If I don't get my engine up to 3krpm a some point th alternator will not be in charg mode.

I had replaced the battery, alternator (and yes even that diode thing in the back) a year ago thinking that was the issue but no dice.
I put a volt meter to the fuse box. Everything was normal meaning(just a tiny drain on one fuse going to the clock).

If I drive around town and don't get the engine at high revs, the alternator will act as a draining device and I'm toast.

When I give the engine a good rev past 3krpm the lights, become energized and the alternator is in charge mode. I know this because I had to jump my car, once started I reved the engine to a high rpm. I drove for like half hour and I was able to restart my engine later.


So what the heck is going on?

loose/incorrect belts is the most common charging failure on these cars...
that and poorly rebuilt alternators.

You could pull out your Alternator and have it tested at a low rpm to see if it will charge. If it charges OK during that test there is a good chance you are haveing poor connections in you wires or grounds.

In the thumbnail below at your Alternator Connector you should be getting Battery Voltage in the 2 big slots that go to the fat Red Wires all of the time.

The Skinny Blue Wire and the small slot are connected to your No Charging Warning light and you will only get voltage there when the Key is on.

You have to go way to the end of the article to the last paragraph where it tells you the importance of the Blue wire, the Bulb and that circuit when it comes to charging.
The Alternator Electrical Connector/Plug will come apart as it snaps together. I would have a good look at it to see if it corroded or gummed up inside or just plan needs cleaning.
http://www.pelicanparts.com/techarticles/914_alternator_troubleshoot/914_alternator_troubleshoot.htm

In the other 2 pics I made up a electrical Test Cord to plug into my Alternator that acts like the circuit you would have if your Key was on. You need a least a 3 watt bulb in the Blue Wire Circuit to mimic your No Charging Light. The Red Arrow points to the Bulb.
The above would tell you if you have any wireing problems getting the positive voltage to your Alternator.

Next would be to clean the grounds connections.

Attached Thumbnails

     

__________________
84 300D, 82 Volvo 244Gl Diesel

Thanks guys. It is not charging at low rpm. I know this for a fact. If it is the altenator, what component would it be?

Also if it isn't thr alternator is there a charging circuit somewhere on the car?

If it is not charging then the idiot light will be on provided that it is not blown.
It does not sound like you have tested anything with a volt meter. If you have, please post your results in volts, not opinions.

Turn the ignition on, does the idiot light glow?
Start the car & rev to about 1000rpm, has the idiot light gone out?

With no accessories (lights, fans etc) turned on.
With the car turned off what is the battery voltage?
With the motor running at about 1000rpm what is the battery voltage?
With the motor running at 3000rpm what is the voltage?

At 2000rpm with as many things turned on as you can, what is the voltage?

Does your car have a WVO system fitted ?

Answer the above questions with numbers or yes/no & the problem can be diagnosed properly.

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

I was suggesting a method to find out why it was not charging at low rpms:
The point I was trying to make about pulling the Alternator and having it tested was that the Test Leads and the Ground on the Test Bench are going to be in better shape then the ones on your Car.

If the Alternator works at low rpms (like 750 rpms) on the Test Bench that means you have a problem in your wiring (to include the Ignition Switch) or the Battery (take the Battery with you when you have the Alternator tested and have the Battery teseted also).

__________________
84 300D, 82 Volvo 244Gl Diesel

Hi All,

New to forum...been lurking since I got my 1982 300SD a few months ago. Great forum, very helpful, thanks all! This is my first MB and my first diesel, but I am learning a great deal here!

I am experiencing battery drain/non-charging. The battery light has been coming on intermittently since I got the car.

Long story below, followed by what I've done (if you just want to skip to #1 below)

I was told by PO it had a brand new battery. No corrosion on terminals. When the problem was first experienced, I removed the alternator and had it bench tested at the local parts store. Checked out OK. Upon re-installation, the problem went away for a while...then, it came back. Once again going on and off at different times. My girlfriend is the primary driver of the car, so I am not sure how long it had been happening. I did notice it seemed like the light would flash when going over RR tracks, etc. We then began to experience a no crank situation (the car would eventually start after 2-3 no cranks). Upon a closer inspection of the alternator, I noticed the plastic mounting clip on the voltage regulator was cracked on one side. I figured this may be the cause of the non charging and the flashing of the battery light, so I changed the VR (I also cleaned the alternator harness at that time). I charged the battery and everything was great...for a few weeks. Most recently, the battery light would glow dimly on upon start up and occasionally shine bright all during the same trip. We ended up getting stuck at with a dead battery at Circle K.

To the best of my recollection, here is what I have done:

1. Changed voltage regulator

2. Had alternator bench tested twice (both check OK)

3. Had battery tested out of car (check OK)

4. Had charging system test at local auto parts store with the little hand held unit they use (battery check OK, alternator test bad at only 12.06V - although the car was not really warmed up and the guy was kind of in a hurry)

5. Did the ground terminal/ammeter test for drain with engine off: 10mA (approx, using cheap multimeter) which I assume is the clock. Also did the test light/ground terminal check and couldn't see any noticeable drain.

6. Cleaned engine ground to body contact(the twisted cable). Was unable to remove contact from motor for cleaning because it needed a special wrench.

6 1/2. Checked belt deflection (approx. 1/2" w/ thumb pressure) and visual inspection of belts (1035mm belts look almost new)

7. Most recent voltage tests (after 2nd removal of alternator for bench testing and full battery charge):

Across battery, engine on at idle: 12.4 (No increase at approx 3000RPM)

Across battery, engine off:12.55

Also performed alternator harness test from post #3, got battery voltage at both red wires (with neg. lead on voltmeter ground to body and pos. to one wire at a time). Did not however perform test on blue wire yet.

I took it on a ten minute test drive and was still getting a dimly lit battery light.

Battery light is off when ignition off, comes on when ignition on, stays on when running, will vary from dim to bright. Had occasion periods during trouble shooting when battery light would stay off when engine running and everything would appear to be normal.

Please advise if this post is too long or confusing, this is my attempt at being thorough.

Any suggestions welcome! Thanks! Also, I will be away from internet for about a week coming up. Please forgive any delayed replies to any posts.