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alternator warning light.


dor

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Yesterday I stopped the engine, then restarted it five minutes later.  It now wouldn't charge and the ignition light was glowing dimly.   As I removed the alternator I noticed that the wire to the W terminal had come off, but don't know if this resulted from moving the alternator while taking it off, or whether it came off to cause the problem.  The engine does quite a shake as it stops, so it is possible that if the lucar connector was close to coming off, this might have been enough to do it.

My question is:  Would the warning light still glow dimly if the W wire was disconnected, or does this indicate that something has blown inside the alternator (diode?)?

I fitted my spare alternator and that works fine.  I suspect the dim warning light means the alternator has blown, but would like some confirmation that it is not as simple as the wire coming off.

I didn't refit the alternator to see if checking the W wire was the problem as it was getting late and I needed to charge, which is why I fitted a known good alternator, rather than risk refitting the original one and finding it still wouldn't work and then having to change the alternator again.

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The W terminal is surely the one that usually associates only with driving a rev counter.

It plays no part in the normal charging (or indicator light) operations.

As many alternators have a W terminal present, but with nothing connected to it, I would not expect a wire falling off it to damage the alternator, (unless something more than that happened).  It only moves you to the same place as my alternator is already at - a W terminal, but with nothing wired to it.

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7 minutes ago, Tony Brooks said:

I suspect a field diode has blown as long as you still had plenty of amps and volts, in fact the volts may have gone higher than normal. If you lost amps and volts then it would be a main diode failure. Which ever it is it's a new/overhauled alternator.

Voltmeter showed no change in the voltage displayed.  Tacho didn't move off the stop.   I'm sort of fairly sure it is a diode gone, but wanted to be sure.

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OK, thanks for your replies.  Looks like a visit to David edge.

Looking at the Vetus wiring diagram, it shows the W terminal connected to just the tachometer.  I was thinking it was the one that was necessary to energise the field coils via the warning light, which on reflection I think goes to the D terminal.

Edited by dor
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2 hours ago, dor said:

Looking at the Vetus wiring diagram, it shows the W terminal connected to just the tachometer.  I was thinking it was the one that was necessary to energise the field coils via the warning light, which on reflection I think goes to the D terminal.

Also, think about it... if the wire from the warning light fell off then the light would never illuminate. 

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1 minute ago, WotEver said:

Also, think about it... if the wire from the warning light fell off then the light would never illuminate. 

Yes.  I did think about it, but wondered if the lamp may have received some current from something else on the panel.  Having now looked at the panel wiring diagram I can see that the W wire is the only way to supply that lamp.

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55 minutes ago, dor said:

Having now looked at the panel wiring diagram I can see that the W wire is the only way to supply that lamp.

Do you mean that?

I would expect  W to have Tacho connected, and not be connected to waning light.

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I think at least some Vetus alternators have an extra 12V+/sense wire on a Jap alternator so I would be taking a good look at the manual. Hitachi (I think) alternators may have an energise wire separate from the normal W one. Just shows how important it is to get all the info into a question.

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