Tach

Outboard designs up to 14'

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cusoak
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Location: Senecaville Ohio

Tach

Postby cusoak » Mon Aug 01, 2016 4:28 am

I want to get a tach for my 1961 Merc 45 hp motor.
Was wondering if you all could help me in deciding which one to buy.
I want one that is simple to hook up and use.
Any ideas.
Jeff

BillW
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Location: Seattle, WA

Re: Tach

Postby BillW » Mon Aug 01, 2016 11:26 am

Jeff,
I have two boats with vintage Merc motors. They each have Faria tachometers; they are cheap, look good, and seem to work well.

There are two wires from the stator down to the rectifier. Either of these, at the rectifier terminal ( in other words before the
rectifier) is where the sender wire attaches; then to the tach.

There is one slight difficulty here, and that is you must set the selector switch on the back of the tach to the appropriate number
of alternator (stator) poles. If that engine was overhauled, then the re-builder (maby you) will know if it is 8 poles or 12. Otherwise, you may have to find out by trial and error; I did that twice. Try 12 poles first and if it seems to be correct (5600 ish rpm) then it is correct. If it reads
a couple thousand rpm low (3600 ish) then it's wrong; change to 8 pole.

You have to get twelve volts somewhere. On one of my boats with an aftermarket ignition switch, it was easy to get it at the switch. On the other,
with ignition switch in the original control box, I chose to put an on-off switch on the dash. Run 12 volts to that, and hook your engine instruments
(tach, fuel, voltmeter) to that. The problem with this is that if I forget and leave that switch turned on, then the battery will run down.

A picture of the dashboard of my Squirt with these Faria instruments can be seen at the Boat Builders Blog, Bill & Linda's Squirt.

Bill

gdcarpenter
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Location: Raleigh, North Carolina

Re: Tach

Postby gdcarpenter » Mon Aug 01, 2016 4:36 pm

One technical difference for correct switching of tach as I understand it.

The 2 yellow wires coming from the stator to the recifier each carry current from only HALF of the windings in the stator.

Your 'pulse' tach should be set to half of the number of poles on your stator. (i.e. 12 windings set pulse to "6")

I beleive you have an 8 pole/winding stator so tach should be set for four (4) pulses.

Then you can dial in your prop!
This is my first, last and only boat build.

http://www.gdzipbuild.blogspot.com

BillW
Posts: 110
Joined: Sun Oct 05, 2008 8:03 am
Location: Seattle, WA

Re: Tach

Postby BillW » Mon Aug 01, 2016 5:50 pm

Well, you are correct; that's what is actually happening. However, in the case of a Faria brand tach, the selector switch
wants to know the number of poles of the stator. Notice this picture; if an 8-pole, then you select position 3 on the switch.

Probably other brands require dividing in half, as you say.

An expert could look up under there with a flashlight and tell which stator it is. But not me.
RSCN5001.JPG

cusoak
Posts: 928
Joined: Sat Dec 26, 2009 8:49 am
Location: Senecaville Ohio

Re: Tach

Postby cusoak » Tue Aug 02, 2016 5:39 am

When I bought my doner boat it was a 1969 Glastron. It originally was set up with a 70 hp merc.
I took all the gauges out and have a tach,
Here is a picture of the back and front I would like to use it if you think it would work.
Jeff
Attachments
tach a.jpg
0802160819b.jpg

BillW
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Location: Seattle, WA

Re: Tach

Postby BillW » Tue Aug 02, 2016 7:35 am

This one is exactly like gdcarpenter said; the switch is marked in pulses, not poles. Looks like you are now set to 4-pulse (8-pole).

That is quite likely correct for your Merc 400, but it could be 6-pulse (12-pole). I have one of each. Just don't exceed the range of the
instrument, if it's not correct the first time.

When the 12 volts is applied, the pointer should return to zero.

cusoak
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Joined: Sat Dec 26, 2009 8:49 am
Location: Senecaville Ohio

Re: Tach

Postby cusoak » Sat Aug 13, 2016 4:24 am

So that I am understanding the wiring correctly I need to splice into one of the yellow wires that come from the stater to the rectifier and run it to the tach and then add 12 volts also.
Is there any chance of shorting out anything after it is hooked up.
how about a drawing ?
Jeff

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Roberta
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Location: East Troy, Wisconsin

Re: Tach

Postby Roberta » Sat Aug 13, 2016 4:55 am

Stator wire to send, ignition energized wire to ign, ground to grd, and a light source to the post by the light.
Roberta "Queen of the Boat Builders"
Built Zip "Oliver IV", Super Spartan "Jimmy 70", and Torpedo "The Glen L".

