# Automobile Tachometer on an Outboard

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## Introduction: Automobile Tachometer on an Outboard

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.

## Recommendations

• ### Rockets Class

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• ### Casting Contest

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## 3 Questions

0

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

0

Awesome!!!!

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

0

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.

1

How did u find the original resistor?

0

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