Engine change and tach compatibility

[farmertoad]

I've been reading over this sight for the past few weeks trying to solve my own problems through the troubles of others and have learned quite a bit in a short time. I've also developed new questions because of the new knowledge. Talk of tachs has me wondering if my readings are bogus and the cause of my troubles. I've been trying to solve idle speed issues on my '84 125 force. My boat had a Johnson 115 that I roached when the VRO system failed. Bought the Force and installed it with the original tach. Now I'm wondering if I'm getting true readings????
Thanks for sharing your knowledge!

 

[Barnacle_Bill]

Re: Engine change and tach compatibility

Probably not as your Force has a 20 pole alternator and your Johnson had a 12 pole alternator. You need one of these: http://www.iboats.com/Tach_O_B_Chry...1494696--**********.894354262--view_id.134619

Umless there is a 10P setting on the back of your tach.

 

[QuadManiac]

Re: Engine change and tach compatibility

Probably not as your Force has a 20 pole alternator and your Johnson had a 12 pole alternator. You need one of these: http://www.iboats.com/Tach_O_B_Chry...1494696--**********.894354262--view_id.134619

Umless there is a 10P setting on the back of your tach.

Shouldn't it be a 20P setting?

 

[Barnacle_Bill]

Re: Engine change and tach compatibility

Shouldn't it be a 20P setting?

No it would be 10 as the "P" stands for pulse and not poles. For example A typical 12 pole alternator sends 6 pulses to one side of the rectifier and 6 to the other, hence the setting of 6P since the tach is only connected to one side. Do I make sense?

 

[QuadManiac]

Re: Engine change and tach compatibility

Absolute sense. Thanks. You are saying that a 20 pole stator puts out a sine wave whose frequency is RPM x 10, correct? 10 maxima and 10 minima.

So, if i were to bridge rectify the output, and not filter it, with a battery or a capacitor, I'd have 20 pulses, relative to zero volts, lol. One to remember, although I'll probably never come across a use for it.

 

[Frank Acampora]

Re: Engine change and tach compatibility

I wouldn't swear to it since I have never unwound a 20 pole stator, but I don't think all 20 poles are used for alternator. There are multiple windings on the core. Several windings are dedicated to charging the modules and the rest are used for alternator output. That coupled with the fact that I have a tach which uses setting "3" for one alternator and "4" for the other (I forget which is which) leads me to believe that a 20 pole stator has less than 20 poles dedicated to alternator. But as I said--I wouldn't swear to it.

Come on--Some of you electronics guys educate us!

 

[Barnacle_Bill]

Re: Engine change and tach compatibility

Absolute sense. Thanks. You are saying that a 20 pole stator puts out a sine wave whose frequency is RPM x 10, correct? 10 maxima and 10 minima.

So, if i were to bridge rectify the output, and not filter it, with a battery or a capacitor, I'd have 20 pulses, relative to zero volts, lol. One to remember, although I'll probably never come across a use for it.

Thats all a rectifier is, a full wave bridge rectifier. I made one once using 4 ECG116 diodes.

 

[QuadManiac]

Re: Engine change and tach compatibility

Thats all a rectifier is, a full wave bridge rectifier. I made one once using 4 ECG116 diodes.

Ya, I'm aware... nm, i was just musing on the 10 waves out of the 20 pole stator. I've been thinkin bout a bridge and then 3 terminal regulator, with maybe a pass transistor to get the current required, as a 'universal' rectifier/regulator to solve everybodies woes, lol. Once again, just musings.