Tachometer question

[BayouRunner]

I have a 1990 MERCURY 115 ELPTO, Model number 1115412JD...when I took out the tach that was in the boat, a Performance Plus Magnum 150, it had 3 posts on the back. Can a 4 posts tach work somehow...and if so, how? Or does it have to be another 3 post? I wanna say the one that was originally in the boat, had a post for power, a post for ground and a post for the sending unit for the tach??? As soon as I find where I put it, I can get exactly what the posts are labeled as.

 

[Silvertip]

Re: Tachometer question

Those three terminals are correct for nearly every tach manufactured. A tach needs +12 volts when the key is on and that post will be labeled "+12V" or "I" (ignition). It also needs ground and will be labeled with a ground symbol or "-". It also needs a pulse signal from the engine and that post will be labeled "S" (send). Most tachs also have a fourth terminal for the internal light and it will be labeled "L". That terminal can be jumpered to the +12 volts terminal if you want the light on anytime the key is on. If not, you connect it to the "L" terminal on any other gauge. Same for the +12 volts and ground terminals (if there are any other gauges). What is more important with tachs is that it matches the stator on your engine. Tachs normally have a small dial on the back that has a number of settings. You outboard places 6 pulses on the send line for each engine revolution. Therefore that dial needs to be set to "6P". You cannot use an automotive tach on an outboard.

 

[BayouRunner]

Re: Tachometer question

Ya lost me at hello.

The symbols or markings on the back on the tach I took out had none of the markings you mentioned. That's probably not THAT big of a deal, but it needed to be pointed out. Whatcha lost me on was the dial on the back. I remember seeing a dial, but don't remember seeing any markings by it. And, if I'm not mistaken, there was the power line, the ground and 2 lines going to the one post marked tach or maybe it did say Send. There should only be 1 line to the Tach post, correct?? I have GIT to find that old tach so I can tell ya exactly what it has on the back. Lemme go look...hold on. HAHA!! I found it! Ok, on the back of the tach I took out the boat, there are 3 posts. One located at about the 10 o'clock position says BATT, so I'm guessing that's power. At about the 2 o'clock position, that post says GND, so I'm gonna guess that's the ground. And at the 6 o'clock position, that post says SENDER, and again, I assume that's the sending unit wire for the tach. I could SWEAR there was 2 wires on it. Maybe my dementia has set in early? What color would the wire to that post be, ifin you know? And I'm guessing any lights to other gauges would hook to the BATT post? The fuel gauge is the only 1 I'm worried about, and that has a light on it from a lil fuse panel to the side of the gauges. But to get the gauge to actually work, it was hooked to the back of the tach, I'm assuming to the BATT post.

 

[Silvertip]

Re: Tachometer question

BATT = +12 volts when the key is on. GND = Ground = Negative terminal of the battery. SND = SEND = tach signal from the engine. That wire is part of the big cable that runs from the engine up to the helm. It will likely have a bullet connector on the end. The light terminal on the gauges are normally jumped from one gauge to the next but the source of power is the NAV light switch. Some folks wire to the "A" terminal on the ignition switch but that means the gauge lights are on any time the key is on. The TACH or SEND terminal on the tachometer should have only the one wire on it. BATT or 12V terminals can have two or more. It just depends on how the boat is wired.

 

[BayouRunner]

Re: Tachometer question

Gotcha! It won't be to hard to figure out which wire goes to the SEND post, there's only 2 to choose from. Luckily I found one online (new) for $41!!!!

 

[BayouRunner]

Re: Tachometer question

Silver....I got my tach in!!! (It's a shame how the simpliest things make some people giddy with excitement...anyway). I'm a lil confugled. My tach that I got from Marine Trading Post outta N.Fort Myers Florida is a TuroTest 0 - 6000 rpm tach. On the back, at the top left side it has written TA-S-01 to the right of that is a gold/brass/bronze colored post, with nothing written by it. Under that, towards the left side it has written 1 = 12 Pole Alternator...2 = 4 Cylinder 4 cycle...3 = 6 Cylinder 4 cycle...4 = 8 Cylinder 4 cycle. To the left and a lil below that, it has a lil dial with 1,2,3,4 circling it. The "pointer" is towards the 2. Bottom middle is the 12v terminal...to the right of that is the S terminal..and to the right of that is an abstract looking Christmas thingy (I'm assuming that's the ground). You mentioned up there, in an earlier response to this thread that "You outboard places 6 pulses on the send line for each engine revolution. Therefore that dial needs to be set to "6P"." That's 1 area where I'm confugled. There is nothing on the tach that I have that says ANYTHING about 6p. Also, I got the power line and the line that works my gas gauge hooked to the 12v, which I know is working, cause when I turn the key, my gas gauge works. I have what I know is the ground, hooked to the abstract Christmas tree looking thingy. My 2nd confugledness is I don't know which wire is from the sending unit in the engine! I looked in the Clymer manual I have for my engine (I know, I know...prob not the best), and see nothing about the wire that would be leading to the instrument area what color it is or where it hooks on the engine. I'm thinking that wire might be bad???? If I can find it, I'll just run a new one and that'll be that, I hope. So, basically what I'm asking is...is my tach the one I need and set up correctly...where does the wire come from on the engine and/or what color is it....how do I test the line, other than having it hooked to the tach? Should it have some kinda current on it when the engine is running? I have a 1990 Mercury 115..model # OC247555.

 

[Frank Acampora]

Re: Tachometer question

You need to check on your with a Mercury dealer or over at the Mercury forum: A lot of Mercs (and all Force engines) used a 20 pole alternator and a special 20 pole tachometer is necessary for these engines. NO setting on your Turo-Test tach will work with this IF your engine is 20 pole. Sorry to be the bearer of possible bad news.

 

[Silvertip]

Re: Tachometer question

Your engine has a 12 POLE alternator which produces 6 PULSES on the tach send line so you select the #1 setting you described. An alternator outputs "alternating current" so if you were able to see the signal it would appear as a sine wave going positive for 90 degrees, then dropping to zero for 90 degrees, then going negative for 90 degrees and then back toward zero again for 90 degrees. Therefore those 12 pules (six positive and six negative) results in just 6 "complete" pulses. Yup -- long answer to a short question. But now you know. I don't know off-hand what color the tach send wire is on your engine but likely a gray wire.

 

[BayouRunner]

Re: Tachometer question

Wow...that is confusing! So basically what you're saying is I have the right tach, but change the switch position from 2 to 1? If the pointer is in the wrong position (mine was in the 2 position), would I get no reading on the tach? Or a skewed reading? Where does the send wire go on the engine? If by chance it's bad, I should just be able to run a new one..correct?

 

[Silvertip]

Re: Tachometer question

The tach would work but the reading would be incorrect when set to 2. The tach wire connects to the stator at the engine.

 

[BayouRunner]

Re: Tachometer question

THAT'S the 2 main things I needed to know. Unfortunately, it is currently raining in south Louisiana, so I can't get to the boat to see what's what. I'm thinking the wire from the stator is bad or something, cause even hooked to the #2 spot, I got no kinda reading at all, much less an incorrect one. I don't HAVE to have it fixed today, or by the weekend, I just want it to want it. I would like to have it by NEXT Saturday, I have a sac au lait (crappie to the non southerners) the 12th. The old tach I pulled out was working, albeit I think with misinformation, so I don't understand why the new one isn't. I'll letcha know what I find, as well as post some pics since I Vaselines the boat. TOTALLY different looking boat! (I know, a temporary fix, but a fix non the less). Tach, find my leak and I can somewhat relax and enjoy myself without worrying.