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Author Topic:   Tachometer Problem with Change of Motor
Larrydl posted 05-11-2006 07:20 AM ET (US)   Profile for Larrydl   Send Email to Larrydl  
My two-year-old old boat came with a 70-HP Johnson two-stroke. The Johnson tachometer worked perfectly. I had a local boat dealer exchange the 70-HP for a 90-HP four-stroke Yamaha. Now the original tachometer works fine up to 3,000 RPM and sticks there. It won't go any higher.

The Yamaha boat dealertold me that I will need a new 12-pole tachometer, but they don't sell them. Weird, heh? The parts guy suggested I go to eBAY to get one. Please tell me what that 12 pole means. I am not an electrician.

On the back of the Johnson tachometer, there is an 8-pin wiring [connector]. The tachometers that I see for sale on eBAY do not have that 8-pin connector. Please tell me how hard this exchange [may] be. Doing the exchange of the tachometer looks easy. I just worry about the electrical hook ups.

Thanks, Larry from Delaware

seahorse posted 05-11-2006 07:53 AM ET (US)     Profile for seahorse  Send Email to seahorse     
You have the BRP System Check tach with the 4 warning lites. The following is the wiring diagram to make it a tach only. Check the switch on the backside to make sure it is in the #6 position and not #5.

On a Yamaha, yellow wire is 12V+, green is the tach signal, and of course black is ground.

System Check Gauge Wiring

If you just want to use the tachometer function of the gauge, here's the pin configuration (looking at the back-side of the gauge, holding it upright):


Purple wire goes to pin 1- 12V
Black wire goes to pin 2- grnd
Gray wire goes to pin 3 - tach

Grounding these pins, will activate the following:

Pin 4, turn on no oil lite
Pin 5, turn on low oil lite
Pin 7, turn on check engine lite
Pin 6, turn on hot lite

Pin 8, is used to activate the warning horn, it supplies a ground to the horn circuit when any other warning circuit (pins 4 thru 7) is grounded

Larrydl posted 05-11-2006 02:14 PM ET (US)     Profile for Larrydl  Send Email to Larrydl     
Thank you for your reply, but I took your advise
and looked on back of my tach.
There are no adjustments.
Just 2 wires that plug in back of tach, with
a 8 wire harness that comes from motor.

Do you think I should buy a new tach with adjustment
for 12 poles.?? What does 12 poles mean?

jimh posted 05-11-2006 04:07 PM ET (US)     Profile for jimh  Send Email to jimh     
The number of poles refers to the number of coils and magnets in the alternator.
Larrydl posted 05-11-2006 04:33 PM ET (US)     Profile for Larrydl  Send Email to Larrydl     
Thank you Jim..
Any tips on wiring up a new tach.
I probably need to change to a new one,
but I don't think the wires will match up.
I have a volt meter.
How would I know which wires go where?
Chuck Tribolet posted 05-11-2006 06:06 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
tach purple to motor yellow (+12 when ignition is one)
tach black to engine black (ground)
tach grey to engine green (tach signal).

Check that it's wired that way now, and that the switch
is set correctly.

If you need to replace the tach, the replacement tach will
have instructions that tell you which wire is which.


Larrydl posted 05-11-2006 06:16 PM ET (US)     Profile for Larrydl  Send Email to Larrydl     
Thank you Chuck.
So, I better not buy a cheapie on Ebay that does not
have instructions.
Chuck Tribolet posted 05-11-2006 09:21 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Check the manufacturer's website -- they may have PDF files of
the instructions.


jimh posted 05-11-2006 11:40 PM ET (US)     Profile for jimh  Send Email to jimh     
On most outboard motors the tachometer signal is developed from some mechanical rotation of the flywheel, and therefore there is no correlation between the number of power strokes (two-stroke versus four-stroke) and the tachometer signal. The tachometer has to be calibrated for the number of pulses per revolution that are developed by the mechanical rotating device triggering the tachometer. For most outboards it comes from the charging circuit.

The charging circuit generally consists of a single coil winding (stator) which is distributed among a multiple pole or field-piece assembly. Magnets are embedded in the flywheel and rotate around the fixed coil assembly. The magnets are arranged with a N-pole and S-pole pair.

Imagine that there were only a single pair of N-S magnets in the flywheel. This pair would rotate over the stator and induce a pulse as it passed by each pole or field-piece of the stator. If the stator had ten poles, there would be ten pulses per revolution. Typically there are multiple pairs of magnets in the flywheel, spaced in such a way that they correspond to the number of poles in the stator. In this way, each magnet pair is exciting one pole. This has the effect of making the voltage and current in the coil winding stronger, but it keeps the frequency the same as if there were just a single magnet rotating around and exciting the poles.

On smaller engines there is a limit to the number of poles and magnets due to the smaller size of the flywheel. On larger engines (more horsepower) the flywheel is larger and there is room for more coil poles and magnets.

Larrydl posted 05-12-2006 09:39 AM ET (US)     Profile for Larrydl  Send Email to Larrydl     
You sure do know your stuff.
I am a retired Protocol Officer from the White house
and retired from US Army.
I have had many boats, but I never had problems with
a Tach.
My boat dealer keeps on insisting that I can change
this Tach myself and they do not sell Tachs.
They assume everyone is an electrician and a mechanic.
Well, because all of the help that I got here,
i think I can do this myself.
Of course it is very, very easy to take out old one
and put in a new one.
I had a 70 hp 2 stroke and now a 90 hp 4 stroke.
By what you told me, I can't understand why my old
tach is no longer working. HPs are close.
Weird how my tach gets to 3000 rpms and just gets
stuck there until I throttle down.
The needle moves smoothly under 3000 rpm.
My gauges are off white and it looks like most tachs
have a black face.

Well Jim, thank you again.

JAMES NORMAN posted 06-04-2008 05:20 PM ET (US)     Profile for JAMES NORMAN  Send Email to JAMES NORMAN     
I just had 1998 70 hp 4 stroke worked on. The vapor seperator was replaced. When I crank it the RPM guage doesn't work. What could they have not connected and where?

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