 year old (at least!!) A4 tach? No luck so far!


Do I buy a whole new tach system?
NO

I don't REALLY need a tach. Am I driving a Porsche on a windy mountain road?...or am I putting around Puget Sound when the wind dies? Hmmmm

I will sleep on it!
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Old 05-23-2022, 12:27 AM
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I don't mean to beat you up when you're down but . . .

In your opening post you said the tachometer instructions called for an alternator connection yet thus far you've tried everything BUT what the instructions said to do.

Quote:
***Will someone post a thread on how to get a "W" tach sense output from my 34 year old (at least!!) A4 tach?
No luck so far!
It seems to me that was covered pretty clearly in post #2. Have you even asked an alternator shop what an AC tap would cost? My tachometer was clearly identified for alternator connection (picture) when I bought it and I figured if the diesel guys could do it, it couldn't be that tough.

. . . and it wasn't.
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Old 05-23-2022, 12:50 AM
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next step..

thanks......the instructions for my tach just show either using a "w" wire from the alternator or a "pulse sensor". Zero details beyond that.

I will try to contact an "alternator shop" tomorrow.

Thanks for your input.
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Old 05-23-2022, 01:12 AM
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Hey wait, not so fast. Take a closer look at your Motorola alternator. If it's like the one pictured in this post it already has an AC tap. Look carefully at the arrow pointing to a post at the 2 O'clock position, clearly marked 'AC Tap.'

edit: or am I incorrect that you have a Motorola?
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