Automobile Tachometer on an outboard

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 3Remove and replace resistor with proper rating

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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 4Completion

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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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Cheap Tachometer for your bike

Caution .... LOOOOOONG

Some of you might remember my post a while back about being able to use a Car Tachometer on our bikes, since Honda for some reason does not put one on their Shadows.

Tubes_ Rock came up with a great theory of accomplishing this, although it was just that - a theory, which hadn't been tried.

He said, in essence, that one could try to trigger a Car Tach from both coils of a bike's V-twin engine by running a wire from each coil primary via an isolation diode to the Tach trigger wire. This way you'll get a true rpm reading since the tach is triggered by both coils, yet the coil primaries are isolated from each other by the diodes.

I bought a nice little Car Tach for $ 19.- (vs $ 150.- or more for a bike specific one) and thought I'd give it a shot.I hooked it all up but was hesitant to start the engine, because now both coil primaries are connected together, just isolated from each other by 2 diodes. I figured if a diode blows, there goes your ICM, since the coil primaries are connected to the ICM.

I pm'ed Tubes and he he said why don't you put a resistor in series with the diodes to reduce the chance of damage to the ICM in case of a catastrophic failure. Also the ICM coil primary outputs apparently have some diode protection of their own built right into the chip.

I felt more comfortable with that and and started the engine after inserting the resistors, hoping for the best.

Well, to make a long story short ---- it worked like a charm     

Tubes had suggested a resistor value of between 4.7K to 10 K and it worked beautifully with a 10 K resistor in series with the diode.Since I had been experimenting anyway I decided to increase the resistor value, giving even more protection to the ICM.I found out that a 100K resistor still works flawlessly and decided to go with that.

I incorporated the diode/resistor combo directly into the harness between the ignition coils and the Tach ( heatshrink works super).I then tested the whole thing on a nice hour's ride and it works great, looks great and makes my wallet grin from ear to ear.   

Bike Tach ...... $ 150.- and up

Car Tach ....... $ 20.- plus 80 cents for 2 diodes, I had resistors, wire, heatshrink laying around in my garage anyway.

Instructions are simple - run a wire from the front cylinder coil primary(neg terminal) to the anode(no band - pos end) of the diode and solder. Solder one side of a 100K resistor to the cathode(band - neg end) of the diode. When soldering a diode or any other semiconductor, I always clamp a heatsink(hemostat) between the component and the solder joint for protection.Repeat the same procedure for the rear cylinder coil.Solder both free ends of the resistors together and connect to the Tach trigger wire. Then of course you have to connect the Tach's ground, ilumination and switched positive for power according to the Tach's enclosed instructions.The diodes are 1N4004's ( 400 V PIV) which is more than enough to resist the kickback voltage of the primary coils.

This mod should work with most Tach's made for distributorless car ignitions.If it doesn't, you could try to lower the resistor value a bit.

For my Canadian friends I got the Tach on sale at Princess Auto (Powerfist brand ...read ... made in China)No matter - works A1