Recently when driving my 88 300 SEL the battery light would light up very dim at idle RPM. I suspected the battery, so I checked and tested it. No problem with the battery.
Next I checked the current from the alternator to see if it was charging. It was putting out around 45 amps and keeping the voltage above 12 at the battery. So I figured everything might be OK.
Well about a week after that the red battery light would come on from time to time more constantly. However sometimes you could shut it down, start it back up and the light would go out.
Driving back home one night the battery light got brighter and brighter. I also noticed the headlights going very dim. Then the ABS light came on. I knew I had trouble then When the ABS light came on I touched the brakes and the radio which was on went off. let off the brakes and the radio came back on. Next the SRS light came on and the radio died completely. I drove another mile or so and the voltage level droped so low that it would not run anymore. Coasted a couple miles (downhill) to a nice spot to pull over. Keep in mind that I still have the lights on which are getting dimmer and dimmer. When I got stoped I turned everything off, and spent 10 minutes or so going over all the fuses, etc.
Knowing that it wouldn't crank I got back in and cranked it up. I couldn't believe it. It turned over very slow, but it did crank. I had my amp meter with me so I checked, and the alternator was putting out 50+- amps. The indicator lamps were all back to normal and everything seemed to be fine. I went home with no more problems.
The next morning I started it and checked the voltage and amperage. This time the light was back on again, battery voltage was slowly falling, and the alternator was only putting out 2 amps or so.
So that seems to indicate the alternator as the culprit. Although I can't understand why it would be an intermittent problem.
I think I'm going to pull the alternator tommorow and try rebuilding it. What do you think, guys.??
I hope it's not something else in the electrical system that is causing the alternator not to put out.

It's entirely possible your alt. has had it.

Then again, depending on it's age, you may be able to get away with a new voltage regulator. A lot of times, this is all that's wrong. Easy to check.

If the car will start, what kind of voltage are you seeing with the VOM set to the 20 scale, red lead on + terminal; black lead on - terminal?

On a cold motor, you should see 13.9 or better. When warm it may be around 13.5. If this isn't happening, pull the voltage regulator. Easy to do. It takes longer to setup a 300-SEL for this than to actually do it.

- Pull front of car up on ramps.
- Remove large encapsulation pan underneath. 300-SEL pan is secured with four-8mm screws.
- Underneath alternator, you'll see 2 phillips screws securing the VR. Remove the 2 screws and pull out the VR. It comes right out. Look at the brushes - carbon looking and spring loaded - there are 2. When new, they are about 1/2" long. I'll bet yours are worn down.
- If worn down, install a new set. They go right back in without issue.
- Reinstall 2 phillips screws.
- Reinstall encapsulation pan and start up. Check volt on battery now.

Here's a photo of the VR. You can click on it to expand.

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1HB00BPJD1HB00BVI7&year=1988&make=MB&model=300-SEL-006&category=F&part=Voltage+Regulator

This part can be ordered online from here and elsewhere or look for a local supplier if time is an issue. The original part is Bosch.

When it came time to replace my Bosch alt., I used an AL166X(100amp) vs. the std. AL66X(80amp). I now see 14.30 on a cold motor. With the 80amp alt., I never saw anything > than 14.0. I'm not sure if the AL166X plug will work on the '88 300-SEL.

My 2 cents.

__________________
'91 300-SEL

Thanks, 91'
Best I remember the voltage was in the 12.5-13V range. I will check it in the morning before I get started on it.

That was at the very least a buck's worth there 1991300SEL, But really really thorough!! Wow! I am adding you to my favorites list!!

Sounds to me like your brushes are a little short and are intermittantly contacting the commutator on the armature of the alternator. Also the brushes or brush could be binding in its shaft and not moving correctly. When the voltage regulator calls for voltage those brushes must be making good contact with the commutator or you have low amperage draw or insufficient charging...My 2 cents...

__________________
83 300CDTurbo 307,000
2005 Honda Element 266,000
56 Nash Ambassador Country Club Special 34,000(under restoration presently)

http://www.peachparts.com/shopforum/car-audio-multimedia/125099-123-install-extreme-dynamat.html

Well, the brushes were worn just a bit. see pic
When I removed the screws from the VR it didn't even have any spring pressure against it. Brushes only extended 3mm or so beyond the end of the case.
I put some new brushes in it and evrything seems to be working good now. Voltage is up to around 13.85.

Thanks,
BMJR

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Those were in fact well worn brushes.

Now, just an observation, if those were just worn a bit, remind me in the future if you tell me the MB your trying to sell me at that time was only driven a little bit! Ha. Ha!

Isn't it great when little problems stay little, because friends are willing to share their help and knowledge!

The wonder of the net!