What kind of Tachometer will work

brooksville_rebel

Petty Officer 1st Class
Joined
Aug 12, 2011
Messages
240
Man its worse then getting my kids to eat vegetables to find a tachometer for my engine. I have emailed several places and either no answer or tell me I need to go to the boat place I bought it. I bought it off a little Egyptian lady that used it for tours up and down the river. Pretty dang sure she has no clue either. And many of the tachs I see say 4 stroke engines or 2 4 6 8 cylinder engines. Mine is 3. Then asks what pulse/poles.

I have a 1987 Mercury 3 cylinder 2 stroke 70 HP engine. SN 0B228735

I searched the online service manual (from this site http://boatinfo.no/lib/mercury/manuals/70-75-80-90-100-115.html#/0) and from what I could tell there's one gray wire that comes from the engine to the tach but I couldn't find that pulse/poles answer.
I could use some direction and help in finding one PLEASE.
Thanks.
 

brooksville_rebel

Petty Officer 1st Class
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Aug 12, 2011
Messages
240
Thanks very much. ! One thing I still don't get is some say 4 stroke engines , others say 2 4 etc cylinders. Some don't say anything at all.
So what do I need to make sure matches up ?If 3 cylinder is not mentioned do I go 4 cylinder ?
Thanks for the help. Appreciate it.
 

gm280

Supreme Mariner
Joined
Jun 26, 2011
Messages
14,585
As GA_Boater so kindly provided a nice chart, the number of cylinders has nothing to do with it. The pulses come from the stator. So most any 2 cycle marine tach will work. Just select the pulse number on the back of the tach and happy boating.
 

GA_Boater

Honorary Moderator Emeritus
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May 24, 2011
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49,038
4 stroke and 4/6/8 cylinder are settings for I/O's and inboards. The "P" settings are for outboards, like your motor is 12 pole stator which uses the 6P (pulse) setting.

Some tachs are universal and have selections for almost every possible motor, both auto-based and outboard.
 

Silvertip

Supreme Mariner
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Sep 22, 2003
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28,758
To carry this one more step, Tachometer manufacturers don't like to make more versions than they need to so counting pulses from the stator on an outboard means one tachometer with various pulse settings works nicely. A 1, 2, 3, 4, 6, or 8 cylinder outboards could all have 6 pulse stators so the number of cylinders is of no concern. 6 pulses would be produced with each engine rev regardless of the number of cylinders. I/O and inboard engines however have automotive style alternators so tachometers count pulses from the ignition system which is determined by the number of cylinders the engine has. Therefore a tach has a setting for the number of cylinders as well but it relates to four stroke auto type engines.
 

brooksville_rebel

Petty Officer 1st Class
Joined
Aug 12, 2011
Messages
240
Thanks. That helps. Now I will start one on my trim cuz that one has me totally baffled. Learning a lot from these boards and you gentlemen
 
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