Automobile Tachometer on an Outboard





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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    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 beats buying a $150 jr dragster tach for my race mowers.when i would rather spend less than $50

    how did you mesure how many ppr the engine puts out?

    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.

    It may be possible to test this with a timing light but I would not be able to suggest a procedure.

    *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?