Tachometer Question

[Jim Hawkins]

I want to hook up a tach to my 1987 Johnson 30HP. I don't want to mess with another disposable solid state tach and don't want to spend a lot. I'm looking at this on ebay,

http://www.ebay.com/itm/281703748236?item=281703748236&viewitem=&vxp=mtr

It says it works with most 4 cylinders but mine is 2 cylinder. Does it really matter as long as I have that grey signal wire coming off the motor?

 

[joeanna]

I think this is their website; http://www.vdo.com/media/182623/flc_marine_brochure_viewline_en_2014.pdf
page 6 is the newer tachs. Hope that helps.

 

[Jim Hawkins]

Thanks but I didn't find anything there to answer my question.

 

[MH Hawker]

yes it matters you have to set the pole for it to work right

 

[Jim Hawkins]

Thanks MH, do you know how many poles a 1987 30HP Johnson has? I assume that's to do with the alternator not cylinders, right?

 

[MH Hawker]

no idea on it, the motor manual should tell that

 

[GA_Boater]

Jim - I'm not completely sure that tach will work on an outboard, but this one will - http://www.ebay.com/itm/Sierra-Mari...ash=item1c68529ea0:g:-I0AAOSwmtJXV4g7&vxp=mtr

 

[GA_Boater]

Your pole question - 10 poles;

 

[spoolin01]

I looking for the same thing long ago, I recall there were aftermarket analog tachs with multiple pole settings.

 

[Jim Hawkins]

Thanks guys, and GA_Boater, you are indeed a Supreme Mariner. I'm going to capitulate, call iboats tomorrow and let them tell me which of their fine products to purchase.
BUT, meanwhile I sure would like some more education on this. What exactly are the poles? I assumed it was like the coil pickup on the alternator but 10 seems like an awful lot for my little motor. Sure would like to learn something from all this.

 

[Silvertip]

Here's the long answer: An outboard tachometer counts the number of pulses being produced by the stator on the engine. Those pulses (not poles) are converted to an RPM value because the number of pulses is directly related to engine rpm. A 10 POLE stator has 10 poles, each of which has copper windings on it. As the magnets in the flywheel pass by each pole electrical pulses are created. Since the pulses are essentially a modified AC waveform, the pulse looks like a sine wave going positive for 1/4 of cycle, then going back to zero for 1/4 cycle, then negative for 1/4 cycle then back to zero for 1/4 cycle. So the tachometer should have a setting on the back that allows a single tach to work on engines with various stator designs that may have 8 - 10 - 12 or more poles. The relationship between POLES and PULSES is 2. Your 10 pole stator produces 5 pulses for each engine revolution. Therefore the setting on the tach should be "5". Because there are so many different engine designs such a 1, 2, 3, 4, 6, and 8 cylinder engines, battery and magneto ignitions, and two and four stroke designs, it is far simpler to design a tach to work from the stator since the number of cylinders and engine design are then not a factor.

 

[Jim Hawkins]

Thank you Silvertip, I understand perfectly. I once built a generator for a windmill and wound all the coils and cast the stator myself and understand the setup. I just had no idea my motor had 10 poles. I thought motors like this usually only had 1 coil and 1 magnet. Thanks for teaching me something.

And today I called the fine folks at iboats and ordered,

http://www.iboats.com/Eclipse-Tacho...063675--session_id.741806039--view_id.1161431

 

[GA_Boater]

Thanks for supporting iboats and the forum.

To add a picture to what Silvertip said, it's easy to tell how many poles a stator has. I think this one is for your motor. It has 8 copper wound poles and 2 white ones for 10 poles in total. The white ones are dual purpose - to generate AC with the other 8 and as a trigger for ignition spark timing as the 1 coil and 1 magnet you mentioned. You have 2 cylinders firing 180 degrees apart, so 2 trigger coils 180 degrees apart. The same basic stator is used on motors with more than 2 cylinders with added timing circuits.

Not much shorter than Silvertips, but isn't the pic is worth 1000 words? :smile.

 

[Jim Hawkins]

Supreme Mariners, the lot of you!