Alternator, Voltage Regulator / Tachometer Question.

I have a Pilgrim 40 with a 100 hp Westerbeke main engine.
I am having trouble with my tachometer. The needle fluctuates back and forth while running my engine. Sometimes it is steady, but most of the time it bobs back and forth for 0 – 1500 RPM (or that speed my engine is running). The weekend while moving slowly, I could easily hear my engine running. Every time the tachometer would work the engine would change tunes (you know what I mean) like a slight load was being put on the engine. Then when the tachometer would stop working the engine would change tunes again.
Could this unusual tachometer operation be related to a bad alternator or voltage regulator?

Sounds like a loose wire or a loose ground.

Clean everything , and remember the alt tach is not very accurate.

A slipping belt will also change the readings .

I may have to remove and clean all the connections on the alternator. __________________

Clean the terminals on the back of the Tach , and be very wary of any splices in the circuit.

If you have some handy.... when you clean the contacts on the alternator and the tach...wipe them off with isopropyl alcohol..... it will eliminate any residual film or corrosion you can't see.

Alfton said:

The weekend while moving slowly, I could easily hear my engine running. Every time the tachometer would work the engine would change tunes (you know what I mean) like a slight load was being put on the engine.

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Some tachs require the alternator to be putting out current
If you batterys are charged fully I have seen the tach shut off as you describe

Try to load up you 12v system by turning on a few things,and report back

Stray-Cat said:

Some tachs require the alternator to be putting out current
If you batterys are charged fully I have seen the tach shut off as you describe
Try to load up you 12v system by turning on a few things,and report back

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This is exactly what it sounds like to me too. Another charging source can cause the same behaviour - genset running and charging the batts through an inverter/charger, solar charger. If the tach takes its signal off the alternator then the alternator needs to have the field energized for the tach to work.

If my generator is running and the battery charger is on my tach will cycle on and off as you describe.

JAT said:

If you have some handy.... when you clean the contacts on the alternator and the tach...wipe them off with isopropyl alcohol..... it will eliminate any residual film or corrosion you can't see.

Click to expand...

Or, use some electrical contact cleaner from the home center or electrical supply store. It's made for the purpose and comes in a convenient spray can.