1975 Johnson 115 HP tach settings

[lawyertob]

I just bought a bunch of used gauges for my 115ETL75E and now I am trying to figure out what settings the tach needs. The tach has the settings up to 2, 3, 4, 5, and 6 pole, but I have no idea how many poles, my motor has. Well, actually, I looked in the manual and think it might be 8 poles, which would be just my luck.<br /><br />Anyway, I am hoping that one of the OMC gurus like Dhadley, OBJ or Joe Reeves can answer this off the tops of their head. But anyone else who knows the answer feel free to jump right in...but no guessing correct answers only! <br /><br />$0.01 for your thoughts,<br />Joe

 

[Trent]

Re: 1975 Johnson 115 HP tach settings

You have 12 poles and you need to set the tach to 6P. On some 115 models if the tach falls to zero above 4000 RPM's you will have to install a 2000 ohm 1/2 watt resistor connected from send to ground terminal.

 

[lawyertob]

Re: 1975 Johnson 115 HP tach settings

Trent,<br />Thanks for the reply. Okay, I did even more studying of my manual last night and it says that I have a 6 amp alternator with an 8 pole stator. Here is the diagram:<br />

<br /><br />The only place it says anything about the number of poles is in that one diagram.<br /><br />Now in the magneto diagrams it shows a bunch of poles and charge coils...does that have anything to do with it? <br />

<br /><br />I am generally mechanically inclined, but I have only been doing the boat thing for a short time. I have read repeatedly that these Johnson motors take a 6 pole setting, but I cannot figure out WHY. <br /><br />For that matter, I can't figure out why I set it to 6 if I have 12 poles. How does that affect the readings? And if I do have 8 poles do I set it to 4? Part of the confusion might be confusing poles with pulses...but I don't have a good handle on that yet.<br /><br />Oh well, at least I am smart enough to admit that I need help. But I am still stubborn enough that I have to know why the answer is what it is. <br /><br />Another $0.02 worth,<br />Joe

 

[Trent]

Re: 1975 Johnson 115 HP tach settings

Let me just referr you here... web page<br /><br />Maybe this web site will explain alot to you better than I can.?<br /><br />You seem to be a smart guy. And this info is pretty good. Just look around on the site.I think you might find what your really looking for? Trent

 

[lawyertob]

Re: 1975 Johnson 115 HP tach settings

Originally posted by Trent:<br /> Maybe this web site will explain a lot to you better than I can.?. . . . Just look around on the site.I think you might find what your really looking for? Trent

Trent,<br /><br /> Thanks for the link, I had been there before, but forgot about the basic theory links on the top left. Sure enough a couple of links later I got to this page tachometer pole / pulse settings which explained that it is the pulses per revolution not the poles that determine the setting. It also lists the poles and pulse settings for most engines.<br /><br /> Now, the only problem is that this site, and several others that I have looked at, all say I have 12 poles and 6 pulses....but my manual clearly says 8 poles and is silent on pulses, but I would assume 4.<br /><br /> Oh well, at least I know that my tach will work, now all I have to do is figure out what setting. That is still better than thinking that I didn't have the correct setting available.<br /><br />Thanks again,<br />Joe

 

[tvpear12]

Re: 1975 Johnson 115 HP tach settings

set your tach to 6p. this will be correct. no need to use a life line. The schooling is cheap, but the learning will cost you.

 

[lawyertob]

Re: 1975 Johnson 115 HP tach settings

Originally posted by tvpear12:<br /> set your tach to 6p. this will be correct. no need to use a life line. The schooling is cheap, but the learning will cost you.

TV,<br /> Here we go again WHY do I set if for 6P if I have an eight pole stator? Take a look up at the first pic I posted, it says 8 poles.<br /><br /> Now, I admit that I am a bit rusty on magneto theory, but I think I understand the whole pulses thing now. Don't get me wrong, I am not saying you are wrong, I just want to understand why you are right I guess it comes from my time in the Navy...we were taught to understand how it works...not just to believe that it works. This method can be burdensome in the beginning, but saves a lot of trouble in the long run. Well, I guess I will have to diagram the whole thing out and see how many pulses I get from one revolution...gets kinda complicated with all those magnets and poles, but hey that is half the fun. <br /><br />Thanks,<br />Joe

 

[lawyertob]

Re: 1975 Johnson 115 HP tach settings

Oh, one more thing. I did some stumbling around in the iBoats parts finder this morning and found out that you can get a new stator for almost every motor EXCEPT mine. Maybe this is another example of this motor being a weird year and different than everything else.<br /><br />Another $0.02 worth,<br />Joe

 

[gaugeguy]

Re: 1975 Johnson 115 HP tach settings

Tachometers count positive pulses. If a stator has 12 poles it has 6 positive and 6 negative poles (just like a magnet) but the tach is only concerned with the 6 positive pulses. My chart only goes back to 1977, so I can't give you a definite answer. I would put it in the 6p position and try it, if it is reading too slow change the setting to the 4p position, it won't harm the tach.

 

[lawyertob]

Re: 1975 Johnson 115 HP tach settings

Originally posted by gaugeguy:<br /> Tachometers count positive pulses. If a stator has 12 poles it has 6 positive and 6 negative poles (just like a magnet) but the tach is only concerned with the 6 positive pulses. My chart only goes back to 1977, so I can't give you a definite answer. I would put it in the 6p position and try it, if it is reading too slow change the setting to the 4p position, it won't harm the tach.

Gaugeguy,<br /><br /> Thanks for the reply. I didn't think it would hurt the tach, but it is nice to have confirmation. For safety's sake, I think I will set it for 4P. That way if I get to 5000 rpm and I have a lot of throttle left, then I can assume that I can switch to 6P. Otherwise, if I set it to 6P and it should be 4P, I could get into an overspeed situation and not know it. I am going to assume that the motor has the proper prop on it.<br /><br />Thanks again,<br />Joe