tachometer question for johnson outboard

[djwalters]

I have a question about tachometers for outboards. I have a 1977 johnson 140HP 4 cyl. that the tach quit working on and a 1984 johnson 70HP 3 cyl. that I want to add a tach to. Are all marine outboard tachs the same? I read somewhere that there an ajustable setting on the tach? I assume for different number of cylinders? If above not true, where do I start to get the info I need to see what tachs will work with my motors? What years / makes interchange? How they hook up? Thanks so much, Dave

 

[rickdb1boat]

Re: tachometer question for johnson outboard

Finally, some decent weather here in St. Louis! You need a Tach that is designed to work with a 12 pole alternator system. Most Tachs have a switch on the back to set it for the number of poles, as they can work with a number of different motors. Yours would require a Tach that is capable of being set to the "6" position on the back. Or a Tach that is 12 pole only(Some OMC tachs are only 12 pole). Here's some more reading:<br /><br /> web page <br /><br />Head to: Installation Instructions:<br /><br />Outboard Tachometers: <br /><br />Other Outboards w/Alternator

 

[djwalters]

Re: tachometer question for johnson outboard

I have a new tach that has the following on the back. "C" = Cyl. "P" = Pulses ----- 1st setting has 2P/4C next has 3P/6C next has 4P/8C then 5P and 6P. According to this, I don't believe it will work with my 3 cyl or 4 cyl if I need a "6" setting? This is a NEW tach. Maybe won't work with older motors? How do you convert poles to pulses? Thanks again, Dave

 

[djwalters]

Re: tachometer question for johnson outboard

Sorry, I commented before clicking on the link you provided. Now I believe it will work on both motors if I am reading the directions from Teleflex correctly? The 4C, 6C, and 8C evidently pertain to 4 cycle gas automotive engines. The 6P setting is the one I need. Thanks so much for your help, and yes, this was a dandy day here in St. Louis. Dave

 

[rickdb1boat]

Re: tachometer question for johnson outboard

If it has a setting for 6 pulse, it should work...

 

[jhebert]

Re: tachometer question for johnson outboard

The calibration of the tachometer is not related to the number of cylinders in the engine. The calibration is related to the number of poles in the alternator. It is not unusual for the same number of poles in an alternator to be used among engines of various number of cylinders.<br /><br />I try to explain in detail how electrical energy is produced by the permanent magnet alternator commonly found on outboard motors in<br /><br /> http://continuouswave.com/whaler/reference/PMA.html <br /><br />This will also explain the concept of number of poles in the alternator. I think you will find reading the article to be informative on this topic.