I just had the situation where a faulty voltage regulator prevented the alternator from charging the battery. That problem was solved with a new regulator; but what needs fixing now is that the charge indicator lamp never came on, even though battery voltage became so low that the car couldn't run. With some warning, I could have dealt with this fault before my wife got stranded!
I need some help here: 1) what is the threshold point at which the lamp should be turned on? 2) what is the best way to test that it's truly operating at the threshold? and 3) if need be, how can the threshold point be changed to give an earlier warning?
The lamp does illuminate on ignition as it should.

Thanks,
Barrie

When the brushes fail on the alternator, the light will not come on because the circuit is completed by the voltage regulator. (Go figure)

If the light comes on when the key is first turned on, it is OK.

------------------
Benzmac:
1981 280GE SWB
1987 16V
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

Thanks Benzmac.
Do I understand you correctly that 1)if the brushes fail, thus causing the alternator to fail, the alternator failure light will fail to light, and 2) there is no way around it. Are you sure this is German engineering?

The circuit does go through the alternator but I'm still not clear what is being measured, and at what point it triggers the dash warning light. Can anyone help on this?
I naively assumed this circuit monitored the battery voltage to warn when it was no longer receiving a charge.