Intermittent Alternator Warning, no lights

[wingreen]

I have a 1993 CDi220. The alternator/battery warning light comes on intermittently, just for a second or two, then goes out. Recently this has been happening more and more (although it seems to happen more at speeds over 60). Had it checked by local independent who says everything checks out but agrees that, as its an intermittent fault, its hard to check. They said to bring it back when light permanently on then they can check! Car runs fine, no problems EXCEPT, if I'm driving at night, when the warning light comes on, headlights go out! This is only for a couple of seconds at most before warning light goes out and lights back on again - but, clearly, its not right.
I've read quite a few threads about this warning light but nothing quite like this problem (though might have missed it!).
Any thoughts? In particular, any suggestions as to what my local independent garage could do to diagnose (and cure) the problem without having to wait until its a permanent fault?
Thanks

 

[EDZ649]

Sound like you could have a broken earth strap somewhere.

 

[wingreen]

Really? (Thanks for the prompt reply) That doesn't sound too bad (and presumably could be checked in some way?)

 

[Dieselman]

Poor connection to or faulty front SAM would be my guess.

 

[wingreen]

The SAM possibility sounds more serious (and expensive!). Interestingly, no-one has yet suggested it has anything to do with the alternator though

 

[EDZ649]

The SAM possibility sounds more serious (and expensive!). Interestingly, no-one has yet suggested it has anything to do with the alternator though

It could well be the alternator but the fact that you have taken it in to a garage one would assume this is the first thing they checked and because they couldn't find anything you are looking for other possibilities, hence the replies on here.

 

[wingreen]

Ah, OK. What the garage told me that they couldn't check the alternator - or, put it this way, when they checked it was giving out 14v (I think) which meant it was OK. They did say that, as it was an intermittent fault (at least according to the warning light) the chances of them being able to test it at a time when the fault occurred was remote. So they said, bring it back when warning light stays on, then we can test it

 

[wingreen]

Update: Driving for a few hours during the day I got the warning light coming on for a few seconds on a regular basis. I was worried about what would happen when it got dark (I had a long journey!) but, amazingly, once I put my lights on, the warning light didn't come on once! Is there a clue to the problem in this?