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Old 09-09-2019, 09:00 PM   #1
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Lehman 120 Alternator Capacitor?

While trying to figure out why neither my FB nor Salon helm tachometers are working, I came across this capacitor that is not attached to anything. Anyone know what this is originally for and if it needs to be wired into something?

(For some reason when I attach any pic, it rotates it on it's side...)

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Still haven't figured out the Tach's either...
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Old 09-09-2019, 09:13 PM   #2
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Are you batteries charging? What do your voltmeters show, they should be 13+ volts.
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Old 09-09-2019, 09:18 PM   #3
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That is hard to guess not knowing where the wires go, but at 20000 MFD with 50 WVDC and located in the engine room...

...I'd guess it was once used off of an alternator as a noise filter.

But, that is just a total wild guess.
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Old 09-09-2019, 09:21 PM   #4
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Everything is working fine (except the tach’s -but this capacitor hasn’t been hooked up even when they were working). It appears to be unnecessary but wanted to know if it could be identified with an original purpose before I removed it.
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Old 09-09-2019, 09:35 PM   #5
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Brad,

I can't promise you. I really can't. I'm just /totally/ guessing...

20000 MFD is a pretty big capacitor in terms of its capacitance. But, with a 50V DC working voltage (WVDC), it wasn't used on anything 120VAC, e.g. an air conditioner. It is in the engine room attached to an engine. And, 50VDC is a nice rating for something that will actually be used at 12VDC.

The upshot is that I don't think it was there to help kick-start a motor.

With a 20000 MFD capacitance, it would be right there in the range of what one would use to filter DC coming off of an alternator back in the day when really good DC power was needed for computers and electronics.

If I had a guess, and this is a total guess, that looks to me like a late 1970s to early 1980s, maybe mid 1980s capacitor. I'm thinking the boat once had Loran-C or some other sensitive navigation electronics and that capacitor was put in as part of the installation.

Then, sometime thereafter, the electronics were long gone and the alternator needed rebuilding, and they never hooked it back up. Maybe the Loran died, or was moved to a new boat, or sold for fuel money. Or, or, or.

I guess what I'm saying is that the cap is about right for filtering an alternator for sensitive electronics, I think. And not right for much else I can think of that would be right there on the engine. And, some early 1980s era stuff was right there in the sensitive electronics category.

So, if it were me, I'd take it out. It isn't doing anything now. And, it isn't doing anything that needs done today. And, for all we know, someone disconnected it for a reason.

But, this is just me guessing at it.
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Old 09-09-2019, 09:46 PM   #6
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Also, what isn't working about the tachs? In other words, so they all read wrong? Read inconsistently? Not move at all? Twitch or shake but not move? Not move until after the throttle advances? Etc?

They come off of the alternator? (I don't think Lehman's normally come form an auxiliary drive).

So you have a twin or single (I can't see what type of boat)? If twin, are both sides bad?
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Old 09-09-2019, 10:50 PM   #7
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Brad
I have a similar but smaller capacitor wired to the alternator on our Lehman. I’ll check the size and how it’s connected next time I’m at the boat. Never been able to figure out why it’s there. I assume noise suppression.
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Old 09-10-2019, 02:50 AM   #8
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Itís a noise suppressor.
If your alternator is not charging do to the battery bank being fully charged on shore power. The alts will read zero. Try this.
With engine stopped.
Shut off battery charger
Run a large load like a bow thruster or windlass for about thirty seconds to bring battery voltage down below 12.7 volts
Start the engine.
Are the tachs now working?
Does the alternator bring voltage back up above 12.7 in a few minutes?
If yes to both, you are done. Normal.
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Old 09-10-2019, 10:04 AM   #9
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Quote:
Originally Posted by BradC View Post
While trying to figure out why neither my FB nor Salon helm tachometers are working, I came across this capacitor that is not attached to anything. Anyone know what this is originally for and if it needs to be wired into something?

(For some reason when I attach any pic, it rotates it on it's side...)

Attachment 93799

Still haven't figured out the Tach's either...

If both tachs aren't working and they were before, its likely that either the alternator is not working or the tach feed between the alternator and the tachs has broken.

As has been stated its absolutely true that if your battery(ies) is totally charged - the alternator may not come on and this will cause the tachs to display nothing. This can especially happen if you have solar panels connected. In that case - putting a load on that battery should cause the alternator to turn on.

The alternators on a Lehman are usually set up with a field power switch that is activated by oil pressure. That pressure activated switch should have 12VDC to one side of it (all the time as long as the battery switch for that engine is on) and once there is oil pressure - apply that 12V to the other side which should then go directly to the alternator field winding. The oil pressure activated switch is pretty close to the alternator so its easy to trace.

Its also easy to tell if the alternator is working by testing the battery voltage when the engine is running. It should be at least 13+ volts unless the battery is extremely low, but even then it should come up after a little running.

If you can see the voltage rise as the engine is running, then the alternator is working and the tachs should also be working. At that point, its a problem with the wiring to the tachs. On my boat and probably most/all there is a separate power wire to the tachs which also needs to be on for them to work - and of course a return (ground).


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Old 09-15-2019, 01:46 PM   #10
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I appreciate all the great information here. I've removed the rogue capacitor as it does not seem to serve any purpose, as High Wire and others indicated, and now moving to my next issue in another post.

I've started another thread to trace the Tachometer issues here:

Ford Lehman 120 Alternator Tach connection

The engine had a mechanical tach. At some point that was disconnected and a Balmar 110 amp alternator with smart regulator was installed. The tachs now are electronic and come off the Balmar MC-612.

I'm not going to continue this thread as it was specifically for the capacitor question. Thanks to everyone for your input. I look forward to fixing the Tach issues next...
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