240D battery light - PeachParts Mercedes-Benz Forum

vahe · #1 09-01-2009, 09:45 PM

The battery on my 240D 77 is fairly new, when I started the motor this afternoon I noticed that the battery light at instrument cluster was on, fairly dim, but it was getting brighter at times and back to dim.
The battery is ok and fluid level is fine, obviously electrical issue, can someone tell me where to start to troubleshoot.

Vahe
240D 77

leathermang · #2 09-01-2009, 10:38 PM

I know nothing about electricity...
Do not even read the rest of this post.

It may be that the resistor or something like a zener diode is getting weak... resistors usually just give way but if it is an electrolytic it can leak and be more gradual..
that system is supposed to be balanced ( a comparator circuit ) so that if you are taking more out than it is making that light comes on... the best bet would be to put a regular gauge into the system to watch for a problem... someone will know how to wire that up... they do not call them IDIOT LIGHTS for nothing... You never hear the phrase ' IDIOT GAUGES ' do you ?

Oracle12345 · #3 09-01-2009, 10:40 PM

check out your alternator, voltage regualtor. Normal voltage is 13-14 volts anything under or over this spec is not normal. Also the charging voltage shouldnt be spiking.

leathermang · #4 09-01-2009, 10:44 PM

How can you tell if the charging voltage is " spiking " ?

Oracle12345 · #5 09-01-2009, 10:53 PM

If the charging volts changes at really fast with postive and negative numbers rather than gradually holding between 13-14.7 volts. The faster you go obivously its going to be near 14.7 volts but at idle it should around 13 volts.

the voltage regualtor on the w123 is external which means it can be replaced instead of buying a new alternator

qpoiz · #6 09-01-2009, 11:02 PM

I had a similar problem with my 240D. Batter light would glow dimly constantly, but when accelerating, it would glow brighter and sometimes bright enough to light up the cabin.

Turned out to be the battery harness. Two of the cables coming out of the alternator plug had come lose.

I found this out after weeks of talking to a mechanic, searching for a new bracket, and finally ending with a dead battery multiple times.

barry123400 · #7 09-01-2009, 11:08 PM

Check that your alternator belt has some tension.

Brandon_SLC · #8 09-01-2009, 11:31 PM

Quote:

Originally Posted by vahe

The battery on my 240D 77 is fairly new, when I started the motor this afternoon I noticed that the battery light at instrument cluster was on, fairly dim, but it was getting brighter at times and back to dim.
The battery is ok and fluid level is fine, obviously electrical issue, can someone tell me where to start to troubleshoot.

Vahe
240D 77

Do your headlights seem dim when you are driving at night? My first guess would be the voltage regulator. But as others have said it could be caused by many things. Poor ground, loose connection at battery, starter or alternator. So check for loose or corroded connections first.

From my own experiences, if no connections are loose or corroded:

I'd give it about a 70% chance of being the voltage regulator.

I'd give about a 25% chance of it being a bad alternator.

And a 5% chance that you may have a Black Forest Gremlin.

leathermang · #9 09-02-2009, 12:00 AM

How does having a digital voltmeter compare to an analog one when looking for the ' spiking' ?

compress ignite · #10 09-02-2009, 01:24 AM

Digital : You have to watch to see the digits change.
Analog : the Needle Has the St. Vitus Dance Syndrome.

leathermang · #11 09-02-2009, 08:53 AM

Compress Ignite,
So there is not some ' averaging' on either which might make one miss ' spiking' ?
Is one more accurate for this measurement than the other ?
Where are the best places to hook up the measuring instruments for this test ?