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Alternator Charging at Speed

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I had a weird incident with my alternator when driving at speed. You are encouraged to turn on your headlights when driving on country roads in Australia to improve your daytime visibility to overtaking cars.


As I headed out of the city, I thought to put my lights on and as I pulled the switch there was a momentary flash from the ‘L’ charging light. I was probably doing about 50-60mph at the time (say about 3,000 rpm). I thought it was odd but there was no issue with the lights when I checked at the next stop and didn’t see the light again at that stage. 


Later on, having stopped for a break, I set off again. This time, I forgot to put the lights on when leaving. I remembered once I was going at 70-80mph (just over 4,000 rpm). When I pulled the switch, the L light came on and stayed on as long as the lights were on. Having flicked them on and off a few times, I slowed down in preparation to take a look and tried again when the engine speed had slowed. This time, I got the one flash of L and then it went out and stayed out at all speeds after that. 


The alternator is an AL41X 65A and when it occurred there was no squealing from the belt and no other loads other than my CDI ignition, 123Tune and a GPS (no stereo fitted). The engine had been running for over an hour, so I expect that the battery would have been charged at that point. 


I can imagine that putting the lights on a speed may add a lot more drag to the alternator but it seems incredible that this would be enough to stop it charging without effectively stalling the belt. 


Could it be the regulator? Might put the multimeter on it while driving tomorrow and see if I get the same effect. 

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I think you need to take a few readings or it's just guess work, I had weird charging issues from a bad connection on the back of the alternator, I removed the plug and connected with spade terminals to ensure a good fit, prior to that I also had issue with a dodgy regulator, but that was the opposite effect of boiling the battery!

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