How to "wire" a new tachometer to the alternator

[bigoledave]

Greetings,

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

https://www.amazon.com/gp/product/B089Y16XTL/ref=ppx_yo_dt_b_search_asin_title?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!

[ndutton]

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.

[joe_db]

It is way cheaper and less effort to just buy a tach intended to connect to the coil

[bigoledave]

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
....

[ndutton]

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.

[joe_db]

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.

[joe_db]

https://www.moyermarineforum.com/forums/showthread.php?t=11410&highlight=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.

[ndutton]

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.

[joe_db]

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.

[ndutton]

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.

[bigoledave]

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.

[joe_db]

This is my tach:

https://www.ebay.com/itm/85mm-Marine-RPM-Tachometer-Gauge-LCD-Tacho-Digital-Hour-Meter-Backlight-3000-RPM/293174849810?hash=item4442953112:g:~C8AAOSwX0VdQ98H

$29.95

[bigoledave]

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"

[bigoledave]

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...

[joe_db]

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.