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Old 23-06-2020, 19:04   #1
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Tachometer and voltmeter "bounce."

Hello, I have twin Westerbeke 44A's and I've been chasing an odd issue for a week or so. So the port engines tach and voltmeter dip down and then return to normal about every 18 seconds.



So whats been done so far:

The alternator has been rebuilt and bench tested good.

The tach was replaced for a different issue, so it's a new tach.

The panels (admiral version) were swapped and fault followed.

Battery isolator tested good.

Batteries have been discharged and then tested and fault remains.

Preheat solenoid, circuit breakers, senders have been replaced.

No obvious grounding or wiring issues.

Belts are correctly adjusted.


Anyone have any idea's or seen something like this before? Would running a new ground from the back of the tach to the battery make any difference considering I've already rung out the ground?



Anyone had a alternator that benched tested good but went intermittent installed? Swap alternators?



At higher RPM's the tach stabilizes but the voltmeter shows the same drop. The voltage drop is real, you can measure it at the engine starting battery. Makes me think the internal voltage regulator on the alternator is bad, but on the bench there was no drop.
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Old 23-06-2020, 19:29   #2
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Re: Tachometer and voltmeter "bounce."

Quote:
Originally Posted by billnb View Post
Hello, I have twin Westerbeke 44A's and I've been chasing an odd issue for a week or so. So the port engines tach and voltmeter dip down and then return to normal about every 18 seconds.



So whats been done so far:

....

The panels (admiral version) were swapped and fault followed.

...........
No obvious grounding or wiring issues.

....................


Anyone have any idea's or seen something like this before? Would running a new ground from the back of the tach to the battery make any difference considering I've already rung out the ground?


............
I would be looking closer to the wiring in the panel in the suspect panel.

Do you have a schematic of the panel you can post?
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Old 23-06-2020, 19:29   #3
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Re: Tachometer and voltmeter "bounce."

Seen that kind of thing often, and most of the time it’s a bad ground, as it’s more than one gauge, it’s probably a panel ground, often things in a panel will share one ground.
I have no idea why but even on the real complex electrical aircraft I used to work on, almost always intermittent electrical issues were loose or “bad” as in high resistance grounds.
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Old 23-06-2020, 19:40   #4
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Re: Tachometer and voltmeter "bounce."

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Originally Posted by Wotname View Post
I would be looking closer to the wiring in the panel in the suspect panel.

Do you have a schematic of the panel you can post?

Attaching a copy.



What I mean by the fault followed is that when I swap the pigtails that go to the starboard panel with the port the starboard panel (now showing the port engine) displays the fault and the port works fine. So it wouldn't be the panels at that point.
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Old 23-06-2020, 19:42   #5
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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, itís probably a panel ground, often things in a panel will share one ground.
I have no idea why but even on the real complex electrical aircraft I used to work on, almost always intermittent electrical issues were loose or ďbadĒ as in high resistance grounds.

This is what others have told me too, which is why I was thinking about running a new ground from the tach. Shouldn't all the gauges (oil pressure and temp) show the bouncing though if it were a grounding fault?
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Old 23-06-2020, 20:21   #6
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Re: Tachometer and voltmeter "bounce."

Quote:
Originally Posted by billnb View Post
Hello, I have twin Westerbeke 44A's and I've been chasing an odd issue for a week or so. So the port engines tach and voltmeter dip down and then return to normal about every 18 seconds.
.................
At higher RPM's the tach stabilizes but the voltmeter shows the same drop. The voltage drop is real, you can measure it at the engine starting battery. Makes me think the internal voltage regulator on the alternator is bad, but on the bench there was no drop.
How much is the voltmeter dip? A volt or so or many volts?
How regular is the intermittent fault? Every time the engine is run or just whenever?
How long does the dip last? A second or less or many seconds?
How regular is the 18 seconds period? Always or most of the time or it depends?

Apart from the tach stabilising, does the fault follow the same pattern (period and magnitude) from idle though to cruising rpm?

Lots of question I know but the fault is unusual and the more data the better for a remote analysis.

ATM I thinking you are getting a periodic voltage drop from the alternator and the tach is following this as the tach voltage output of the alternator reflects the voltage OP of the alternator. At higher RPM, the hysteresis in the tach circuitry masks the repeating voltage drop.

But why????

Anyhow the answers of the above questions might help (or not ).
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Old 23-06-2020, 21:01   #7
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Re: Tachometer and voltmeter "bounce."

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How much is the voltmeter dip? A volt or so or many volts?
How regular is the intermittent fault? Every time the engine is run or just whenever?
How long does the dip last? A second or less or many seconds?
How regular is the 18 seconds period? Always or most of the time or it depends?

Apart from the tach stabilising, does the fault follow the same pattern (period and magnitude) from idle though to cruising rpm?

Lots of question I know but the fault is unusual and the more data the better for a remote analysis.

ATM I thinking you are getting a periodic voltage drop from the alternator and the tach is following this as the tach voltage output of the alternator reflects the voltage OP of the alternator. At higher RPM, the hysteresis in the tach circuitry masks the repeating voltage drop.

