Tachometer starts at 2000 then drops to zero, engine doesn't change.

[bartpkelly]

I have a Yanmar 3HM, and the tachometer reads off the flywheel. The manual says about 700-900 rpm. It starts at 2000 then drops all the way top zero. The engine itself has no change in sound, so it hasn't changed its RPMs. and certainly not to zero. This happens within 4 minutes after starting from cold.

I need help on what to do. I did check the first connection and it seemed secure and clean. No loose wires on the way to the flywheel that I could detect. Since it is the flywheel the alternator wouldn't make a difference right?

Any tips or advice would be great. 3 photos attached:
1) The sensor at flywheel
2) Back of the tachometer
3) Throttle and stop (Governor?) when the engine is not running.

 

[Don S/V ILLusion]

First thing to do is check continuity of the wiring and ensure good connections by removing and cleaning each. Just looking at them can be misleading.

 

[bartpkelly]

Will do that tomorrow and see what happens.

 

[kloudie1]

The pin and socket quick connectors are the usual suspects. The one you have pictured with the wires from the flywheel pickup is prime, but there is usually another one behind the Yanmar instrument panel . The soc,it's can be cleaned with a very small wire bottle brush or a small rolled up piece of fine (180_220 grit) sandpaper.

 

[dlochner]

The wiring is the prime suspect. However, the sensors do fail. The service manual should have the testing procedure, which involves measuring the resistance across the sensor terminals.

 

[bartpkelly]

The wiring is the prime suspect. However, the sensors do fail. The service manual should have the testing procedure, which involves measuring the resistance across the sensor terminals.

Thanks, I am pretty green, completely green on electrical as far as measuring resistance so I am hoping cleaning and replacing will do.

 

[kloudie1]

Here is the schematic for the tachometer.. it shows the wiring, and how to test to see if the sender is OK.. The tachometer instrument itself on these old engines are notoriously inaccurate and sticky,
The Yanmar tacho is pretty expensive but a decent replacement (VDO Vision 333-163) is around $175.

 

[Nichole340]

Where does the Red Wire go in your photo? Could that be a possible solution?

 

[bartpkelly]

Where does the Red Wire go in your photo? Could that be a possible solution?

Having a hard time tracing that red wire. Seems the previous owner enjoyed snipping wires as there are several dead ends.

I cleaned the Sensor connections and fired her up: Absolutely no readings. Took off the sensor attachments and reattached again and ...Success for a few minutes, she read the RPMs at what I assume was correct. Shut her down, adjusted the idle, and when restarted she read at 800rpms (great) and then it dropped to nothing and never moved again. I will try to test the sensor and then buy a new tach most likely.

Newbie question...when using the multimeter on the sensor...is the engine running or is it off? Manual says to test it but not very clear for someone who has no idea. I think it should be running...but let me know if I am right or wrong.

 

[JoeWhite]

Yes, running so that you can read the pulses probably 1000Hz.

 

[bartpkelly]

Yes, running so that you can read the pulses probably 1000Hz.

Thank you!

 

[dlochner]

Newbie question...when using the multimeter on the sensor...is the engine running or is it off? Manual says to test it but not very clear for someone who has no idea. I think it should be running...but let me know if I am right or wrong.

The first measurement is the resistance in the sensor, this is done with the engine not running and the wires disconnected.

Measuring the output voltage is tricky, it is an AC current that is not easily read with most multi-meters which are designed for 60 hertz.

The sensor works by sending a pulse each time a flywheel gear tooth passes by, thus the frequency will be variable based on the rpm.

 

[bartpkelly]

The first measurement is the resistance in the sensor, this is done with the engine not running and the wires disconnected.

Measuring the output voltage is tricky, it is an AC current that is not easily read with most multi-meters which are designed for 60 hertz.

The sensor works by sending a pulse each time a flywheel gear tooth passes by, thus the frequency will be variable based on the rpm.

Whew, thank you. That makes sense.