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.

Check with your local dealer.

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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    3 Questions

    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?

    Thanks for this article

    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.

    How did u find the original resistor?

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

    Comments

    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