BillW
Posts: 110
Joined: Sun Oct 05, 2008 8:03 am
Location: Seattle, WA

Re: Tach

Postby BillW » Sat Aug 13, 2016 5:49 pm

Don't "splice" into one of the yellow wires, but make your tach wire with a ring terminal and attach it under one of the yellow
wire screws at the rectifier.

Positive 12 volts, from either the ignition switch in your control box, or put an on-off switch dedicated to the tach, with +12 volts (red) from the batt
to that switch, then "ign" terminal on the tach.

"gnd" means negative side of the batt, or any other ground, like the engine block.

cusoak
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Joined: Sat Dec 26, 2009 8:49 am
Location: Senecaville Ohio

Re: Tach

Postby cusoak » Sat Aug 20, 2016 5:36 am

Any chance that I could screw up and burn out anything.
What should I be careful in doing.
Jeff

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Roberta
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Re: Tach

Postby Roberta » Sat Aug 20, 2016 5:56 am

I would disconnect the positive battery terminal while working on the system.
Roberta "Queen of the Boat Builders"
Built Zip "Oliver IV", Super Spartan "Jimmy 70", and Torpedo "The Glen L".

BillW
Posts: 110
Joined: Sun Oct 05, 2008 8:03 am
Location: Seattle, WA

Re: Tach

Postby BillW » Sat Aug 20, 2016 9:25 am

To clarify a little: usually the power (+12 volts) is taken from the ignition switch so that power is turned on and off with
starting and shutting down the engine; for that reason the tach terminal that receives the 12 volts is marked ign.

It is probably the white wire on the ignition switch, but to be sure, use a voltmeter to check. With ign on, is there 12 volts
(approx) between white and black (ground) terminals of ignition switch? With ign off is there none? If so, wire directly from
the ign switch white terminal to tach ign terminal.

This assumes you have the old "square" control box with key switch on the aft end of the box. If you have an aftermarket key
switch in the dashboard, the 12 volts is probably the purple wire. It will also turn on and off with the key. Again, check with voltmeter.

If you don't want to mess with this, put a marine type on-off switch in the dash next to the tach, run a wire from the battery
positive terminal to that switch, then to tach ign terminal. Turn that switch on before engine start. Be sure to turn it off upon
shutdown, or battery will run down, slowly.

I think we discussed the other two wires, sender and ground, in the previous post.

Bill

BillW
Posts: 110
Joined: Sun Oct 05, 2008 8:03 am
Location: Seattle, WA

Re: Tach

Postby BillW » Sat Aug 20, 2016 12:56 pm

I forgot to mention the most obvious and easiest way; must be old age setting in.

Do you already have other instruments in the dash, example fuel gauge or voltmeter, and do they turn on and off with
the ignition key?

If so, then wire from "ign" on the fuel gauge direct to "ign" on tach. You then have 12 volts to tach which comes on
with the key.

Then, the other two wires as we said before.

cusoak
Posts: 928
Joined: Sat Dec 26, 2009 8:49 am
Location: Senecaville Ohio

Re: Tach

Postby cusoak » Sat Aug 27, 2016 5:27 am

I do have a key switch in the dash that runs volt and fuel gauges. So getting 12 volts there would not be a problem.
I want to make sure that when wiring I do not do something that is going to short out what all ready charges my battery.

Jeff

gdcarpenter
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Joined: Tue May 05, 2009 12:18 pm
Location: Raleigh, North Carolina

Re: Tach

Postby gdcarpenter » Sat Aug 27, 2016 12:40 pm

Hi John,

You have received great way most instructions for hooking up your tach. I have mine connected to my Mark35A just as indicated and all works great.

The 'signal' wire connected to one of the the yellow stator leads at the rectifier has virtually no effect on the charging system. It just uses the 'pulses' of generated current/voltage to 'trigger' the tach.

Get the tach going and you can better evaluate your prop performance, though I suspect you are very close propwise.
This is my first, last and only boat build.

http://www.gdzipbuild.blogspot.com


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