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Old 24-09-2020, 06:52   #1
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Yanmar Tach Reading Way Too High

My 4jh3 tach is reading WAY high, like a 1,000+ over. This is a flywheel pick up unit and I have not replaced the sensor or the tach. I have had issue with it in the past and they have all been just corrosion on the sensor. It had been reading low so I cleaned the sensor connections and replaced the spade connectors on the pick-up wires. Now it reads real high and will even peg out the meter at 1/2 throttle. Since the tach is just reading pulses from the flywheel I don't understand why/how it can suddenly read high.
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Old 24-09-2020, 17:23   #2
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Re: Yanmar Tach Reading Way Too High

No one?
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Old 24-09-2020, 18:23   #3
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Re: Yanmar Tach Reading Way Too High

My 4JH4E tach is driven by the alternator. When I upgraded to a Balmar alternator the pulse count changed so the tach read too high. The Yanmar (VDO) tach has a button on the back that lets you adjust the tach and how it interprets the alternator pulses. The instructions are basically this
https://j109.org/docs/vdo_programmab..._hourmeter.pdf

If yours really goes off the engine you likely have a different setup. If instead it does come off the alternator that is how you would change it.

I suppose in either case, the question is why did it suddenly start reading high.


This might be relevant too: https://www.cruisersforum.com/forums...h3e-23223.html


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Old 24-09-2020, 18:45   #4
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Re: Yanmar Tach Reading Way Too High

Bad sensor connection making and breaking with the engine vibration?
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Old 25-09-2020, 11:09   #5
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Re: Yanmar Tach Reading Way Too High

Mine does as well (3ym30) but only occasionally and then it suddenly goes off scale. I have to stop the engine to reset it which can be a pain. Looking fwd to more replies to you Sailorboy.

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Old 25-09-2020, 15:41   #6
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Re: Yanmar Tach Reading Way Too High

Mate had a similar problem on a Volvo. Try unscrewing the sensor bit by bit while someone watches the gauge. We suspected slight wear in the main bearing which moved the flywheel closer to the sensor.
Can't remember exactly but was it supposed to be 30 thou clearance? Not sure how you are supposed to check that though!!
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Old 25-09-2020, 15:50   #7
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Re: Yanmar Tach Reading Way Too High

To my understanding of how th sensor and tach work I don't see a sensor clearance causing a high reading. I can see it causing a low one, but not high.
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Old 25-09-2020, 16:01   #8
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Re: Yanmar Tach Reading Way Too High

FWIW, the Yanmar GM and HM series tach sensor has a M18 x 1.5 thread; I THINK the 4JH3 has a similar thread.

The clearance should be 1.3mm (~50 thou).

To measure, screw the sensor in until it just touches the ring gear and back out almost one full turn.

The smaller the gap, the greater the sensitivity so I suggest SB1 try resetting the gap.
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Old 25-09-2020, 16:11   #9
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Re: Yanmar Tach Reading Way Too High

Quote:
Originally Posted by sailorboy1 View Post
To my understanding of how th sensor and tach work I don't see a sensor clearance causing a high reading. I can see it causing a low one, but not high.
If the sensor and tach were precision instruments I would agree however I wouldn't call the Yanmar parts 'precision'.
The sensitivity of the sensor is function of the clearance and ring gear speed so regardless of the theory I would suck it and see.

If you do try it, please report back .
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Old 25-09-2020, 16:24   #10
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Re: Yanmar Tach Reading Way Too High

Quote:
Originally Posted by sailorboy1 View Post
To my understanding of how th sensor and tach work I don't see a sensor clearance causing a high reading. I can see it causing a low one, but not high.


Don’t know a thing about these, but if the clearance is off (or something’s dirty or otherwise marginal) I could imagine the signal from a single pass of the sensor crossing the ‘sensor is here, sensor is not here’ threshold multiple times. This would look like higher RPMs.
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Old 25-09-2020, 17:44   #11
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Re: Yanmar Tach Reading Way Too High

Here is the thing:
- the tach was working fine
- due to a leak above the sensor it was shorting and reading low
- fixed the leak, clean the sensor contacts, put new spade connectors on the wires
- now it reads high

I am resistant to messing with unscrewing and missing with sensor clearance till last resort.
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Old 25-09-2020, 18:07   #12
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Re: Yanmar Tach Reading Way Too High

Quote:
Originally Posted by sailorboy1 View Post
Here is the thing:
- the tach was working fine
- due to a leak above the sensor it was shorting and reading low
- fixed the leak, clean the sensor contacts, put new spade connectors on the wires
- now it reads high

I am resistant to messing with unscrewing and missing with sensor clearance till last resort.
Maybe it was the faeries messing with you.

It ain't my job to tell you how you should 'fix' your tach problem. There are many approaches; all I can do is suggest how I might fix it and I really like the simple cheap approach in the first instance.

Me, I would loosen the sensor locknut, back out the sensor 1/4 turn, retighten the locknut and try it. If that didn't work, I would try 1/2 turn.

Still no difference, then I would wind it in until it touched the ring gear and back out 7/8 of a turn.

Still no good, I would try to measure the AC volts while connected (at the sensor end and also at the tach end). If the tach end is over a volt or so (AC, RMS), I would be thinking the faeries have smoked the tach or sensor into over reading.

Or just buy some new parts .

I am very interested in knowing what you plan to do.
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Old 27-09-2020, 03:42   #13
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Re: Yanmar Tach Reading Way Too High

At a guess you have an inductive/magnetic pickup that relies on a ferrous toothed wheel passing the pickup to generate pulses. Two things spring to mind that result in the behavior you are seeing.
  • The pickup is vibrating and generating false pulses. Could be loose in its housing (lock nut loose?) or its bracket could be loose or not rigid or the tip of the sensor has come loose.
  • It is important to get the correct gap between the sensor and wheel so that the voltage range of the signal generated is within range of the display otherwise the display could be wrong.
Clearly other things could be wrong, the electronics which converts and reads the raw signal (which could be in the sensor or at the display) could malfunction, but these 2 areas would be where I would start.
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Old 27-09-2020, 03:55   #14
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Re: Yanmar Tach Reading Way Too High

The reason that the clearance of the pickup to the wheel is important is that a magnetic pickup generates a voltage proportional to speed and non linearly inversely proportional to gap. I have seen such sensors generate spikes of over 50V. If too high a voltage, i.e. too small a gap, the electronics which cleans up the waveform and converts it to a form to be displayed can saturate or fail leading to the input to the display being incorrect.
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Old 27-09-2020, 04:20   #15
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Re: Yanmar Tach Reading Way Too High

You said you cleaned the sensor? What with & did you remove the sensor to do it?
Cleaned the contacts? What contacts exactly? I believe it's simply a magnetic pulse & no contact what so ever.
Keep us informed please?
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