Automotive tach on an outboard

[kyleg001]

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.

This motor has a 12 pole stator, creating 6 pulses per revolution.

I purchased a cheap $30 Sunpro tach from Autozone that has a selection for
4, 6, 8 cylinder motors.

Unlike outboards, which run off of the stator, these 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.

I disassembled the tach, 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.

I hope this is helpful and if you have any question let me know.

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

*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.

 

[Silvertip]

Re: Automotive tach on an outboard

You got lucky as the ability of the IC and the input circuit to it do not always recognize the very different (but same frequency) waveform. You also did not save 50 bucks by buying an auto tach since it won't last in a marine environment since it is not sealed. You also didn't have to buy a Johnson/Evinrude specific tach. Had you searched and bought a generic outboard tach you would have not have had to make any changes, had a tach that will last, and it would have cost you only a few bucks more than the original 50 dollars.

 

[kyleg001]

Re: Automotive tach on an outboard

You got lucky as the ability of the IC and the input circuit to it do not always recognize the very different (but same frequency) waveform.

Your are correct about the input circuit, I did get lucky, however I have found that allot of universal automobile tach ICs are able to receive a wide range of inputs. I found a cheap one and wanted to let everyone know.

You also did not save 50 bucks by buying an auto tach since it won't last in a marine environment since it is not sealed.

The tach is sealed , came with a good seal between the plastic casing and the front bezel, only have to plug the small hole in the back were the wires come out.

You also didn't have to buy a Johnson/Evinrude specific tach.

I understand that I don't need a Johnson/Evinrude tach.

Had you searched and bought a generic outboard tach you would have not have had to make any changes, had a tach that will last, and it would have cost you only a few bucks more than the original 50 dollars.

I purchased this because I needed a tach for tuning the engine, and I needed it right away. Because there are not really any boat supply stores near me that would carry a tach I would need to order one online, with around a week for shipping

The cost of the tach and resistor was $30.10. I said possibly save you $50 because I averaged the cost of most tachs, that I liked and would work with my motor(approx $78)

I also did this because I love electronics, have seen many posting of people asking if it is possible to use an automobile tach on an outboard, and posts about the high cost of marine tachs, sorry if my previous post came off as being rude.

Cheers,
Kyle

 

[ronsealdeath]

Re: Automotive tach on an outboard

Just wanted to let you know that I followed your procedure and it worked a treat. The cheapest outboard tacho I can get in the UK is about ?40 or ?50. I bought a universal auto tacho for ?12 and some resistors for 20p.

Replaced the resistor for the 6 cyl position and put it all back together. It seems to be accurate but I have no way of testing! My outboard is also 12 pole Evinrude so I assume if yours maths was ok then it should be fine.

Unlike the other poster I think your guide was really valuable and thanks for giving me something to do for a few hours this afternoon! I dont care that its not a posh outboard make that is sealed etc... It was fun to do!

Cheers
Al

 

[Silvertip]

Re: Automotive tach on an outboard

A 10 second search on E-bay revealed a host of outboard tachs for $49 - $58. So much for average $78 cost.

or these

http://greatlakesskipper.com/produc...ria-custom-outboard-only-boat-tachometer.html

or this one for $39

http://greatlakesskipper.com/produc...-6000-rpm-outboard-tachometer-boat-gauge.html

 

[kyleg001]

Re: Automotive tach on an outboard

Let me reiterate, $78 is an AVERAGE cost of a gauge.

Average ? def: the quotient of the differences between the initial and final values of the two quantities that make up the ratio.