Yanmar charging light faintly glowing, 13.8V across battery

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GlennG

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Have a Yanmar 2GM20. The charging light's glowing faintly and I'm pretty certain that it's not charging the batteries fully. When the engine's running at 2000RPM, there's 13.78 across the battery terminals.

Haven't started dissecting things yet; would appreciate pointers.

System's bog standard with a battery switch (switched to both batteries). Batteries are old.

Cheers,
Glenn
 
How old is the alternator?
Could be a sticking brush in the alternator or one of the carbon brushes is almost worn out.
It should be possible to get replacements.
 
It's often caused by one diode in the rectifier array blowing but can also be poor wiring and brushes as already suggested. It's worth unscrewing the end plate of the alternator where the terminals are and cleaning everything up. The fixings provide continuity for a couple of connections.
 
Thanks for that.

The engine was replaced in '99. Like all sailing boats she's not used that much, but I guess it's high time for an alternator service.

I should have removed the alternator yesterday. Will now have to wait for the weekend. Hindsight is a great thing:-)


Cheers,
Glenn
 
GlennG said:
Have a Yanmar 2GM20. The charging light's glowing faintly and I'm pretty certain that it's not charging the batteries fully. When the engine's running at 2000RPM, there's 13.78 across the battery terminals.

Haven't started dissecting things yet; would appreciate pointers.

System's bog standard with a battery switch (switched to both batteries). Batteries are old.

Cheers,
Glenn
Click to expand...
Classic sign of brushes going
Stu
 
GlennG said:
Have a Yanmar 2GM20. The charging light's glowing faintly and I'm pretty certain that it's not charging the batteries fully. When the engine's running at 2000RPM, there's 13.78 across the battery terminals.

Haven't started dissecting things yet; would appreciate pointers.

System's bog standard with a battery switch (switched to both batteries). Batteries are old.

Cheers,
Glenn
Click to expand...

I would check the output at the alternator itself to see if correct. If ok something wrong in the wiring.
 
pampas said:
Chargeing ok, so its the w/l trio gone and not brushes.
Click to expand...

Not convinced it would be charging ok.
It could be barely charging.
I say this from having experienced the same problem.
In my experience increasing the engine revs put the light out but it got progressively worse until the light wouldn't go out at all.
When I dismantled the alternator I found one brush with little wear and the other brush heavily worn suggesting that one brush was sticking.
Applying WD40 to brush and reassembly fixed the problem for a while but returned a couple of months later.
Hence my reason for suspecting brushes.
 
Its difficult to diagnose these problems sometimes even when "hands on"


There may be two "faults" one causing the apparent low volts and the other causing the warning light to glow.

The low volts may be due to a knackered battery .. (worth taking it off and charging it and testing it ashore) or to a defective regulator, rectifier, field diode trio or possibly the brushes.

Not convinced about the brushes ..I suspect not, they are usually easy to inspect BUT the W/S manual does not give a figure for the minimum length!

As suggested check the volts output from the alternator... measure between the case, or earth terminal if it has one, and the main output connection. If significantly different from the reading at the battery terminals then start looking for bad connections( including the negative) between the two.

Do what you feel you can to check the alternator but the time will come when the sensible thing to do is to take it to an auto electrician for proper diagnostic testing.

The waning light glowing I think is probably due to a bad connection somewhere, not necessarily an alternator fault. It should go out once the output volts gets close to the battery volts (which it is obviously doing and exceeding) but it can stay on faintly if a poor connection provides sufficient volts drop to keep it glowing.

I'd start by charging and making sure the battery is Ok .. you say it is old .... and checking all the connections between the alternator and the battery.

Depending on how far you feel confident to dabble with the insides of the alternator the next step might be to get it properly tested.

I have successfully diagnosed and repaired alternator problems ( brushes, rectifier, regulator and even slip rings) but I have also failed miserably to do so and ended up at the repairers.
 
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pampas said:
Chargeing ok, so its the w/l trio gone and not brushes.
Click to expand...

No its not - read OP: "The charging light's glowing faintly and I'm pretty certain that it's not charging the batteries fully. When the engine's running at 2000RPM, there's 13.78 across the battery terminals."

Should be more volts, usually 14.2v on a charged battery. Agree with others - brushes/ diode/duff wiring.
 
Incidentally I have a spare Yanmar alternator (80 Amp.)
It's hardly used as I took it off and replaced it with a Balmar Alternator plus added a second Balmar alternator. I don't know what size is on your engine but this is off a Yanmar 4JH4E.
 
GlennG said:
Have a Yanmar 2GM20. The charging light's glowing faintly . . . .
Click to expand...

