Yanmar 3YM30 where do I find the tacho sensor point?

[Nautilus2]

Need help locating the tacho sensor point. Recently i had new engine mounts installed. All well on test runs except no rpm on TAcho dialface and engine hours not increasing. searched for loose cables, can only find one with twin female 'bullet' connectors at rear of engine near flywheel. cannot see where this connects (if indeed it is the correct tacho cable). would dearly appreciate help with this!

[Wotname]

Belated welcome Nautilus2.

The 3YM30 uses the alternator to sense the tach. The attached is direct from the service manual although I added the blue arrow.

[Nautilus2]

Thanks Wotname! I am not aboard right now, but will check this first thing tomorrow and see if i can match it up. Based in Lombok at present, but a southern Aussie when back on land.

[GordMay]

Greetings and belated welcome aboard the CF, Nautilus.

[Nautilus2]

Thanks GordMay, i've been a keen follower of technical advice for the last few years, and have often found gems of info that have helped me out of a dilemma. first time to post a direct query!

[rgleason]

The 3ym30 (my version 2013) has a hole with plug fo install a flywheel tach sensor.
We needed one due to inability to adjust for real rpms (varied to much) with an older balmar alt with more rotor sections.

[Taipe]

Originally Posted by rgleason

The 3ym30 (my version 2013) has a hole with plug fo install a flywheel tach sensor.
We needed one ....

Can you please share your source for that after-market flywell sensor and whether you were able to use the original Yanmar teach with it? - Thanks!

[Taipe]

[QUOTE=Nautilus2;2974331]Need help locating the tacho sensor point. ....

There have been problems with innacurate tach readings dependant upon alternator pulley dimensions and belt width.
If you plan to remain with using the Yanmar wiring harnass plug-in to the alternator, you need to know if the alternator pulley matches the original that your engine left the factory with. If not sure, an inexpensive Asian brand tach gun will provide an accurate read from your engine pulley. It will come with an adhesive visual marker that is to be applied to the engine pulley and provides the rereflective reference for the digital reading.

[zengirl]

I don't know if this is of any use to anyone else, but the previous owner left this info on the boat regarding my 3ym30.

[rgleason]

The part was available from yanmar in 2014 and installed. I found it in the parts manual and called Mac Boring to confirm.
It fixed our problem getting a actual rpm reading that was correct at all speeds.

[Nautilus2]

Thanks again Wotname, I found the braided cable from alternator, checked and cleaned the bullet connection,it looked good anyway. Now suspecting a broken wire where a tight cable tie held it to other thicker cables, just before the connector. Any way I can test it before I start cutting? Photo below is upside down!!
Also followed the orange wire through the harnesses right to display instrument and all appears pristine. Therefore suspecting the break in the braid covered white wire direct from alternator. It does feel a bit soft.I’m being very cautious as I have no background in this, just learn as I go!

[Wotname]

Originally Posted by Nautilus2

Thanks again Wotname, I found the braided cable from alternator, checked and cleaned the bullet connection,it looked good anyway. Now suspecting a broken wire where a tight cable tie held it to other thicker cables, just before the connector. Any way I can test it before I start cutting? Photo below is upside down!!
Also followed the orange wire through the harnesses right to display instrument and all appears pristine. Therefore suspecting the break in the braid covered white wire direct from alternator. It does feel a bit soft.I’m being very cautious as I have no background in this, just learn as I go!

First up, the braid covered white wire direct from the alternator is probably good. These wires are surprising robust and rarely fail - it is possible of course but unlikely.

The most important thing to do is to ensure the alternator is delivering a load (i.e. current, not just voltage) - some alternator sensed tachos become flunky if the alternator is not loaded. Easy way to check is to turn on a few loads - lights, bilge pumps, fans and what have you. Check the voltage on the batteries is above 13V when the above loads are on and the alternator is running.

If that doesn't help, go deeper and as I don't know your electrical knowhow, I will try to steer a line between explaining quickly compared with pedantic detail.

Confirm continuity of the P wire (this is braided wire in the photo) from the alternator to the back of the tacho.

Confirm the P wire is not shorted to ground (engine chassis, -ve battery wire etc)

Measure the AC volts on the P wire while the alternator is loaded - should be more than a couple of volts (AC); the exact valve isn't critical and it should be consistent if measured when open circuit and if measured connected to tacho.

Measure the DC volts on the tacho + & - terminals, should be 12+ volts DC.

If you do these tests correctly and the tacho still isn't working, then the tacho is likely to be faulty.

If you are unsure that you are doing the tests correctly, report back and I (or someone else) can walk you though each one.

The only way to learn

[Nautilus2]

Ok thanks I’m on to it, electrical knowledge basic.

[Wotname]

OK, good luck and hopefully you will find the smoking gun

[Nautilus2]

Aha!!! Did step 1 and tested alternator, fridge, lights, sub etc on. Increased revs in neutral to roughly 2000 by ear. Voltage Stayed at 12.7.

FYI I installed New batteries a few months ago before I left the boat for 3 months. I wired in the new batteries Very Carefully . Boat on the hard. After this, the engine mounts were done by mechanic, boat returned to water, engine good no vibration, and run for a few hours over a three day visit to the nearby Gillis. With solar power, didn’t notice particular lack of battery power, but in hind sight I do recall wondering about the engine running but voltage not reaching the normal 14 pluS. Was on a deadline to return home, left it till my return now to sort out.

So it seems I have been searching in the wrong place, ?

At the rear of alternator is a two pin I- inset. The cable with the matching pins is not connected. When I connect it and start th engine, the red battery light comes on on panel. I am assuming this is also indicating alternator not charging? I have no idea if this used to be connected before the work was done.

I am happy and grateful for any help you can give me on this, Wotname!