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.
The pictures of the sensor showing the corosion weren't clear enough to show anything. I was to close with the camera, but you get the idea.
Hope this helps and you never need to do either.