But why????

Anyhow the answers of the above questions might help (or not ).

Hey I appreciate the replies man.

The voltmeter will drop from about 14 to 12.7 volts.

It's very regular, occurs around 18 seconds on average, with a spread of 16 to 20s every time the engine is ran.

It lasts about a second or less.

Always that I've noticed it and actually tried timing it.

Yeah the Tach follows the same pattern at low RPM and idle. Up at cruising speed the Tach more or less stabilizes and hardly dips any RPM's.

I'm leaning towards alternator too, it's just weird that it tested good at the shop. They were using analog meters to measure the output, so it seems like they'd see any brief decrease in output.

I think I might just go ahead and run a ground from the panel down to the battery and see if that does anything. If not just swap the Tach. If you have any idea's I'm open to anything!
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Old 24-06-2020, 01:00   #8
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Re: Tachometer and voltmeter "bounce."

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Originally Posted by billnb View Post
Hey I appreciate the replies man.

The voltmeter will drop from about 14 to 12.7 volts.

It's very regular, occurs around 18 seconds on average, with a spread of 16 to 20s every time the engine is ran.

It lasts about a second or less.

Always that I've noticed it and actually tried timing it.

Yeah the Tach follows the same pattern at low RPM and idle. Up at cruising speed the Tach more or less stabilizes and hardly dips any RPM's.

I'm leaning towards alternator too, it's just weird that it tested good at the shop. They were using analog meters to measure the output, so it seems like they'd see any brief decrease in output.

I think I might just go ahead and run a ground from the panel down to the battery and see if that does anything. If not just swap the Tach. If you have any idea's I'm open to anything!
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!
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Old 24-06-2020, 09:06   #9
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Re: Tachometer and voltmeter "bounce."

***As above*** it sounds like the field voltage is being cut off.

If you have solar , wind, or shore power as a secondary charging source it can confuse the regulator into thinking you have an over charge situation and the regulator will cut field voltage. When this happens you will loose alternator output and the tachometer pulse that goes with it. Tach pulse voltage should be between 6-9 volts AC.

Turn off solar panels and turn off battery charger when running the engine.
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Old 24-06-2020, 09:58   #10
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Re: Tachometer and voltmeter "bounce."

Iíve had a "similar sounding" problem with my Yanmar 4JH2E since it was installed in 1998. Every 15-20 seconds, the oil pressure and temperature gauges flicker. Why? Because the hour-meter is apparently winding up an internal spring and drawing "too much" current. Every time the hour-meter increments, the rest of the gauges flicker. Probably an artifact of the under-sized wire in the Yanmar harness.
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Old 24-06-2020, 10:23   #11
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Re: Tachometer and voltmeter "bounce."

I have to agree with a64pilot, first check the ground. It's easy enough to do with a jumper cable. Might not be it but it's simple easy and cheap to check. (I also do a lot of aircraft work and have seen this before)
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Old 24-06-2020, 11:22   #12
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Re: Tachometer and voltmeter "bounce."

Bad ground. Gauges on my boat used an independent, though shared wire to engine as ground. I replaced many units, until oldtimer told me to clean all ground contacts at each gauge, at engine, any connections, and possibly at battery bus. Gremlins lurk anywhere.

Reading static volts was useless, showed battery voltage. Didn't catch intermittent resistance of bad ground.
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Old 24-06-2020, 12:50   #13
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Re: Tachometer and voltmeter "bounce."

Would agree this could be a charge regulation issue. If you have an external regulator how is it programmed? Try disconnecting charging source to see if it disappears, could be interaction between two alternators or alternators and solar panels could even be the way the regulator is designed to work. The fact that the revs change as well could be due to the load changing as the alternator shuts down.
The other area I would look at is the fuel supply. An air leak or fuel starvation could both lead to this by setting up a cycle where fuel runs short, gets to a critical point where the engine slows then corrects. Could even be a sticky breather valve on the tank that builds up some vacuum then releases but stays open at higher revs
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Old 24-06-2020, 13:24   #14
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Re: Tachometer and voltmeter "bounce."

To those who are suggesting a bad ground issue, you have to ask yourself why is the fault so regular. The OP describes it occurring every 18 seconds with a duration of 1 second and is like this every the engine runs. That is not intermittent or random!

I am hoping the OP will come back and tell us what sort of regulation and sensing is used on that engine's alternator, what sort of other charging equipment is connected to that engine's battery and what other loads are connected to that engine's battery. The answer is likely to be hidden in this detail.

It is possible it is a bad ground but unlikely IME.
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Old 24-06-2020, 14:50   #15
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Re: Tachometer and voltmeter "bounce."

In my case, it wasnít a bad ground, probably just small wires in the harness and maybe a poor connection. Bypassing the ground wire didnít eliminate the problem. Itís been doing it since 1998 and never gotten worse. As speculation only, Iíd guess that the alternator excitation voltage is supplied by the same source as the gauges, so you MIGHT see some fluctuations in alternator output. In my case, the OEM alternator supplies only the start battery, so I never thought to look at it carefully.
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