[C320-list] Engine pod questions

[surprise thompson87.com via C320-list]

My 1999 boat with Yanmar 3GM3F engine has developed an erratic tachometer. Does anyone have any advice about how to diagnose the problem?

Perhaps relatedly, but perhaps not, I’ve noticed that when the engine is running the engine pod volt meter is pegging out way past the marked scale. It drops slightly when I turn on the blower but still reads way above the 14.3 volts I’m seeing on the batteries per my Victron battery monitor. Is it possible that the alternator is putting out a substantially higher voltage than the batteries are seeing?

Scott Thompson
Surprise, #653

[Jack Brennan via C320-list]

At the rear of your engine is a sensor with two wires connected. These feed your tach. Pull the wires off one at a time and clean the connection. Make sure the connections are tight.

If that doesn't make a difference, it's likely a ground connection in the instrument panel at your steering pedestal. Tighten all of the (usually) black wires and make sure the connections are clean.

Lastly, the tach could be bad. I had to replace the one on my 1998. The sensors go bad only very occasionally.

Jack Brennan
Sonas, 1998 Cataliina 320
Tierra Verde, Fl.

________________________________
From: C320-list <c320-lis...@lists.catalina320.com> on behalf of surprise thompson87.com via C320-list <c320...@lists.catalina320.com>
Sent: Sunday, October 12, 2025 9:16 PM
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Cc: surprise thompson87.com <surp...@thompson87.com>
Subject: [C320-list] Engine pod questions

[KEN GEIGER via C320-list]

We have the same or very similar symptoms on 765, 2000.  Following with interest.
Ken GeigerNorthern Dream. #765

Sent from AOL on Android

On Sun, Oct 12, 2025 at 9:18 p.m., surprise thompson87.com via C320-list<c320...@lists.catalina320.com> wrote: My 1999 boat with Yanmar 3GM3F engine has developed an erratic tachometer. Does anyone have any advice about how to diagnose the problem? 

[--Bill via C320-list]

Here is a link to how a tach works on many small diesel engines https://answerpoint.blog/w-terminal-alternator-tachometer-wiring-guide

I don’t have a Yanmar but i think it’s similar. I have a Beta 28 and had to replace my instrument. It had to be calibrated to the specific alternator pulley sizes in order to read correctly.

—Bill Ackerman
#240 Tortuga 1995

> On Oct 13, 2025, at 1:30 AM, KEN GEIGER via C320-list <c320...@lists.catalina320.com> wrote:
>
> We have the same or very similar symptoms on 765, 2000. Following with interest.

[SARA SCHROEDER via C320-list]

I had the same problem with the tach bouncing all over the place and the culprit was a loose alternator belt. Once the belt was properly tighten up it stopped bouncing around.

Sara
Wandering Star
Seattle
#707/2000

[surprise thompson87.com via C320-list]

The Yanmar tachometer gets its reading from a magnetic flywheel sensor, not from the alternator, and so the link below does not apply.

T. Scott Thompson
Catalina 320 “Surprise,” #653 (1999)
surp...@thompson87.com

[--Bill via C320-list]

Thanks for the clarification. I wasn’t sure which type is on the Yanmar.

—Bill Ackerman

> On Oct 13, 2025, at 3:46 PM, surprise thompson87.com <surp...@thompson87.com> wrote:
>
> The Yanmar tachometer gets its reading from a magnetic flywheel sensor, not from the alternator, and so the link below does not apply.

[Michael Stumpfoll via C320-list]

Your boat could have the tach connected one or the other way, from the hall effect sender with two wires, or a single wire to the terminal on the alternator.
Thanks,
Mike Stumpfoll

On Oct 13, 2025, at 4:31 PM, surprise thompson87.com via C320-list <c320...@lists.catalina320.com> wrote:

The Yanmar tachometer gets its reading from a magnetic flywheel sensor, not from the alternator, and so the link below does not apply.

[Jeff Smith via C320-list]

Scott,

Two thoughts:

1. You can get a contactless tach to check the reading you are getting from the instrument pod tach. My engine pod tach does not work at low rpms, but I used the contactless tach to set my minimum engine rpms to 1,000 (I have the Perkins M30). You could read the rpms using a contactless tach and compare to the engine pod. It is not exact, but within 100 rpms would be close.
2. You can also check the alternator by using your multimeter and checking the output coming out of the alternator. Obviously, you should be careful when doing this. I believe alternators can put out up to 14.7 volts. If you are getting more than 14.7, you may need to have your alternator repaired/replaced (likely an issue with the voltage regulator). However, I would think the battery monitor would accurately reflect the voltage the batteries are receiving (testing the back of the alternator should confirm).

Good luck,
Jeff

[John Beck via C320-list]

Is it a Hall effects sensor?
John Beck
Sail on # 518
Jb


On Mon, Oct 13, 2025 at 4:31 PM surprise thompson87.com via C320-list <

[Troy Dunn via C320-list]

John-

The 30GM30F uses all analog gauges and sensors. The tach sensor appears is
a simple magnetic sensor that senses the teeth on the flywheel and just
sends analog pulses to the tachometer which it converts to an rpm reading.

Troy Dunn
Hull #514