Tach sensor

scout568

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Joined
Aug 21, 2012
Messages
21
Re: Tach sensor

do these go bad pretty often and are they exspensive, I've been working on my choke and notice today when the engine was running the tach said 4000 and it was only at idle so i rev it up and back to idle and it came back down but still seems to running high it says motor is running at 1500 now and i know its not. Time for a new sensor?
 

NHGuy

Captain
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May 21, 2009
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3,631
Re: Tach sensor

I forget where but I just got a discontinued new old stock (NOS) tach that matched my instruments, online. I think it was $20 plus shipping. I could not beleive how cheap it was. I have seen em for $75 on lots of places.
 

Don S

Honorary Moderator Emeritus
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Aug 31, 2004
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62,321
Re: Tach sensor

do these go bad pretty often and are they exspensive, I've been working on my choke and notice today when the engine was running the tach said 4000 and it was only at idle so i rev it up and back to idle and it came back down but still seems to running high it says motor is running at 1500 now and i know its not. Time for a new sensor?

Probably time for a new tach, there is no "sensor", the tach just picks up the pulses from the coils negative terminal. If the coil was bad, the engine wouldn't run.
Most tachs have a switch on the back so you can set the number of cylinders for your engine. Might try turning that setting switch back and forth a few times and see if the tach settles down.
 

scout568

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Aug 21, 2012
Messages
21
Re: Tach sensor

Hey nice score on the tach for $20, and Don thanks for the info, so really the tac just picks up the amout of voltage running thru the coil and that determins the reading, faster engine speed eaquals more voltage wich makes a high rpm reading correct. ok so then can I just go to an auto parts store and by an after market tach for a V6 if I need one?
 

wellcraft-classic210

Master Chief Petty Officer
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Sep 22, 2010
Messages
839
Re: Tach sensor

A tachometer measures voltage pulses that go from 12volts to near o volts on the negative side of the coil with each spark plug fire. 4 pulse cycles per revolution on a 4 stroke V8 etc.

They do require a power & ground source so they can sense the signal on a third wire. You may want to verify that befor you purchase a new one. But --as Don said above-- Its probaly time for a new tach.

An auto tach should work in pricipal -- A marine tach may hold up better to moisture induced corrosion / either type shoud be kept dry.
 

scout568

Cadet
Joined
Aug 21, 2012
Messages
21
Re: Tach sensor

Thanks for all the info, the funny thing is I went out and fired the boat up and tapped on the tach and it started reading fine, checked the wiring on the back its all tight so sound like I need to find a new tach in the near future thanks again
 

04fxdwgi

Senior Chief Petty Officer
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Jun 10, 2011
Messages
754
Re: Tach sensor

Thanks for all the info, the funny thing is I went out and fired the boat up and tapped on the tach and it started reading fine, checked the wiring on the back its all tight so sound like I need to find a new tach in the near future thanks again

Try moving the selector on the back of the tach back and fourth, from 8 to 4 cyl and back again a few times. The contacts get crapped up sometimes and moving it through the ranges a few times cleans em up, sometimes.
 

edsmax

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Joined
Jul 2, 2012
Messages
14
Re: Tach sensor

OP - listen to Don and 04fxdwgi. Just remove your tach and spin the circular engine selector switch in back. It is labeled 8cyl, 6cyl, etc. The contacts get dirty of time and will cause an erratic reading. Spinning the selector a few times usually will clean up the contacts and will fix your issue.
 

scout568

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Joined
Aug 21, 2012
Messages
21
Re: Tach sensor

Thanks guys will try this and let ya know if I have any more issues with it
 
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