Oldsaltoz is spot on.

As an ex RAC engineer on the Lucas course I did, I can assure you it is a poor earth which is caused by high resistance through one of your main heavy current leads.

This can be on the earth side and although your alternator is bolted directly onto the engine, the poor earth fault can be on the earth lead from the engine back to the battery.

It can also be on the live heavy current side back to the battery.

You can test for this fault using a mulitmeter, a cheap one will do. Set it to 12 volts or 15 volt range and check every joint on the charging circuit between the wiring and the terminal to which it is attached.

There should be zero volts at every joint/junction. In the attached diagram, you can see that there is 4volts on one of the tests. This terminal needs to be broken down cleaned and re-attached using standard engineering practice, ie vaseline to stop cathodic action/corrosion and hence high resistance.
 
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lenseman said:
Oldsaltoz is spot on.
As an ex RAC engineer on the Lucas course I did, I can assure you it is a poor earth which is caused by high resistance through one of your main heavy current leads.
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Well I hate to disagree with you but here goes.
We need to know one other thing from the original poster.
Is the Alternator light glowing faintly or brightly when the key is on but the engine not running?
If the light is glowing brightly then it won't be an earthing issue.
 
Talulah said:
Well I hate to disagree with you but here goes.
We need to know one other thing from the original poster.
Is the Alternator light glowing faintly or brightly when the key is on but the engine not running?
If the light is glowing brightly then it won't be an earthing issue.
Click to expand...

I fully agree whole-heartedly with you but he stated "glowing faintly" but the poster would only be referring to this condition with the engine running. He does go on to state at 2,000rpm?

If it was a phase diode (or two) the alternator would be far too hot to touch (>+60°C) and the ignition light could be hard on or totally off but this is not the fault.

If the alternator is cold or just warm, it can be a brush and it is always the outer brush unless the centre brush has drilled through the copper slip-ring. :)

But back to the problem in hand . . . . glowing faintly at 2,000 rpm. :)
 
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lenseman said:
I fully agree whole-heartedly with you but he stated "glowing faintly" but the poster would only be referring to this condition with the engine running. He does go on to state at 2,000rpm?

If it was a phase diode (or two) the alternator would be far too hot to touch (>+60°C).

If the alternator is cold or just warm, it can be a brush and it is always the outer brush unless the centre brush has drilled through the copper slip-ring. :)

But back to the problem in hand . . . . glowing faintly at 2,000 rpm. :)
Click to expand...

Glowing faintly at 2000 revs is how I read it to. Which is why I didn't understand why you're convinced as per your previous post that it's a poor earth connection. (And then you've just said it could be the brushes.) Confused....
 
Talulah said:
Glowing faintly at 2000 revs is how I read it to. Which is why I didn't understand why you're convinced as per your previous post that it's a poor earth connection. (And then you've just said it could be the brushes.) Confused....
Click to expand...

Glowing faintly will always be a bad earth, never a brush.

A brush fault will always be lamp on or lamp off never glowing.

What causes people to think it is caused by the brushes is the fact that to inspect the brushes, they remove the battery live terminal (or earth terminal) remove the alternator. The brushes are then inspected, repaired or changed and everything is returned to the engine and the battery terminals are then reconnected.

Everything is good, no glowing lamp and it is therefore assumed that the brushes we at fault when in fact disturbing and reconnecting battery terminals or alternator body has in fact effected a cure. :)
 
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lenseman said:
Glowing faintly will always be a bad earth, never a brush.

A brush fault will always be lamp on or lamp off never glowing.

What causes people to think it is caused by the brushes is the fact that to inspect the brushes, they remove the battery live terminal (or earth terminal) remove the alternator. The brushes are then inspected, repaired or changed and everything is returned to the engine and the battery terminals are then reconnected.

Everything is good, no glowing lamp and it is therefore assumed that the brushes we at fault when in fact disturbing and reconnecting battery terminals or alternator body has in fact effected a cure. :)
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What would be the consequence of one brush making a good contact and the other not?
 
Like I said.
Its difficult to diagnose these problems sometimes even when "hands on"


Lensman has shown the essentials of checking for poor connections, but don't overlook the possibility of a poor connection within a crimped terminal.

I don't agree with him that the glowing warning light could be due a poor connection in the negative although it could be on the positive side.
A poor connection on the negative could give rise to the low volts reading however.

There is not likely to be a poor connection on the main battery cables, that is between battery and starter solenod or between battery negative and engine block as that would show up when starting, giving a sluggish cranking speed.
( If cranking is sluggish that's where to look first though!)
 
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