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Old 08-03-2016, 02:02 PM   #1
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Tachometer not following directions

So I retired and now I have lots of time on the boat. Wife likes it quiet at home so I am often asked to leave. Lucky me, I can go to the boat. It's given me some time to look at a problem I have had for a long time. The starboard tach. Ocean Alexander 38 twin Cummins 5.9's. Magnetic tach sensor on the flywheel. Both alternators are 120 amp run by Balmar AR5 regulators externally. The alternators are new, and the problem was there with the previous alternator. Tach's are VDO.

Now comes my issue. Port tach works as it should. Starboard tach has a mind of it's own. If the boat is pluged into shore power the stbd tach stays on zero. If I unplug the shore tie and wait for a couple of minutes the tach will come on. If I am underway and start the generator the starboard tach goes to zero. The alternator is still putting out solid power. I do have an inverter, but I have tried it with the inverter off and on, makes no difference. I replaced the oil pressure sensor to no avail. It's just one of those irritating things that you watch while your underway and wonder about what the hell isi wrong. Not that anything ever goes wrong on a boat!
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Old 08-03-2016, 02:13 PM   #2
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Dick, Perhaps reading of my recent adventures with my tachometers will be of interest as well assistance. Pariclarly if you have a 'single wire' alternator(s).
Your situation sounds much as my issue.
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Al-Ketchikan-27'Marben Pocket CRUISER

What happen with my tachometer!!!
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Old 08-03-2016, 02:41 PM   #3
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The difference seems to be the sensor. The Cummins uses a magnetic sensor on the flywheel and yours uses the alternator. Mine runs fine as long as you don't have more than normal float voltage on the batteries. If the inverter/ battery charger is involved, the tach quits. I have ample output from the alternators and the batteries are always up to the right voltage. I can put out a solid 180 amps of 12 volts if I have to.
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Old 08-03-2016, 02:58 PM   #4
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Do you have pictures of the back of the tachs?
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Old 08-03-2016, 05:08 PM   #5
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Going to the boat tomorrow, I will pull one of the tach's and take a picture. I'm wondering if the battery charger on the inverter is putting out a higher voltage than the regulator likes and shuts down turning the tach off? Just another thought. Don't think the voltage is adjustable on the Trace inverter.
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Old 08-03-2016, 07:04 PM   #6
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If your tachs are getting the signal from the magnetic pick up on the flywheel it should have nothing to do with the alternators. The magnetic pickup generates its own pulses and the tach uses that and what ever 12 volt supply it has to produce the readings. The 12 volt supply to the tachs normally come through the start key in the "on" position. My guess is something in that ignition supply is getting messed up.

I switch my boat from alternator fed tachs to magnetic pickups.
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Old 08-03-2016, 08:13 PM   #7
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Quote:
Originally Posted by Dick Geving View Post
If the inverter/ battery charger is involved, the tach quits.
Hi Dick, I would believe the other Dick posting may be close to the issue. In our case, with the engine running and the tachometer not registering, then with the inverter on line and a demand such as the toaster, the tachometer rose to the correct RPM. Turn off the toaster and the tachometer dropped once again to zero.
The next morning underway with the thinking the tachometer was out for the trip home, I was presently surprised to see the tachometer registering the correct RPM and it has to this point and will I am sure. In our case, I believe knowing the alternator tachometer post is internal I will use the word "Pre-charged" ,activated itself under a steady consistent load. Other than that Dick it remains 'Magic, pure magic' and I will not challenge magic!!
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Old 08-03-2016, 10:55 PM   #8
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Filled with great information I will open the floorboards tomorrow and find that miserable probem with my new found knowledge. Thanks guys for all you guidance. Having a great dinner tonight of fresh Dungeness crab, french bread and a small oak finished white libation.
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Old 08-03-2016, 11:03 PM   #9
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You say you have magnetic sensors at the flywheel.
Misreading can often be caused by ferrous debris collecting in the sensor face. That debris will interfere with the signal.

Each time a flywheel tooth passes the sensor an electrical impulse is produced. Ferrous debris will dampen that signal so the sensor/tach cannot read it.

Mark the leads on the sensors.

Pull the sensor, clean it and reinstall. Screw it back in , carefully, untill it touches the flywheel. Then back it out about 1/2 turn to one turn which should leave about 0.020" or 0,5 mm. gap and set the locknut.

Also be sure the power wiring to the tach is in good shape. If the tach doesn't get good power it may cause a misread.
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