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?

[Stu Jackson]

Have you checked the other end of the wires, at the back of the tach?