Seeming Alternator failure without warning light.

Post Wed Dec 07, 2016 5:34 pm

Have been an observer on this board for some time but due to the collecting of valuable notes, I have not required tech expertise until now. Car is an 89 m40 316i which I've owned for 20 years with well over 300k on it and which has been very reliable. Last night though on the way to 5 a side, the stereo went off itself and the dash lights dimmed--voltmeter showed battery discharging. Unusually though, there was no warning dash lights and the bulb is working as I started the car this morning after charging the battery overnight. The ignition lights go off once the car starts as if all is normal but the battery is not being charged as per voltmeter. It is obviously the alternator but unusual not to have a warning light. I changed the brushes in it some years ago and recall at the time flashing lights prior to that. Has anyone come across this type of alternator failure?

Post Tue Dec 13, 2016 4:59 pm

Just a follow up on this to say that the problem here was the voltage regulator. The solder holding one of the brushes in its housing had become detached so was not making good contact. This problem does not show up in the dash warning light. I had expected to buy a new alternator as this one is original but no. Was like an electric car the last few days but with a very limited range.

Post Tue Dec 13, 2016 9:23 pm

Sorry, I missed the OP on this one.
The warning light has the battery voltage on one side of it and the output of the D+ terminal (blue wire) on the other side of it. Any difference between these two voltages is what makes the lamp glow.
With the ignition switch turned on, battery voltage is applied to one side, and the other side is earthed through the regulator's electronics, so the bulb lights. When the engine is started, a slightly higher battery voltage, increased by the alternator's D+ output, is still applied to one side, but the alternator will also be now producing a similar voltage on the B+ terminal, so no voltage is now across the bulb, and it goes out.
In your case, the dud regulator must have been failing to earth the B+ side of the bulb, and just leaving it open circuit, so the bulb could not light.
The alternators fitted to E30s are of very good quality, and rarely fail, even at high mileage, except for worn brushes, so it's no surprise that a repaired regulator pack cured yours.

Post Wed Dec 14, 2016 12:26 am

I see, thanks Brian. While no visible indication from the warning light, the system did start to withdraw facilities--the stereo/amp being the first to shut down followed by dimming lights. I managed to get around 10 miles in the dark which at least shows the battery to be in good shape. I would imagine a lot of good alternators are dumped because of wonky regulators.