Tach bounce down every 8 to 10 seconds or so?

[smokey73]

I've read the posts on a tach bouncing, possibly due to a bad diode. My symptoms don't seem to match those previously discussed so hopefully someone can help me out.

It doesn't seem to be any different if its at low or moderate rpm. It is not just a bounce of a few rpm, it almost goes to 0 but there is no difference in the engine sound so it has to be in the alternator or the tach itself. The drop is very quick and then back up. I tried to upload a video but it is too big. I'll see if I can edit it and put it up in a few minutes.

 

[Rich Stidger]

There can be lots of faults that could cause this but I think the first thing I would look for is a loose wire between the tach and the alternator. You could connect a temporary jumper between the tach and the alternator to see if that stops the problem. Then you would know what direction to proceed.

 

[smokey73]

Here is the video as an attachment. Sorry, couldn't seem to imbed it directly. I did not take the battery switch through off with the engine running and this is not a new installation. Any ideas on the problem would be appreciated. I wouldn't think a loose wire would be this consistent and for such a short duration.

 

[sailcapt340]

On my 340, it was corroded connections at the top of the Tach Sensing Unit, located behind the engine and on top of the bell housing.
It's cramped in that spot back there, but I just pulled the two wires off the sensor and found they were green and corroded. A little steel wool on the prongs and a small file in the slots of the wire connectors and it stopped the swings (bounce) you are describing.
There are a few pictures on my media page you can look at to see the sending unit.
Somewhere in the archives, a man wrote how to check the current and what readings you should get on a volt meter at the tach itself. Should be a simple search. I'll try again myself also.
I have to say doing the cleaning of the connections saved me from removing the sensor and checking if it was worn or corroded. Lot less work.
Hope this helps.

 

[sailcapt340]

Smokey73,
Type in (yes I said type) "2000 340 Tachometer". It has the article written regarding this subject.
Along with the readings you want to see. It's a place to start.

 

[smokey73]

Not sure if you mean type it here on this Forum site or on Google search? I couldn't find anything on this site that had an article or readings but maybe I'm looking the wrong way.

Thanks

 

[Benny17441]

What is the usefulness of a tach in a sailboat? You are not going to over rev the engine because the pitch of a proper prop will not allow it. To glance and see if the engine is running? Perhaps if there is deafness. I use it mostly when performing maintenance in the boat to adjust the idle speed and check for engine accelerating to maximum RPM. In reality I find I do not glance at the numbers on the dial but just at the needle movement. My ears tell what is the appropriate fast idle, the vibration what is my optimum cruising RPM. Engine noise and boat speed tell me all I need to know. I went for years with a faulty tach that would not read over 2,200 RPM and never missed not having a good one. Underway the only use I have for my tach is a visual confirmation of what my other senses are telling me. Do check for loose wires or dirty pick up points but do not loose any sleep over the tach.

 

[sailcapt340]

Yeah , it's a little hard to find but here's a copy, from this sites archives

My Update

After spending sometime speaking with others and online, here is what I have found.
Most, were ready to suggest I just replace the sensor and be done with it. Or for that matter, both sensor and tach. The parts are easily available at any number of places on line.

About a year ago, Mr. Bill Kneller of s/v Rhapsody (J/30) wrote:

The tachometer uses a magnetic pickup sensor on the flywheel - mounted on the port aft side near the top of the bell housing. The connector at the sending unit should be checked for loose terminals.

1. Check the connections on the back of the meter.
2. Check the voltage on the wires on the back of the gauge - black is ground, red/black is 12v. If there is not 12 v there, investigate.
3. Check if sender is fitted loosely.
4. Measure the internal resistance of the sender - should be 1.6 +/-0.1 KOhms.
5. Measure the output voltage of the sender - to be 1v or higher.

What I found on my boat was...corrosion at the sending unit connections.
First of all the unit itself is well hidden from sight, so you really have to be looking for it. The tach itself was fine, no corrosion on the wires or connectors.
The sending unit was a different story all together, at first sight, it appeared to be fine, nothing really out of the ordinary. After remove the wires from the sensor I found the corrosion on the sensor connecting blades and inside the wire connectors.
With just a little steel wool and some cleaning, all is well. I was ready to replace the wire ends if needed but they cleaned up nicely and there are no signs of deterioration on either side. So I went with the clean option.

