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Old 12-19-2012, 08:58 PM   #1
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Trouble shooting malfunctioning tachometer

This perhaps should have gone in the Electrical forum, but I was not sure, and so will put it in General. The port tachometer on my Gulfstar 36 has quit working. The engine is a Perkins 4-236. The port engine has a tach both at the helm and on the bridge, and neither of them work. That tells me that probably it is down at the engine, since not likely both of them would go bad at the same time (and they are not the same age). Never did any trouble shooting on a tach before, and not sure where to start, so any suggestions where to start would be much appreciated.

John
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Old 12-19-2012, 10:12 PM   #2
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Our starboard tachs (both helms) used to rarely work. We recently had the engines serviced and now the tachs work. I'll ask my husband if he remembers what the mechanic did... I don't recall.
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Old 12-19-2012, 10:23 PM   #3
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Thanks. I have been reading up, and now know that on a diesel engine the tach works off the magnets in the alternator. I didn't know that! Didn't even realize that tachs were different on a gasoline engine and a diesel engine! So much to learn, and so little time (especially if the world ends day after tomorrow, which I suppose would obviate the need to get the tach working).
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Old 12-19-2012, 10:49 PM   #4
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"now know that on a diesel engine the tach works off the magnets in the alternator."
Not always. A lot of them use a little generator to drive the tach or an electronic fly wheel tooth counter. Follow the wires from your tach and see where they go.


Stewart Warner Tach Sender
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Old 12-20-2012, 01:57 AM   #5
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O got the same problem, both tachs in the fly don't work as well

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Old 12-20-2012, 11:38 AM   #6
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Most diesel tachs work off the AC output of the alternator. If the alternator is not producing any output for whatever reason there will be no signal for the tach. At the dock one of my engine tachs is erratic when the battery charger is charging the battery since that alternator regulator has cut way back.

Could be a bad regulator, is the battery being charged, what is the voltage output of the alternator?

There should be a connection on the back of the alternnator for the tach. Look up the alternator on the web and it should show the connections.

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Old 12-20-2012, 12:22 PM   #7
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What BobH said. It's a good indication that the port alternator is not putting out for some reason. Check with a volt meter which should rise above battery voltage after you start the engine. It could also be just a connection between the tach and the alternator. This wire should be grey in color.
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Old 12-20-2012, 12:37 PM   #8
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There are three types of tachs on a diesel. Magnetic, Alternator or a gear drive generator.

Yanmars drive the tach from a magnetic pick up on the bell housing.

So blanket statements don't work, because there are a lot of Yanmars out there.

Now the good news. Both the magnetic and alternator driven have wires that attach to either the alternator or the magnetic sensor. Both commonly have the same problem. The vibration of the engine may cause the wire to be loose in the clip or the clip it self may be lose on the spade it slides on to. The clip or spade may also be oily or dirty. So check the wire for a good connection and clean up the contact.

Some alternators will not show a charge until the rpms raise the voltage high enough. But I do not think that affects the tach. The diesel alternator tachometer is hooked up the AC tap of the alternator which is before the regulator. This connection can be marked as: AC, AUX, S, R, TACH, or nothing at all.

On the back of the tach here are a few voltages that may be useful. The voltage between the B+ and the Grd. terminals on the back of the tach should be 12V-14VDC. The voltage between the Sig and Grd. should be 2 VDC. If no 12V then find and fix the feed or trace the fuse. If no 2V that means it isn't getting up to the tach from the alt or the alt isn't producing the 2V. That should be easy to check.

Be careful around a running engine while checking the alternator out put.

Hope this helps.
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Old 12-20-2012, 05:45 PM   #9
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The problem is that the tachs inside are working well.

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Old 12-20-2012, 06:24 PM   #10
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The tachometers for one engine consistently read 400rpm lower at the flybridge than at the lower helm; the lower is obviously the correct reading.
Any clues where I should start investigating? Can it just be tired wiring?
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Old 12-20-2012, 06:31 PM   #11
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Do you have an engine synchronizer?
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Old 12-20-2012, 06:32 PM   #12
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Cam both, just all in the fly with problems? I'm piloting by the consume and speed. I don't know if I can fix or must change.

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Old 12-20-2012, 09:16 PM   #13
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jwnall, Same engines with alternator-driven tachs and had the same problem on the port side. I noted that whenever I tied the batts together and increased the load by operating a high draw device (heater or coffee pot on inverter, windlass), the tach would spring to life. I found a loose connection at the alternator and recabled the boat. The port still does not match the accurate stbd at times, but when the load increases, it's most accurate. That port alternator is slated to be replaced with a high output Balmar soon. I'm hoping it goes well and I don't lose my tach readings.

Vinny, you sound like this is an area of expertise for you. If you don't mind, I might PM you during that project if any hurdles arise.
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Old 12-20-2012, 09:33 PM   #14
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Quote:
Originally Posted by BruceK View Post
The tachometers for one engine consistently read 400rpm lower at the flybridge than at the lower helm; the lower is obviously the correct reading.
Any clues where I should start investigating? Can it just be tired wiring?
Look at the backside of the tach....see if there is a setting dial. Also look to see if there is an adjustment screw that is recessed and accessed through a small hole...

One other item....is there any indication of water intrusion into the tach?
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Old 12-20-2012, 09:42 PM   #15
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Check and make sure that through the boats wiring....both tachometers are NOT charging the same battery banks. If they are, then the strongest one will cause the weaker one not to energize and produce electricity. One easy way to check this is to connect a separate battery to the alternator 12V pole and ground....after removing the other cables...and start the engine. If it does not cause the tach to bottom out at 0 rpm when turned on....you either have a defective tach....or have a defective alternator. With the engine running the tach connection should be putting out over 4 volts....

Also, check the connection at the AC or W (I think its W), and look for continuity to the connection that feeds the actual tachometer. Also, make sure that the tachometer is getting 12V from the ignition circuit.....
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Old 12-20-2012, 10:05 PM   #16
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Checked w my husband and the mechanic just tightened up some connections. Good luck, lots of great troubleshooting ideas on here.
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Old 12-20-2012, 11:47 PM   #17
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Quote:
Originally Posted by FlyWright View Post
Vinny, you sound like this is an area of expertise for you. If you don't mind, I might PM you during that project if any hurdles arise.
No expert, just what I would do to trace down any electrical problem.

Folks have a tendency to jump to conclusions.

Bu PM me if there is something I might be able to answer.
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Old 12-20-2012, 11:59 PM   #18
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No expert, just what I would do to trace down any electrical problem.

Folks have a tendency to jump to conclusions.

Bu PM me if there is something I might be able to answer.
You sure seem to have a better understanding that I do. Thanks!
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