Tachometer needle "dips" frequently - SailNet Community

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  #1  
Old 05-27-2009
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Tachometer needle "dips" frequently

Hi all,
I think this is an electrical problem, but I'm not sure where to start. The Motorola tachometer on our boat has a needle (obviously), an hour meter, and a little "clicker" (I don't know what it's called) indicator, which clicks whenever the fuel pump is on, switching between plain white (shown) and white background with a black line. See pic--the red box highlights the clicker thingy:


I'm not sure exactly what that thing is, so any info is appreciated.

Anyway, back to the problem:

I noticed recently that the tachometer needle has started to "dip" by 100-300 rpm every time that clicker "clicks". The actual engine RPM remains steady even though the needle dips. Also, all the other gauges (oil pressure, volt meter, etc.) remain steady.

Is this just some sort of loose connection? Might it be inside the tach? Any suggestions?

As always, I appreciate your help!
-J
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Old 05-27-2009
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If the tach is alternator-driven this could be an indication that your brushes are worn and getting to the end of their life. However I don't have an explanation for the coincidental "clicker" connection. My first thought was that it may simply be an indicator that the hour meter is powered up and counting.....

Have you tried searching Motorola for tach data?
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Old 05-28-2009
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i would bet you have a failing connection on the ground wire on the tach or at the other end of it. when the clicker light comes on it is over loading the weaking ground wire connection and it drives the tach lower. this is a simple test, get some wire pull the tach out and run a new ground. if this cures it, you just need to find where the connection is getting lose or corroded
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Old 05-28-2009
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Loose fan belt?
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Old 05-28-2009
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I don't think it's the alternator, simply because it's not that old, and nothing else is showing any symptoms. The possibility that the "clicker" is indicating the hour meeting is running is definitely possible. I'll also check to see if it's a ground connection problem.
Thanks!
-J
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Not the alternator just a loose belt.

There is some kind of pulse generator in the alternator that sends a signal to the tachometer for RPM and when the "clicker" comes on it may pull enuff power to load the alternator and allow the belt to slip and thus your wavering tack.

Try turning on something on and off with about 5 amp load to see if this affects the tach to verrify the loose belt idea.

Quote:
Originally Posted by josrulz View Post
I don't think it's the alternator, simply because it's not that old, and nothing else is showing any symptoms. The possibility that the "clicker" is indicating the hour meeting is running is definitely possible. I'll also check to see if it's a ground connection problem.
Thanks!
-J
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Quote:
Originally Posted by timebandit View Post
Not the alternator just a loose belt.

There is some kind of pulse generator in the alternator that sends a signal to the tachometer for RPM and when the "clicker" comes on it may pull enuff power to load the alternator and allow the belt to slip and thus your wavering tack.

Try turning on something on and off with about 5 amp load to see if this affects the tach to verrify the loose belt idea.
Hmmmm, something with a 5amp load--I don't think I have anything on board that draws that much! No fridge! Maybe all the cabin lights at once? It's certainly worth a try.

Thanks timebandit!
-J
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How about a water pump?

Cigar lighter?

Quote:
Originally Posted by josrulz View Post
Hmmmm, something with a 5amp load--I don't think I have anything on board that draws that much! No fridge! Maybe all the cabin lights at once? It's certainly worth a try.

Thanks timebandit!
-J
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Quote:
Originally Posted by timebandit View Post
How about a water pump?

Cigar lighter?
No lighter, but we have a water pump--DUH. Apparently, I'm dense this morning. Thanks again...
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Old 05-28-2009
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Check where the tac attatched to the alternator, or the back of the tack. Lookis like a loose wire.....been there done that one too.... ..i2f
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