Tach Question

[fish2day]

I'm replacing the Tach on a 1991 Johnson 200hp. I have a good OMC Tach off another boat but instead of having a switch to move from 4, 6, and 8 cylinders, It has 3 brass screws with the cylinder #s above each screw. What connects to the screw? Jumper wire ?? from the pulse?? Any help appreciated.

 

[Silvertip]

Re: Tach Question

You probably have a tach from an I/O which uses an automobile derived engine. Tachs for those applications are "number of cylinder" dependent and the signal comes from the ignition coil. Tachs for outboard engines are not cylinder dependent because they get their signal from the charging system (a 12 pole/6 pulse alternator). So the 4-6-8 cylinder is not applicable for your outboard.

That said, you can try connecting the PULSE line from your engine (the wire that was on the "S" terminal on the old tach) to the 6 cylinder terminal. That however will cause the tach to read double what the engine is actually turning. A 6 cylinder 4-stroke (car engine) produces 3 pulses per engine revolution, a 4 cylinder = 2 and an 8 cylinder = 4. Your 6 cylinder outboard (actually any number of cylinders) produces 6 pulses per revolution so the tach will read double. You can also use the 8 cylinder to cut the error but the real solution is to use the correct tach. For this reason outboard tachs are very generi but some also have provisions for 4-stroke engines.

 

[whistler66]

Re: Tach Question

Silvertip, I'm reading this with interest as your post always are informative! Are all outboard engines regardless of whether they are 2 or 4 stroke, age or brand 12pole 6 pulse engines? If someone had an outboard tach with switchable modes would he switch to 12 for the poles or 6 for the pulse? Hope this don't mess up fish2days question. If so let me know an I'll start a new thread.

 

[Silvertip]

Re: Tach Question

I cannot speak for "all outboards" and "all four strokes" so my comment applies to using an auto style tach on a two stroke outboard. You need to know how many poles the alternator has and divide that number by two to get the pulses. Some Force engines had a 15 pole alternator that took a special tach. Some Mercs had 10 pole alternators and would be set to 5 pulse.

 

[NYBo]

Re: Tach Question

Actually, most Force outboards have 20-pole stators.

 

[Silvertip]

Re: Tach Question

Yup -- brain phart.

 

[whistler66]

Re: Tach Question

Is there a way to determine the number of poles an alternator has by looking or ???? I have a 2010 F-50 Yamaha 4 stroke and I'm concerned about my older OMC tach settings. Thanks in advance,

 

[Silvertip]

Re: Tach Question

You can't tell how many poles an alternator has unless you pull the flywheel. The easy way to do it is to call a dealer and ask "how many poles" or what should the pulse setting be. You will not harm anything with an incorrect setting. The tach counts pulses and translates that to an RPM indication. The worst that will happen is the reading is inaccurate. But chances are you have a 12 pole alternator. The other method is to use a shop tach to verify the operation of the installed tach.

 

[whistler66]

Re: Tach Question

You're a good man Silvertip! Thanks, You gave me something to look at and check on!