nervous tachometer

[steven f.]

I have a Westerbeke 21a (24 hp) in my 81 H33. The engine has 1400 hrs on it and the engine was installed new in 1985. My question is in reguards to the tach. Several months ago I noticed that while motoring nicely around 1500 RPM's the tach's needle would just jump up to 2000, the engine did not change at all, just the tach. This occured several times and the amount of the jump would be from 200-500 RPM. Well now the tach is reading way off, its high by about 800 RPM's. I use my boat a lot and am (like most of you) intimately familiar with my boats engine noises, both the good and the bad one's so I'm sure my actual RPM's are within normal limits. All my other guages are correct and all the electrical guages are giving the correct voltages. I'm guessing this is probably the tachometer itself but just wondering if it could be somehow related to the alternator, ideas are welcome, thanx.

 

[Gordon Torresen]

Tachometer jitters

Your guess is probably right but you might want to make some checks first. Your alternator driven tach counts the AC cycles at one of the diodes in the alternator. It is hard to see how you could be getting a consistantly high number of pulses. It could be that there is some rf noise being generated that the tach is sensing. It could be a loose connection that is generating random pulses. You should confirm the the ground (-) connection on the tach is good and that the alternator is securely grounded to the engine block through its mounting bolts. all other wire connections should be clean and tight. There is also a calibration potentiometer (or dip switches) that could have dirty contacts. These can sometimes be cleaned by operating them up and down. If you do that, you will probably want to check the accuracy with a hand held tach. If you install a new display you will have to calibrate it, too.