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Old 26-04-2012, 07:57   #1
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tach reading

My tach doesn't read rpm all the time. The problem started last year and when onto the "when I get around to it list". I'm had the same type of problem in the past and it was just some corrosion on the wire terminals for the sensor so the other day I cleaned those.

But this problem is different and I don't really understand why it would do what it it doing. The tach reads fine on start-up when the engine is cold. But after the engine warms up it just reads zero. So it seems that the issue is a temperature related one. I can not believe that the wires themselves could possibly heat up enough to result in enough resistance to stop the current for the sensor.

Since it works on cold start-up I feel I can disregard a lot of the troubleshooting items such as voltage to the unit, wiring connecions etc. And the only other thing to check would be using a syroscope (which I don't have anyway) when the engine is running and warms up to check the output of the snesor.

Is it possibe that the flywheel sensor itself could heat up a point that it stops working?

Anyone else ever seen this as I hate replacing sensors based on the "if" principle of troubleshooting?

PS - the circuit has nothing to do with the alternator and has nothng to do with charging.
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Old 26-04-2012, 08:02   #2
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Re: tach reading

What sort of engine?
What happens if you turn the engine off after it is hot (thus no tach operation) and then restart a two or three minutes later?
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Old 26-04-2012, 08:30   #3
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Re: tach reading

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What sort of engine?
What happens if you turn the engine off after it is hot (thus no tach operation) and then restart a two or three minutes later?

Not that I feel it matters but the engine is a Yanmar 4JH3E.

And if I turn it off and wait a few minutes the tach is still zero. But the next day it is fine on start-up till the engine gets warmed up again.
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Old 26-04-2012, 08:56   #4
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Re: tach reading

I agree it does sound like a heat issue and I agree that if it is, then wiring / connectors are unlikely to be the issue.
Certainly the sensor could be affected by heat even if this is not common. Perhaps you could apply heat to the sensor (in situ) when the rest of the engine is cold as see what happens. Or remove sensor, heat it up and pop it back in quickly. Or apply freezer spray to the sensor after if has stopped working - this may be the easiest test - you can get the spray from electronics outlets - well you can in this country anyway - YMMV.
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Old 26-04-2012, 09:21   #5
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Re: tach reading

Had an intermittent tach on my 4JH3-TE. Didn’t seem to be temp related though. It was corrosion on the connector wiring at the connection to the sensor. Cut back about an inch of wiring and it was cured. Might help. Good luck with it.
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Old 26-04-2012, 09:24   #6
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Re: tach reading

It sounds like a heat problem but not like you are thinking. I would bet it is a bad wire connection which disconnects (opens) as the metal expands with heat.

First try cleaning all your connections and if that does not work then you may need to recrimp. Check all your connections with an ohmmeter when they are hot before recrimping.
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Old 26-04-2012, 09:26   #7
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Re: tach reading

It will be wiring or plugs / sockets 100%. 4 wires are involved 2 from the sensor (you can check at least the DC resistance about 250 ohms from memory) the other 2 provide the 12 volts. Most likely the fault is at the engine end where heat & oil causes the wires to become brittle with age. Just replaced all the wiring at the engine & fitted a better type of plug on the loom to the control panel on one of my Yanmars (& a solid state relay for the starting circuit )

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Old 26-04-2012, 09:45   #8
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Re: tach reading

The quick connector on the wiring harness could be the culprit. I basically bypassed the entire connector on my 410, replacing it with proper butt connectors. Easy job.
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Old 26-04-2012, 13:56   #9
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Re: tach reading

Remove the sender (24mm wrench from memory) and clean any filings from the end and retry. These senders do not have a depth adjustment and need to be kept clean.
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Old 26-04-2012, 16:04   #10
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Re: tach reading

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Originally Posted by NoTies View Post
Remove the sender (24mm wrench from memory) and clean any filings from the end and retry. These senders do not have a depth adjustment and need to be kept clean.

How would that match the conditions I gave?
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Old 26-04-2012, 16:08   #11
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Re: tach reading

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The quick connector on the wiring harness could be the culprit. I basically bypassed the entire connector on my 410, replacing it with proper butt connectors. Easy job.
On my last boat I think on 1 of the harness connectors at least half had been bypassed. The probem was that everytime one needed to be bypassed the wires were too short so had to add wire at the same time. If the tach didn't work at all I would jump right on this and I'm sure will be checking this along the way. At least the engine access on the 410 is great compared to most boats!
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