How to "wire" a new tachometer to the alternator

**bigoledave** · 05-16-2022, 04:28 PM

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

**joe_db** · 05-16-2022, 07:04 PM

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.

**bigoledave** · 05-19-2022, 05:13 PM

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/ref=sspa_dk_detail_2?pd_rd_i=B07NYD3CPK&pd_rd_w=3dld9&pf_rd_p=ed02e65e-bf3f-46c8-aedd-152f4fca56ac&pd_rd_wg=dmgyX&pf_rd_r=MTZFM0M2T2WPPC749JFV&pd_rd_r=4fd31693-f752-4ea5-946e-0d1e4d227cd3&s=industrial&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEzQVIzRkZVQU0wTVZFJmVuY3J5cHRlZElkPUExMDQ0MTU4MkUwV09NMDZDT040SSZlbmNyeXB0ZWRBZElkPUEwMTA2ODI4MTQ2Q1BLWEZCUUYwWiZ3aWRnZXROYW1lPXNwX2RldGFpbCZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU&th=1

Comments on which one should work with this tach appreciated.

https://www.amazon.com/gp/product/B089Y16XTL/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Thanks

**W2ET** · 05-19-2022, 06:34 PM

Maybe you should ask Indigo?

Bill

**Dave Neptune** · 05-19-2022, 08:12 PM

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

**southcoasting** · 05-22-2022, 10:24 PM

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.

**bigoledave** · 05-22-2022, 11:24 PM

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!

**ndutton** · 05-23-2022, 12:27 AM

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.

***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.

Attached Files

**bigoledave** · 05-23-2022, 12:50 AM

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.

**ndutton** · 05-23-2022, 01:12 AM

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?