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Old 24-06-2020, 14:58   #16
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Re: Tachometer and voltmeter "bounce."

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Originally Posted by a64pilot View Post
Seen that kind of thing often, and most of the time itís a bad ground, as itís more than one gauge, .
Good call. I had the exact same problem with my generator and found that the ground was loose.
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Old 24-06-2020, 15:02   #17
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Re: Tachometer and voltmeter "bounce."

I would firstly check for loose connections.

then I would check the starter switch for alternator excitation circuit shorts or loose contacts. It may not even be connected at all. Some alternators will self-excite and have a small magnet for the purpose--but most do not. That sometimes results in the alternator not starting at all, or running as it pleases--but usually once an alternator begins to charge it continues to do so.

Then I would check the voltage regulator and its wiring...and possibly replace it with a remote adjustable regulator if it is found to be faulty.
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Old 25-06-2020, 14:10   #18
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Re: Tachometer and voltmeter "bounce."

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I don't think the tach wiring has anything to do with the fault but I could be wrong!

The voltage dropping from 14 to 12.7 suggests the alternator output is going to zero volts (zero current) and the voltmeter is faithfully showing the fully charged battery voltage for that second or so. As soon as the alternator is producing current again, the voltmeter is faithfully showing the 14V of the alternator output.

The tach feed from the alternator is the AC voltage across one stator winding. So if the alternator stops producing current then the AC volts on that stator winding will be also zero which is why the tach is dipping. At higher RPM, the hysteresis inside tach doesn't let the tach needle to drop before the AC tach feed comes live again. Again supposition only!

I think you are seeing the alternator output going to zero for 1 second every 18 seconds.

I am assuming the alternator is internally regulated and internally sensed - the resolution schematic you posted earlier is too low for my eyes to make out the fine detail.

Can you transpose the alternators between the engines to see if the fault is internal to the alternator or external to it?

Are there any other charging sources (solar panels etc) connected to that battery of that engine?

Is there any unusual load of that alternator / engine that might be cycling on and off every 18 seconds that is messing with the regulator or the alternate output?

It is a curly one for sure!

So general update: I ran a new test ground from the Tach to the battery directly. This didn't resolve the issue.

Yes, so the alternator is internally regulated and sensed. I can swap alternators between the engines, which is what I plan on doing hopefully sometime today.

As far as other charging sources I have shore power and solar. I've disconnected both during test runs and the issue remains.

I've also purposefully discharged the batteries to around 50% and tried with all other sources of charging off and the issue remains.

I've also tried to run the engine with all loads off at the panel and the issue remains.

During all these test cases the starboard engine works great. (Both are Westerbeke 44A's with Bosch alternators.)

At this point I dunno what else it could be, so I'm going to swap them sometime today hopefully.
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Old 25-06-2020, 16:13   #19
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Re: Tachometer and voltmeter "bounce."

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Originally Posted by billnb View Post
So general update: I ran a new test ground from the Tach to the battery directly. This didn't resolve the issue.

Yes, so the alternator is internally regulated and sensed. I can swap alternators between the engines, which is what I plan on doing hopefully sometime today.

As far as other charging sources I have shore power and solar. I've disconnected both during test runs and the issue remains.

I've also purposefully discharged the batteries to around 50% and tried with all other sources of charging off and the issue remains.

I've also tried to run the engine with all loads off at the panel and the issue remains.

During all these test cases the starboard engine works great. (Both are Westerbeke 44A's with Bosch alternators.)

At this point I dunno what else it could be, so I'm going to swap them sometime today hopefully.
You have certainly eliminated many potential suspects and so far it seems like an alternator problem but I can't think of a rational reason (yet) why a faulty alternator would operate in such a manner.

It might be just semantics but you say you have disconnected shore power and solar and then you say you tried with all other sources of charging off.

Were the other charging sources (if any) turned OFF or DISCONNECTED?

I'm not trying to be pedantic, just trying to to be sure I am fully understanding what you have and what you have already done.

I believe you are on the right path which is to keep getting back to the basics until the problem disappears. The basics being a starting battery, an alternator (internally regulated and internally sensed), the engine loom and the engine panel.

It is very handy that you do have a fully functional system on the other engine with which to compare and contrast and transpose components as required.

Good luck today!
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Old 25-06-2020, 16:18   #20
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Re: Tachometer and voltmeter "bounce."

BTW, are the all the alternator connections clean and tight?

Can you post a clear picture of the alternator terminals once you remove it to assist in the identification of the wiring as I am having trouble seeing the fine detail on the schematic you posted earlier.

Also you do have the model number of the Bosch alternator?
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