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Old 07-07-2015, 10:04 AM   #1
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Tacho Help

Single engine Yanmar 6LPA STP. Tachometer seems to operate OK at first (though the readings are suspect -- seems to read higher RPM than I would expect at a given speed). But after a while it just kind of... fades out and dies.

I suspect (hope) that it's just some connection that needs to be cleaned or tightened. Can anyone direct me to the exact location of the tach sensor on this engine and describe what it looks like. I have the service manual, but it's not very helpful.

Also, if it's not the connection at the sensor or at the instrument panel, where else should I look? Could it be a weak battery? The two batteries are 4-5 years old, well within their 84 month warranty, and appear to be holding a charge OK. But, the purchase surveyor noted that I should probably replace them due to their age.

Thanks.
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Old 07-07-2015, 11:20 AM   #2
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Sorry I can't help you with that engine. I installed one of these on our boat and love it. Seems to be very accurate.

Design Technology, Inc., Home of the Tiny-Tach - Diesel Tiny-Tach Tachometer
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Old 07-07-2015, 12:23 PM   #3
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Sounds like it might be an alternator driven tach? Those are driven from a stator tap on the alternator (and need to be calibrated for belt ratio as well). As the battery becomes charged and alternator output is scaled back stator output can drop too low for the Tach to pick up.

Has it always done this? Is this the original alternator and regulator, or has one or both been upgraded?


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Old 07-07-2015, 12:39 PM   #4
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I don't think it's alternator driven. The service manual shows a general location for the tach sensor by a dashed circle located on the bottom of the engine near the transmission, not near the alternator. Problem is, I can't see the underside of the engine, and if I could I'm not sure what to look for. I'm going to try to find a mirror that I can use to look around under there. Or maybe the manual is wrong or I'm reading it wrong, and it's in a different location.

I was just wondering if anyone familiar with this engine could help me pinpoint the location of the sensor, tell me what it looks like, and maybe offer any other suggestions on what I should check.
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Old 07-07-2015, 01:39 PM   #5
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OK, yes - I looked up a manual on your Yanmar, and it does seem there is a tach sensor in the wiring diagram. Typically these are about 1" around and bolted into the flywheel housing - it picks up the teeth on the flywheel.

Here is one from a smaller yanmar, to perhaps give a hint?


Good luck, and let us know what you end up figuring out!

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Old 07-07-2015, 02:04 PM   #6
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Those engines have a tach sensor on the port side of the flywheel at the 90 degree position.

This website has Yanmar service manuals which show the position.
Yanmar Marine | Barrus

The most likely problem is that the sensor is dirty and just needs a cleaning. If they are like others I have removed, the clearance between the sensor and the flywheel teeth is critical.
Usually to clean, its a matter of backing off the locknut while holding the sensor from turning. Then gently turn clockwise by hand until the sensor just touches the flywheel teeth while counting the turns. (maybe 2-4?) Then unscrew and clean the dust off the magnetic head.
Replace sensor by screwing in by hand and just touching the flywheel. Then back off to the original starting position & tighten the locknut.

Edit
Hmm - the photo above doesn't look like it has a locknut like the Cat sensors I described.
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Old 07-07-2015, 05:30 PM   #7
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Thanks Thomason! Now I know what to look for. I'm heading down to the boat now to take a look and poke around some.

AusCan, thanks for the info, but I think I'll get a mechanic to perform that procedure for me if it comes to that. I just want to check the "easy" stuff first.
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Old 07-07-2015, 08:11 PM   #8
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Got a nice large hand mirror ,type use by little girls to play makeup at the dollar tree and has served me well.
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Old 07-08-2015, 12:17 PM   #9
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Well, I found the sensor and it looks very much like the one in the photo thomasonw provided. It really helps when you know what to look for! It was at the 90 deg position off the flywheel like Auscan said, but on the stbd side (i.e., at 3 o'clock if looking forward). Thanks for the info.

The connections at the engine and at the tach appear to be OK. I started the engine at the dock and ran it in neutral for several minutes at various rpms and the tach seemed to operate normally, though it fluctuated a little at first. I expect that next time I take it out, I'll have the same problem with the tach dying on me. We'll see.
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