# Automobile Tachometer on an Outboard

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You've probably clicked on this link to tell me that you can't do it.

I will say that you can, and it'll possibly save you \$50

I didn't want to have to order a tach for my motor, a 1973 65hp 3cylinder Evinrude.

## Step 1: Find Out How Many Pole or Pulses You Motor Gives Off Through the Tach

My motor has a 12 pole stator, creating 6 pulses per revolution.

Unlike outboards, which run off of the stator, Automobile tachs run of the distributor on older vehicles. On automobiles, 4 cylinders send 2 pulses, 6 cylinders send 3 pulses, and 8 cylinders send 4 pulses per rotation.

## Step 2: Buy and Disasemble the Tach

I purchased a cheap \$30 Sunpro tach from Autozone that has a selection for
4, 6, 8 cylinder motors.

I disassembled the tach, by removing 2 screw from the back and poping off the front beezel with a flat head screwdriver

## Step 3: Remove and Replace Resistor With Proper Rating

the IC in the tach takes a pulse reading in relation to the resistance of its control pin.

That being said I replaced the 10 cent resistor for the 6 cylinder position to 147k ohms(2 resistors, 1- 47K and 1- 100K) causing the gauge to read 6 pulses a revolution, and giving me an accurate gauge reading.

1Pulse = 623.3K
2P = 382K
3P = 258.7K
4P = 195.7K
5P = 163.5K
6P = 147.06K
7P = 138.6K
8P = 134.4K
9P = 132.24K
10P = 131.14K

## Step 4: Completion

I hope this is helpful and if you have any question let me know.

*note: 6P was tested with an oscilloscope, 1, 5, are calculations only, 7-10 are rough calculations. RPM accuracy is based on you doing your own math and the accuracy of your resistor/s

you might want to use a variable resistor to fine tune.

1/4 watt sould be fine, but use 1/2 watt resistors if you want.

Let me know what you think.

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## 26 Discussions

Yes, I took my tach apart and it does not have a resistor like the one in your photo. I do like how u have installed the resistors on the back of the motherboard. Maybe the resistor on mine is a small one that looks like a chip or a square

Awesome!!!!

I am gonna try this. Which parts are the resistors and are the poles on the stator the coils where the magnet circulates?

The resistor(s) can be seen in the image on the tach. If it is an outboard boat motor there should be a tach wire coming from the engine electronics. I would check the engine manual.

It was removed from the board and tested with a multimeter. Also it had striping to represent its rating.

Excuse me sir i have similiar tach with 4,6,8 cylinder option but my motorcycle is single cylinder with 12 pole stator. What kind of modification can i do to accurate the tach sir? Thanks for the help

Hello, iv got a question for you. I have an old 2 stroke aircraft engine that I rebuilt that has four magnets, so 4 poles=2 pulses? It is a 12v system, could a tach from a car be used without any modification? Or could I use a boat tach with the 4 cyl / 2 pulse setting?

i used this on a small engine application twice so far it works great and its pretty accurate.it beats buying a \$150 jr dragster tach for my race mowers.when i would rather spend less than \$50

You could also get a rough idea on the number of ppr by hooking up an oscilloscope to your tachometer wire and doing some calculation with your estimated idle rpms. For example if you measure a 60hz freq. from the tach wire at idle and estimate that the idle rpms are 600 than it can be assumed that you have a 12 pole stator which is 6 ppr. Hope this helps.

i did som researching online and found out that my stator has 4 poles does that mean 2 pulses?

The one in that link states six poles. If that's the case, this would have 3 pulses per rotation. If I remember correctly you should be able to switch the tachometer to the 6 cylinder position and it should be fine without having to make any internal adjustments.

I would suggest trying to find a manual on your engine that informs you of the number of poles in your stator. Your ppr should be half of that.

*note: 6P was tested with an oscilloscope, 1, 5, are calculations only, 7-10 are rough calculations. RPM accuracy is based on you doing your own math and the accuracy of your resistor/s ?

didnt you use a oscilloscope?

The oscilloscope was used to verify the correct reading on the tachometer with a know ppm. It was used to adjust a frequency generator to set a known 600 pulses a minute reference to mimic a 6ppr system at 100 rpms. The generator was hooked to the tachometer to verify its operation with a reading of 100 rpms. Another test was conducted under readings for 200 rpms A formula, that I no longer have, was then used the calculate the required resistance for other pulse systems.

i have the bigger sunpro tach it looks diff in side then the little one that you used so how do i know what resistor to change