Battery Warning Light VW 1.6 D Van - Mr Fox

I have an old 1981 VW Transporter camper, with a 1.6 Diesel engine. For a couple of weeks, the battery warning light started to come on at highish engine speeds, I thought the fan belts must be slipping so I've tightened these up, and checked that the alternator is turning ok, it is.

What could be causing this light to come on ? it goes out eventually when the engine goes back to Idle speed, but can sometimes be seen flickering a little bit all the time. There is never any shortage of power from the battery, it always starts up fine, all accessories work well, suggesting that there is decent power output from the alternator whilst driving.

Any ideas ? I'd be grateful for any suggestions and tests I could do to check everything is ok, before I head off on holiday.
Battery Warning Light VW 1.6 D Van - jc2
I've had a similar concern;was cured by remaking the connections on the back of the alternator.
Battery Warning Light VW 1.6 D Van - Peter D
Worn slip ring brushes flying off at speed. New brush set/regulator could be £20 Regards Peter
Battery Warning Light VW 1.6 D Van - mss1tw
Completely off topic (Well, sort of) How many miles has it done now?

Lots of fond memories of our last one, died of head gasket failure though...
Battery Warning Light VW 1.6 D Van - Mr Fox

It says 88 000 on the clock, but this one is made in 1981, so it's probably on its second trip round, although the engine is quite new.

I tried cleaning up the contacts on the back of the alternator, it had some effect, but the light still comes on at anything other than idle, and takes a few moments to extinguish itself.

The alternator is also quite new, I suspect the connections on the back have something to do with this, as for example if i rev the engine with all electrics running, lights, fan etc. the lights get a tiny bit brighter than at idle, suggesting that the alternator is charging ok, and it always starts up fine.

I will probably have to take it to an auto electrician for a proper check, before I set off touring in a few weeks !

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