Trouble Shooting Yanmar Tachometer for "B" Panel

[dlochner]

The tach for my Yanmar 3jh2e is not working. This tach uses a flywheel sensor to count rpms.

After testing the sensor, I found it had failed. It was replaced and now works.

Digging into the wiring harness, I found the ground to the light and the tach was bad. That was repaired and the light now works.

The tach still reads 0 when the motor is running. How can I test the head to see if it has failed? A replacement tach is not inexpensive. The Yanmar shop manual for the 3jh2e is silent on this.

Any suggestions on how to verify that the tachometer has failed before I shell out big bucks for a new one?

 

[twalker H260]

I would assume you have one wire coming off the sensor and the other connection is the engine ground? (If the tach uses a 3 wire connection - testing can be done using a different method.)

If so you should be able to test if you are getting a signal through the wiring to the tach. connect black lead of multimeter to the ground wire and the red to the tach connection. Set meter to ac volts - make sure you start at a high enough range if it is not auto switching. Start engine - If no chang in voltage on multimeter when engine is running or changing speed I would look at your wiring up to the panel further ( or the sensor at the engine). If you have variable voltage then I would look at the tach. You could likely test the tach itself using a variable voltage bench power supply to mimic the voltage you get testing the sensor wires.

 

[Davidasailor26]

How did you test that the sensor failed? Could you extend whatever test you did to points farther down the wire harness to see if the signal gets lost somewhere along the way?

Access to an oscilloscope might help see what the sensor is doing too, and see what that signal looks like closer to the tach input. Not sure if the 3jh2e signaling works anything like my 3YM30, but this is the signal from my alternator to tach wire -

 

[dlochner]

The signal from the sensor is fine. Measured with the multimeter the voltage is within specs. The measurement was taken at the connector to the Tach. The shop manual has 2 tests for the sensor, a resistance between the 2 terminals and the the output voltage when the motor is running. The original sensor failed the resistance test and was replaced.

After replacing the sensor, I checked the wiring harness. The ground was faulty, so I fixed that.

At this point the tach has DC power and the is getting a strong signal, but the meter is not showing rpms. That makes me suspect the tach itself has failed. A replacement is $300+ without being certain the head has failed I hate to spend the money.

 

[Rick D]

The tach for my Yanmar 3jh2e is not working. This tach uses a flywheel sensor to count rpms.

After testing the sensor, I found it had failed. It was replaced and now works.

Digging into the wiring harness, I found the ground to the light and the tach was bad. That was repaired and the light now works.

The tach still reads 0 when the motor is running. How can I test the head to see if it has failed? A replacement tach is not inexpensive. The Yanmar shop manual for the 3jh2e is silent on this.

Any suggestions on how to verify that the tachometer has failed before I shell out big bucks for a new one?

I may have a tach that I replace which I thought was bad, but was not. If all else fails, I can send it to you use as a test or keep. PM me with details on your tach.

 

[twalker H260]

Came across this PDF. Assuming you have 12v from ign to gnd and getting variable voltage from snd to gnd I would likely assume the tach is faulty. If your able to get a working tach (from Rick D, dockmate or maybe a truck autowrecker?) to try even for a few minutes would give you reasonable assurance of where the problem lies.

http://www.marineengine.com/products/technical_information/Diesel_Magnetic_Tachometer.pdf

 

[dlochner]

I may have a tach that I replace which I thought was bad, but was not. If all else fails, I can send it to you use as a test or keep. PM me with details on your tach.

@Rick D thank you for the offer. We're going to be moving target soon, so shipping across the country will be a challenge. For the next year we have an idea of where we will be, but not when we'll be there. Replacing the tach is important, but it falls just below the mission critical tasks which is our current focus.

Thank you again.