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Squeaky alternator

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Recently noticed that my alternator (Lucas A127) has started squeaking a little at idle. Originally I thought it was because Slindon mud so I hosed it out but the intermittent squerak remained.

What I did notice was that a large squirt of WD40 on the stator plates (the laminated plates visible between the end housings cured it for a minute of two).

So last night i whipped it out and had a good look, no mud visible and the bearings were fine no axial or radial movement, pushed a bit of grease into the bearing at the pulley end and re-fitted it. Started the engine and blissful silence :)

Then I remembered all the dire warnings about not spinning the alternator if it wasn't plugged in, I had forgotten. Stopped the engine plugged in the alternator and started the engine again ... the squeak was back.

The battery light went out so i don't think any damage was done, but I'm curious as to the cause of the noise. My guess is that when the field coils are enegergised there is some sort of vibration or movement (perhaps the squirt of WD40 on the plates had some effect here).

I know that when you load up an alternator with lots of electrical equipment they tend to resist being rotated, hence the squeaky belt (when the belt is old or loose) you can sometimes get when headlights and fans are on that disappears when you switch them off.

Any ideas, the only real clue I have is that with the alternator elactrically disconnected the noise disappears.

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I don't know what causes this but, like so much, it seems to come on with age. Mine does it too but only on cold or damp mornings and I've also had it on other vehicles I've owned. Like you I've found that it is temporarily cured by a squirt of WD40 or similar and have thus assumed that it is bearings or similar. So I'd welcome a definitive answer so I can effect a permanent cure.

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