Quick - stupid question RE: battery light
 

Hi Guys..

since I'm pretty new to diesels I am asking a lot of stupid questions.

Is the battery light supposed to illuminate every single time you get in the car and switch the key to the on position while it glows?


Mine immuminates maybe 5/10 times.

It does illuminate briefly every time I shut the car off.

My alternator is newly installed bosch unit and the car does not appear to have any battery charging issues when I test it with my meeter.

thanks!

It is common to all the vehicles that I have owned that have waring (idiot) lights. When the Key is turned to the fist position all of the warning lights should light up and that is a test to see if the light bulbs in the warning lights are working.

On my mercedes the battery charging warning light particularly useful because if the Bulb is burned out the Alternator is not going to charge. That bulb is part of the Alternators exciter circuit and provides enough resistance for the alternator to charge.

The light should come on when you first turn the key on. At that point it gets 12V from the battery. Once the alternator starts spinning, it's output goes to other side of that lamp and cancels out the voltage from the battery, so the light goes out.

This link describes Bosch system for bikes, but principle is same.

As with many things, Mercedes has a non-typical method to bootstrap the alternator. Several of the warning bulbs ground through the regulator D+ connection, just as the alternator warning light does. So if the bulb in the dash fails, there are alternate sources for bootstrap current. This means that if, for example, the brake warning light glows when you turn the key, the alternator will work fine even if the alternator warning light itself is NG.

My suspicion is that there is a bad trace in your dash pod, or maybe the bulb itself has an intermittent filament. The downside is that if you ever do have a problem with the alternator, your warning light may or may not work.

Occasionally my charge light flickers as the engine stops. It's been this way for as long as I can remember. As far as I know it stays off when the engine fully stops. I haven't lost a battery or alternator to it yet.

Sixto
83 300SD

I've had bulbs where the filament was broken but dangling making contact once in a while.

As for the light turning on at shutdown, I've seen instances on various brands of cars where an operating heater blower or radiator fan acts as a generator when the key is turned off.

"I've seen instances on various brands of cars where an operating heater blower or radiator fan acts as a generator when the key is turned off."

A permanent magnet fan doesn't de-energize in a surge as pure inductor would, because the momentum of the blade keeps it energized. What happens is that when power is removed, it instantly becomes a simple DC generator. At first it will produce 14V, with the same polarity as the original source. As blade momentum is lost to friction, air resistance, or back EMF (if the connection is complete), voltage gradually drops to zero. However, in Mercedes systems, fan power is supplied directly from the battery. So when the switch is open, the fan's generated output would be on the other side of the ignition switch from the indicator light. The current it generates simply charges the battery until it stops.

I think what you're seeing a reverse surge from the alternator winding, powerful enough to jump the gap in the ignition switch. My guess would be a weak trio diode, and maybe carbon tracking in the switch. You might be able to confirm by comparing the ripple currents on B+ and D+. If it's not that, then somewhere a shunt resistor in a relay has gone bad, and it would be like finding a needle in a haystack. Not sure I'd bother fixing this unless it became a real problem.

Yet, from what I have read on the forum that does not seem to be the case. Most who have the charging light bulb fail end up with a dead Battery.
Here is a pic of the circuits on a W123.

And yet, there it is, right on your wiring diagram. The brake wear sensor grounds to the d+ connector thorough the diode. That's why it glows when you first turn the key. The diode is there to prevent d+ from grounding out once the alternator bootstraps. I was also blindsided by this... mercedes is the only brand I've encountered that wires d+ this way. Belt and suspenders: two bulbs have to fail to prevent the alternator from turning on.