voltage to trip the alternator/battery light - PeachParts Mercedes-Benz Forum

JHZR2 · #1 04-29-2010, 11:12 PM

Very non-scientific test, but on my 82, the PO replaced the battery a number of times in the last many years (more than is normal, however the car did only put on ~10k in the last 10 years), but I also have encountered a weak battery after sitting for not too long a time.

I put a voltmeter into the cigarette lighter and am only getting about 12.8-12.9V when lights, radio and fan are on. about 13.3-13.4 at idle with no accessories on.

Seems low, but then again there are losses in the lines going to the cig lighter, and the actual socket could be fouled up dirty too...

Ill put a multimeter on when I get back home but until then, what voltage would typically set off the light? Does it need to be less than 12V?

The battery will charge fully, if given enough time, at 12.8V alternator. Short trips and sitting could cause a flat battery after a while though due to insufficient charge time depending upon use.

Again Ill verify with a multimeter when I get back home, but until then, to fulfill my interest, what voltage makes the light glow dimly and what makes it come on fully?

Thanks!

Diesel911 · #2 04-30-2010, 12:00 AM

Good idea to start by cleaning all the electrical connections that have to do with the Charging System.
Also, are the Alternator Drive Belts Slipping?

Speaking of the no charging light; some members have said that when there brushes were worn the no charging light did not come on.

The Voltage Regulator controls the current going to the field through the Ground. This means that if your Ground connections are poor it will not function correctly.

In order to charge correctly the Light Bulb in the no charging light needs to be at least 3 watts. If the Bulb burns out you will not charge.

The Alternator Connector/Plug being some sort of Brass/Copper alloy get corroded and need cleaning and some times replacing.
The 2 Fat Red Wires that go to the large connector slots should always have Battery Voltage.
The skinny Blue Wire that goes to the small slot will only have Voltage when the Key is on. This Blue Wire is the one that goes to the no charging light.

After you move the wire spring that holds the Alternator Wire Connector on and pull it out there is 2 screws that retain the Voltage Regulator. Arter removing those you can pull the Voltage Regulator out and check the length of the Brushes. 2mm is the minimum allowable Brush lenth.
Different Voltage Regulators also put out different Voltages; some higher some lower.

JHZR2 · #3 04-30-2010, 12:12 AM

The bulb works, the alternator obviously excites and puts out something.

The alternator was also replaced sometime back with a bosch rebuild (I have the receipt).

Ill be checking the voltage at the battery terminals when I get my hands on the MM. The info you gave is useful, thanks very much!

layback40 · #4 04-30-2010, 02:55 AM

The light goes out when the output voltage of the alt is greater than the voltage of the battery.