Me light is still glimmering!

noelex

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I have the same issue on my yacht. I have not had a chance to investigate the problem. The alternators are charging correctly. The yachts electrician claims the light glows when not charging or when overcharging. He claims the glimmer is due an aggressive setting on the regulator resulting in slight overcharging.
Does anyone have an opinion if this explanation is feasible.
Cheers John
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oldharry

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Not so sure about that, Vic. Although the light is not itself earthed, if there is resistance between the alt and the battery it is feeding, then there could be sufficient change of potential at the Ign switch for the w/l to glow slightly as described.

I would start by looking at the engine to battery negative connections, which could put the engine negative at a different potential to the rest of the circuitry - a common fault in marine engines giving strange and surprising results sometimes including glowing charge warn lamps. The simple test is to put a meter across from engine or alt frame to batt Neg. Quite surprising how high a reading you can get from an apparently healthy system!
 

halcyon

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It's probably a field diode failure, allows the lamp to earth every third phase, thus it's light dim. The field diodes provide a voltage to power the rotor windings, and a counter voltage to put out the ign light. Can also get from a sticking brush, ie brush not macking full contact and intermitted output, but a not continuous you can see with a meter.

Brian
 

halcyon

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You would still have a back voltage from the field diodes at the leakage point, thus the back voltage would still be there, thus no light. If you had a very bad short that creates a volt drop in the cable between the alternator and fault, then you could have a dim light, but you need a bad fault.

Brian
 

VicS

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[ QUOTE ]
It's probably a field diode failure, ............. Can also get from a sticking brush

[/ QUOTE ]

Hopefully neither of these are the case as he has already replaced the whole unit but the problem persists.
 

VicS

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[ QUOTE ]
Not so sure about that, Vic

[/ QUOTE ] I assume you are refering to me saying that a bad earth on the panel would not apply. The point is that although there will be a negative connection to the panel for other lights and instruments (Panel illumination and tacho for example) it is not required for the alternator warning light. That would function normally without an earth connection to the panel. As you say, though, a resistance in the altrnator output connection could cause the problem.
 

VicS

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[ QUOTE ]
The yachts electrician claims the light glows when not charging or when overcharging. He claims the glimmer is due an aggressive setting on the regulator resulting in slight overcharging.
Does anyone have an opinion if this explanation is feasible.


[/ QUOTE ] I dont think a defective regulator causing overcharging will cause the warning light to glow. I have had a regulator develop an intermittent fault that caused very serious overcharging for short periods. Sufficiently high to send the 60 amp ammeter full scale. The warning light did not glow though!

Easy enough to check the charging volts though.
 

halcyon

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Then check the voltage either side of the ign light bulb, if warning light connection is higher than the battery voltage, the light will light dim due reverse feed. The field diodes trying to charge the battery via the bulb.

Brian
 

VicS

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[ QUOTE ]
Then check the voltage either side of the ign light bulb, if warning light connection is higher than the battery voltage, the light will light dim due reverse feed. The field diodes trying to charge the battery via the bulb.


[/ QUOTE ]

Thats a good idea. If i understand you correctly you mean that if the alternator side of the bulb shows a small positive reading compared with the battery/ignition switch side there is a reverse fow from the field diodes to the battery. That will indicate a poor connection causing a voltage drop somewhere in the alternator output wiring.
If however the PD across the bulb is the opposite polarity then a defective field diode is a possibility, although unlikely if the whole unit is new.

One reservation though is that some alternators do not have separate field diodes (some SEV IIRC), the field current is taken from the main diodes but is isolated from the battery by a blocking diode in the alternator output. In that case the blocking diode will, create a 0.6 ish volts drop that could be interpreted as a poor connection but i guess 0.6volts is not enough to make the bulb glow even dimly.

Also been thinking about Oldharry's suggetion that a poor connection on the negative side could be the cause. I dont think that will. It has got to be a poor connection on the posive side in the circuit that is parallel with the warning light.

Not sure what the situation would be with a negative regulated set up. I'm not familiar with those and dont even have a diagram to study!
 

oldharry

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<<thinking about Oldharry's suggetion that a poor connection on the negative side could be the cause. I dont think that will. >>

It can: been there and got the tee shirt.

The engine electrical return strap was badly corroded and causing a voltage difference betwen the engine and the batt neg of nearly 2 volts. Starting wasnt brilliant (slow cranking was the reported fault we were looking for, and we noticed the w/l never quite went out) Sorted the starting and the W/L went out too. On that particular installation the return strap only serviced the starter and alternator. Everything else went direct to the batt Neg, and we reckoned the w/l was showing the potential between batt neg and alt neg.
 
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