Tuning the tach. ??

gpfishingdude

Chief Petty Officer
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May 1, 2012
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538
I installed a tach. for the 1986 Mercury 50HP motor on the pontoon boat. When we got it on the water I noticed that it was reading way too high. It read 2000 with the motor shut off and was pegging 7000 rpm at cruising speed. Right now it is reading around 900 rpm with the motor off. Can it still be tuned in by adjusting the number of poles on the adjusting screw on the back or have I ruined this tach.? To adjust the number of poles I have to remove the tach. for access to the back of it. I don't understand what the number of poles is about but I'm hoping that it is some form of multiplier?
 

gpfishingdude

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May 1, 2012
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538
Re: Tuning the tach. ??

I just looked at the instructions again for the Faria tach. installation and now I'm wondering if I have it wired in right. I ran the signal wire to the post on the rectifier that has the yellow and grey wires to it. Should this wire be run to the grey wire without it being hooked up to the rectifier? If I can't get any advice on this by tomorrow I will call Faria and ask them what is wrong with my installation. Have any of you hooked up a tach. to an old 50 h.p. Mercury like this and what number should the pole selector be set at?
 

MH Hawker

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Jul 13, 2011
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5,516
Re: Tuning the tach. ??

The tacks will read with the motor turned off, it normally holds what ever RPM it was at when it was turned off, and with out knowing a bit more about exactly how your motor is set up it is hard to say, how ever the common wire up is a black from ground, a red from the key switch to power the gage and a gray from the rectifier to supply the pulse signal, Depending on what charging stator you have thats how the RPM is set for each motor, mine is 8 pulses and 4 poles, I am sure yours is different, you will have to dig a bit deeper to see what yours is.
 

gpfishingdude

Chief Petty Officer
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May 1, 2012
Messages
538
Re: Tuning the tach. ??

Thanks! I will get out there and try to set the pole adjust today. From what I have read mine should be a 12 pulse and should be set on #5. Sounds like there is a good chance that I haven't ruined the tack. yet.
 

gm280

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Jun 26, 2011
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14,585
Re: Tuning the tach. ??

Thanks! I will get out there and try to set the pole adjust today. From what I have read mine should be a 12 pulse and should be set on #5. Sounds like there is a good chance that I haven't ruined the tack. yet.

NO you haven't ruined the tach. It just needs the correct pulse count set to read correctly. So if you engine has X-numbers of poles (magnets in the stator section) you have to set it to that corresponding number and it will work properly then... I have purchased a Faria tach as well and contacted them for the setting and they told me I had 6 magnets so set it to the number 2 position which corresponds to 6 pulses... I haven't cranked it up yet, but I feel like it should work just fine...
 

gpfishingdude

Chief Petty Officer
Joined
May 1, 2012
Messages
538
Re: Tuning the tach. ??

I found a Mercury site that said mine has 12 pulses so I found the corresponding number to set mine to. It looks like it will work just fine now. It went to about a thousand when I started it and climbed smoothly when I accelerated. I didn't quite understand about setting the pulse count when I first got the tach. so I just skipped that step. I just checked it with the muffs to see if it looked right. Now I need to get on the water where I can open it up and watch it. Thanks for all of the help and for explaining to me where the pulse count comes from.
 
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