Crazy Tachometer

[MapleSugarMan]

Our 1980 Cal 39 has a Perkins 4-108 with a Balmar 9115 dual output alternator and Teleflex diesel alternator type tachometer.

When I first start the engine, the tach doesn't register for about 2 minutes. (Mechanics have told me this is normal with the type of alternator we have.) Then it goes up to whatever rpm the engine is running at and with the exception noted below it has always worked perfectly. It has done this since we bought the boat 3-1/2 years ago.

While I was warming up the engine yesterday, idling at 700 rpm, and the tach giving a normal reading, the tach suddenly jumped to 2800. The engine, however, was still running at the same speed.

This happened once before about a month ago, lasted about 5 minutes, and then the tach went back to normal. This time, no matter how I adjust the engine speed the tach remains pegged between 2800 and 3200.

We have checked and cleaned all the electrical connections on the back of the tach. And we have found and changed a 10 amp fuse that may be part of the alternator-tach circuit. The fuse looked OK, but we changed it out anyway.

We still can't get the tack to read lower than 2800 rpm.

Any ideas?

[captjcook]

I would start by checking/cleaning ground connections, block included. Next, if possible, connect a different tach (I have original 3" that I replaced with a 4"), this lets you know tach/alternator. If tach, replace or depending on type, dip switch or potentiometer (rotary) rpm fine adjust, you might try to operate dips (noting 1/0 code) or rotate screw noting original position in an attempt to "clean" contacts (need non-contact or mechanical tach to re-calibrate) salt air takes its toll. Others may have a more savvy approach....


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[belizesailor]

Could be related to the regulator. Do you have a spare to swap in or the ability to go back to internal regulalor?

[RunawayC47]

I have a Volvo MD2040 with a 60 amp alternator and the tach started jumping around and then goes to zero and the alarm comes on. Occasionally it will jump up even higher than engine rpm. The belt is good and therefore the engine is cooling properly. Also there is no output from the alternator. My other engine is charging properly?

Alternator had been running properly for several days cruising. It happened after starting the engine with batteries at 11.8. While running the alternator to charge my wife also used the inventor putting a strain on the electrical system.

Is this a bad alternator caused by over stressing it?


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[CaptTom]

Don't overlook the possibility of interference. My tachs go wacky when I turn on the 12V florescent ER lights.

[Stu Jackson]

If it ain't at the alternator end, then it's either the wiring (rare) or the connecvtions at the OTHER end.

Check the tach. Depending on what tach you have, use Google to find out how to check it out. All manufacturers have calibration and checkout troubleshooting guides.

Good luck.

[RunawayC47]

Suddenly the alternator worked fine today. Looking at the wiring. I suspect a bad ground neutral connection.


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