Instructables
Picture of 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.

 
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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

Picture of 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

Picture of 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.
L98TPI1 year ago
There are still a lot of older outboards out there (i have two) that use a distributor with one coil. If its a low tension that is non CD (really old) that does NOT use gap less, also known as surface gap plugs. Then you can hook up to the coil as shown in the tachs instructions. If its a CD box distributor combination (used up into the 80's) do not connect any thing to the coil. The voltage there is way high and you might ruin the tach and cause the motor to run badly. In this case you have to depend on the tach out wire that most of them have. In either case, for a four cylinder two stroke set the tach for eight since its a tach for a four stroke, and so on.
slickjazz794 years ago
cool stuff man!