Voltage at the tach was 12.75V, at the wire ends, the same.
Measuring the internal resistance at the sender I had 1.59 Kohms. (so I make the +/- of 0.01 Kohms)

I now have the Tachometer reading back to what it was before. Idle at about 900-1000 and all the ranges in between up to 3000 RPM.

The part costs to replace will range to $135.00 - $150.00 for the sensor and as far as I went $425 - 600 for the Tachometer.

Well worth the exploration and the cleaning in my eyes.

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[smokey73]

I guess I'm a little confused. It was my understanding that the tach for my 1994 Yanmar 4JH2E came off the Alternator rather than a pickup off the flywheel? In either case I'll clean up all the connections.

As to the statement of why a tach. I would agree that using the sound is valid and is a good indicator of the running condition. However, if the tach comes off the alternator then a problem with the tach could also be an early indicator of a problem with the alternator and that seems like a good thing to know. I am away from the boat right now and can't give you the make/model of the Alternator, but it is stock (I believe) for a 1994 Yanmar 4JH2E 50 HP. I am sure there are a lot of those in use by the people on this site so does anyone have info if for this engine the tach comes off the engine or the Alternator.

 

[smokey73]

My mistake!! I checked my Yanmar 4JH2E Service Manual and sure enough, the tach sensor reads the teeth on the flywheel rather than a sensor on the Alternator. That is good since that means I'm probably not having an impending Alternator failure.

So now, when I get to the boat I'll check and clean all the connections on the sensor and tach if necessary and see if it goes away. I can put up with the bounce if I have to since its not likely a major problem and the tach seems to read correctly when it doesn't do the "needle dip." I will check it with a non-contact hand held tach just to be sure but as noted, I can get pretty close even without the tach just by the sound.

Thanks for the tips everyone.

 

[capta]

What is the usefulness of a tach in a sailboat? You are not going to over rev the engine because the pitch of a proper prop will not allow it. To glance and see if the engine is running? Perhaps if there is deafness. I use it mostly when performing maintenance in the boat to adjust the idle speed and check for engine accelerating to maximum RPM. In reality I find I do not glance at the numbers on the dial but just at the needle movement. My ears tell what is the appropriate fast idle, the vibration what is my optimum cruising RPM. Engine noise and boat speed tell me all I need to know. I went for years with a faulty tach that would not read over 2,200 RPM and never missed not having a good one. Underway the only use I have for my tach is a visual confirmation of what my other senses are telling me. Do check for loose wires or dirty pick up points but do not loose any sleep over the tach.

Unless one has perfect pitch, how would one run at the same RPM's each time when operating their engine? This is sort of useful for trip planning, especially if one doesn't want to run at full speed all the time. A tach is also useful for determining fuel consumption a various RPM's and the speed to be expected at those RPM's. Please note that I said "EXPECTED" boat speed, because there are a lot of factors which determine boat speed other than the engine speed.
If your running gear is properly adjusted, there should be little to no 'vibration' from it. Perhaps you need to check your motor mounts, prop balance, alignment and or cutlass bearing.
It certainly isn't a total necessity, but if one can have a tach, why wouldn't one want one on their boat?

 

[Ron20324]

What is the usefulness of a tach in a sailboat? ...

When I'm out for about 15 minutes, I run up to full throttle to make sure I can reach my 3400 RPMs. If not, something needs looking into.

 

[Rich Stidger]

When I'm out for about 15 minutes, I run up to full throttle to make sure I can reach my 3400 RPMs. If not, something needs looking into.

+1 I got the same advice from the East Coast Volvo Tech rep. Run at WOT for 10 minutes every week.

 

[Ken Cross]

You can put an AC voltmeter on the input to the tach while the engine is running to determine if the issue is the pickup, wiring, other tack itself. Also check the DC power input to make sure it isn't intermittent power. I'd look for connector corrosion first because it doesn't cost you anything to check. A marginal connection can look like you describe.
Ken