Alternator? And is this something I can do?
 

Out of nowhere the 78 300SD battery stopped charging. At first I thought the dead battery was because the radio had been left on. I charged the battery and it started fine every time until the time it didn't. Just got in to a dead battery.
Whadya think? Alternator? I haven't done any poking around so I have no better info. Should I start there and is replacing the alternator something I can do? Many of you know what my skills are, but I have never replaced an alternator, ever.

Thoughts? How do I diagnose the problem?
Thanks as always!

Ginny
(first day of homeschool for my 11yo daughter)

Ginny-

You need to get a cheap digital multimeter ($5 at Harbor Freight). Charge the battery or get a new one. When it starts, check to see if you have about 13.5 volts across the terminals. If not you probably only need to check and change the voltage regulator and brush assembly , which is easy.

I've got one of those.
Red to red and black to black?
Thanks!

If You can get It Jump Started...
 

Advance will be able to check the Alternator whilst it's running.

[I suspect your Voltage regulator is Deceased of Old Age.]

If you have a Battery Charger Put the battery on CHARGE 2 Amps, Overnight.
[It MAY take 24 Hours if the Battery Is Dead]
then check the Specific Gravity of Each Cell (Hydrometer)
[Be Careful Battery Acid is MOST TOXIC, especially to EYES!]

If you have NO Digital Multimeter Harbor Freight and Sears sometimes sell
inexpensive ones.

If you get it running What's the output at the battery terminals?
Anything less than 13 volts and the Alternator is sick.

Same thing happened to me, so I went to Autozone and had my battery checked, it was fine. Next day my car went dead, but lucky there was an Advance auto nearby, or so I thought. The salesguy charged my battery for 35 minutes and declared it dead accdg to his device. Same battery Autozone said was fine. Sold me a $100 battery and he kept my "dead" battery. Next he tested my alternator, same device, same result, dead. Sold me a new alternator for $280 and you guessed kept my alternator. Change the voltage regulator first, it's a $30 part. Moral of the story: "Never go to Advance Auto".

It would be nice if it was only the Votage Regulator that Fails.

What is missing is that the Original Poster did not say he No Charging Light was on. Nor did he say that the No Charging Light would come on with the rest of his Warning Ling when he first turned his Key.

If his No Charging Light is not coming on the Bulb could be dead. The Alternator needs the little bit of resistance from the little Bulb in order to charge properly. No Bulb No Charge.
A new Voltage Regulator will not fix the above.

However, he should remove the Voltage Regulator and be sure there is still some Brushes left. More than one Member has said their Charging Light did not go on when one of the 2 Brushes was worn out.

My problem with a dead battery was similar. Alt light came on with key and off when running but batt went dead. I was at Wmart so they checked my batt and declaired it bad so I bought a new one. Several days later and dead batt again. Back to Wmart and they charge batt and say it's good. Next I used meter and found that with ac off charging would go to 13.8V at about 2K rpm but with ac on it wouldn't go past 12.5V which is discharge. But the batt light didn't come on. I changed the reg and same result. Turned out to be some bad diodes in the charging part of alt. The batt light has different diodes than the charging circuit and had 16V at idle and over 20V at 2K rpm. So new alt was the fix. I later found out that there is a alt rebuild shop that only charges $50 to rebuild your alt. Next time I will try the local rebuid first. This was on my 190e. FWIW

Where is the voltage regulator located?

Right on the back of the alternator in these cars.

Here's a pic of the alternator:
http://catalog.peachparts.com/ShopByVehicle.epc?q=&yearid=1985%40%401985&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6193%3AED%7C1 0000135%3AMBC%7C1504%40%40300D&keyword=alternator&subcatid=P:240866@@Alternator&mode=PA

The voltage regulator the black circle thing held in by two screws.

http://catalog.peachparts.com/ShopByVehicle.epc?q=&yearid=1985%40%401985&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6193%3AED%7C1 0000135%3AMBC%7C1504%40%40300D&keyword=voltage&subcatid=W0133-1714889@@Voltage%20Regulator&mode=PA

I wonder about this because they cannot Load Test a Battery unless it is fully Charged.

Arrived to day in the Mail I got at Digital Volt Meter ($6 from eBay) that plugs into the Cigarette Lighter. So I drove around watching it.
With my Headlights on and at idle speed of about 750 rpm I only had 12.7 Volts and my No-charging Light did not come on.

I do not know if on a 1975 300D the Voltage Regulator is in the same place or not. But, on the 78 300SD (Bosch) it is as in the pic. Green Arrows point to the 2 screws that hold it in.

Also apparently some of the Older models came with or aquired Motorola Alternators instead of the Bosch Alternator.

So you might want to post a pic of your Alternator.

If you want to try a functioning alternator I have one I'll let go for cheap. Shoot me a PM.

If you get to the stage of either removing the alternator or the regulator I've got some pictures here that might help

http://www.peachparts.com/shopforum/2668425-post3.html

(Sorry folks I know that that is the next stage)

My battery light comes on fine at start up and goes off right away. Has never come on, to my knowledge, when driving.

Do I have to take off the alternator to get tot he regulator?
PITA, and I've never done that before. . .

I fully charged the battery yesterday and it starts fine today.

No, you just need the car high enough at the front to get under it. The regulator (IF it's like the ones I've worked on, big "if" there) is just stuck into a hole on the back side of the alternator itself and held in by two (Phillips?) screws. Comes right out the back. Either disconnect the battery or else be EXTREMELY careful not to bridge any contacts with your screwdriver; you'll be holding a 12V arc welder if you connect + to - with a tool by accident.

I use a "stubby" screwdriver. The little two or three inch long ones.