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Old 10-04-2018, 16:43   #1
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Yanmar xGMxx Tach Sensor Question

Does anyone know what principle the Yanmar GM series engines tach sensor uses?

I know it senses flywheel RPM and I understood it "counted" the teeth on the ring gear. However I have also being told it detects permanent magnets (1 or 2) that are embedded in the flywheel.

Anyone know for sure which method is used by the Yanmar GM tach?
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Old 10-04-2018, 16:53   #2
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Re: Yanmar xGMxx Tach Sensor Question

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Does anyone know what principle the Yanmar GM series engines tach sensor uses?

I know it senses flywheel RPM and I understood it "counted" the teeth on the ring gear. However I have also being told it detects permanent magnets (1 or 2) that are embedded in the flywheel.

Anyone know for sure which method is used by the Yanmar GM tach?
I'm sure there are no magnets in the flywheel. The inductive sensor magnetically detects each flywheel tooth as it passes beneath it. If you look on the dial of your tach you will see "P = 97" (for a 2GM), which tells you the number of flywheel teeth it is designed for.

Cheers, Graeme
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Old 10-04-2018, 21:44   #3
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Re: Yanmar xGMxx Tach Sensor Question

^^, OK I will have to have a close look at the tach for this number, thanks.

If presumably the P stands for "Pulses" then I surmise there is 97 teeth on the flywheel which intuitively seems correct to me.
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Old 11-04-2018, 03:12   #4
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Re: Yanmar xGMxx Tach Sensor Question

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Old 11-04-2018, 05:46   #5
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Re: Yanmar xGMxx Tach Sensor Question

Thanks Gord, absolutely perfect info!

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Old 11-04-2018, 07:09   #6
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Re: Yanmar xGMxx Tach Sensor Question

Additionally, and maybe too much information, the principle used is the Hall effect, and the magnet is in the sensor...

https://www.youtube.com/watch?time_c...&v=wpAA3qeOYiI
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Old 11-04-2018, 08:33   #7
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Re: Yanmar xGMxx Tach Sensor Question

Following as my 3ym30’s tach (and hour meter) have performed inconsistently for years. Maybe a perennially bad connection.?
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Old 11-04-2018, 17:36   #8
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Re: Yanmar xGMxx Tach Sensor Question

Thanks Gord, I always wondered what that sensor did. My Tacho on our Yanmar 2GM only works sometimes? Any idea why?
Cheers
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Old 12-04-2018, 05:39   #9
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Re: Yanmar xGMxx Tach Sensor Question

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Additionally, and maybe too much information, the principle used is the Hall effect, and the magnet is in the sensor...

https://www.youtube.com/watch?time_c...&v=wpAA3qeOYiI
No, not too much information , perhaps unnecessary for me who has spent a lifetime with electrical / electronic / radio systems but no doubt helpful to some who might be reading this thread. So on behalf of them, thank you.
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Old 14-07-2019, 12:49   #10
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Re: Yanmar xGMxx Tach Sensor Question

Ive got a similar situation. My tach on my Yanmar 2gm20F works perfectly for about 45 minutes while I cruise down the river to the first bridge that I have to open... as soon as I throttle back to idle, the tach stops...goes to zero and wont run again till it everything cools off the next morning.

Ive metered out the orange, and the Blue/Red wires from the Mag sensor when its running and get 1.8v AC at the tach. When it quits, there is zero voltage on those two leads.

The Mag sensor has no moving parts. I just cant seem to figure out whats making the voltage drop to zero. Ive cleaned the plug at the sensor end and the plugs at the panel end. No joy!

The Tachometer is brand new.... its a Faria-Beede model... My previous tach never worked correctly. It always indicated about 1000 rpm low. So I replaced it thinking it was the problem. It always had moisture condensing on the inside of the glass face.

Checked and re cleaned all the connections again this morning and had the same result later when I test ran the engine down the river.
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Old 14-07-2019, 13:43   #11
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Re: Yanmar xGMxx Tach Sensor Question

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Thanks Gord, I always wondered what that sensor did. My Tacho on our Yanmar 2GM only works sometimes? Any idea why?
Cheers
Pull the sensor clips, clean and reinstall..see how that works
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Old 14-07-2019, 16:48   #12
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Re: Yanmar xGMxx Tach Sensor Question

Have to find a 17 year old kid that knows how to turn a wrench and be able to squeeze into the access hatch in the lazarette first! I'm pushing 71...I'm 6ft and 198 lbs. I don't bend like I used to!

Who ever designed a Watkins 29 with such limited access to the back of the engine, and the stuffing box, needs to give his head examined!

I've been able to crawl head first into the engine compartment, better known as the diesel dungeon, and lay on top of the engine...but really can't work on the sensor!

My plan is to ease the lock nut and screw the sensor in till it touches a flywheel tooth centered in the hole. Then back it off a turn.

Im hoping that it's incorrectly mounted and the gap is too great. Failing that, I'll install a new cable from the sensor to the tach.

If these don't fix the problem, I'll pop for a new sensor. Trying to avoid that $130 hit.
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Old 14-07-2019, 16:51   #13
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Re: Yanmar xGMxx Tach Sensor Question

If by CLIPS, you mean the connection plugs, I've already cleaned them with acetone on a little piece of cloth on the tip of a screwdriver.
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Old 14-07-2019, 19:05   #14
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Re: Yanmar xGMxx Tach Sensor Question

As just posted on the other thread which you all have going, ......try pulling the inductive sender and cleaning the magnetic tip. Sometimes these tips get loaded with ferrous particles from the flywheel teeth and don't give a clean flux break and it can cause erratic operation.

Can't speak to the Yanmar sender, but I doubt it's a Hall effect sensor.
The similar sender used by Volvo is a two wire unit with a plastic housing, so no grounding of the case and no voltage fed into the unit. Straight flux change inducing current into the coil.

Typical Hall effect sender would be something like the speedo paddle wheel as built by Airmar. Two wire and a grounding mesh where a 5 volt reference voltage is fed to the unit, the magnet is rotating and the output is pulsed 5 volt.

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Old 15-07-2019, 05:04   #15
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Re: Yanmar xGMxx Tach Sensor Question

Question....since I have limited visibility to the sensor because of how my engine is hidden...do you know how the wires connect to the sensor? Are they connected wtih spade connectors or similar? or are they hard wired into the sensor body?
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