My car is W123 300D 1977

When turn on ignition switch (not yet started) the alternator light (batt red light) is not illuminated but it is illuminated while engine is running .
Alternator light is only dim when engine is run at high speed.
It fails to go out at high idle speed.

What is the root cause of this problem?
How can I check the defect component?

I had a similar issue and did not get it checked. Result, on my evening comute home I was left wondering why my head lights were so dim. Seems I hadn't been charging the battery for days. Wonderful thing about our diesels, we don't need no stinkin' electric juice to keep them running. I was able to get it home and with a brush/voltage regulator replacement (parts set me back $13.50 from parts shop) I hope I'm good for another 100K.

Have your alternator checked.

Mark

Bad Diode?
 

Never had a MB alternator go bad, but with my old Caddies, it was a common occurrence!

Had the same symptoms: light is barely on at normal speeds most likely means that a diode rectifier has gone bad. The alternator will charge but at a greatly reduced amount. You do need to fix it!

Only brushes 're replaced yesterday(1$). It 's back to normal.