Yeah, sounds like you dont have any problematic current draw, and its purely an alternator issue. I dont think the diodes go on them often. I would replace the voltage regulator as its cheap and contains both the brushes and the regulator. I would also check all of the wiring from the alternator to the car.

Your alternator produces 3-phase alternating current, which is then rectified by three diodes into a pulsing DC for your cars electrical system.

A failed diode will usually result in low current output, and frequently will allow the battery to drain back through the faulty diode.

Measuring dc amps
 

Okay.
Thanks for the clarifications, explanations and reassurances.

Clearly there is way too little energy in the form of charging volts being sent by the alternator to the battery. It seems safe to say that the alternator is showing a fault.

It seems that my plan should be to replace the voltage regulator given the data I have been able to collect thus far.

Does anybody know if the three alternator diodes are located inside the voltage regulator assembly or not?

---

Yak
"That's exactly what I had when I first measured it - zero current, but I knew it couldn't be correct since the radio draws some tiny fraction of power to to run its internal clock, as does the clock in the dash - see the note at the bottom of my post.

You could try again and do something to trigger a slightly larger current draw, but not too much, that's why I opened my trunk. I wanted to make sure that the meter was actually reading something. With the trunk open and the light inside it on, the DMM displayed 0.700, so I closed my trunk and it went down to 0.02. Then I turned on the one side parking lights and it went to 1.2, turned them off and it went back to 0.02. These seemed to be normal predictable current readings, so I assumed good values from here out."
***I put the light bulb back in the trunk fixture and with the light on (trunk open), I still measured 0.000 dc amps under the following conditions:
-Car off
-Negative battery cable disconnected from battery
-Black DMM cable touching negative battery cable
-Red DMM cable touching negative battery post
-DMM set to measure up to 10 dc amps
I also used this same arrangement with the headlights on and once again measured 0.000 dc amps.
Again, my DMM only has a 10 amp setting so perhaps this is too small of a quantity to measure. I'm not sure. Maybe the DMM fuse is blown, I will check it.

Thanks again for all of the posts and advice for how to track down and correct the problem(s).

Replaced voltage regulator + alternator - Problem persists
 

I replaced the voltage regulator on the Bosch alternator and the result was:
The alternator was putting out 13.13 volts max. at idle. Clearly, insufficient.

I replaced the alternator with a Bosch rebuilt (65 amp). The issue is:
Despite driving the vehicle 30 miles a day every day this past week, when I started the engine today, the battery light in the instrument cluster remained on at idle. It only turned off after I reved the engine just a bit.

I took various readings today side by side with a 1985 300CD-T (Calif.). Here are the details:

1. For a battery, both vehicles have an Interstate MTP-93 (the one in the 1983 was new in June of 2008, the one in the 1985 was new in September of 2008).

2. With the cars switched off, doors+trunk closed, all accessories (except dashboard clock for both vehicles and digital radio clock in 1985) off, and each negative cable disconnected from the battery, the parasitic drain amperage readings (between the battery's negative terminal and the negative cable) were:
1983: 10 milliamps (0.01 amps)
1985: 30 milliamps (0.03 amps)

3. With the cars switched off, the battery voltage readings were:
1983: 12.32 volts
1985: 12.27 volts

4. With the cars started and the battery light on at first idle (only affecting the 1983), the battery voltage reading was:
1983: 12.14 volts
1985: Does not have this problem

5. With the cars started and just after I reved the engine so that the battery light turned off (only affecting the 1983), the battery voltage reading was:
1983: 14.05 volts (Note: I watched the voltage reading on my DMM climb steadily over a period of about 45-60 seconds until it stopped at 14.05 volts).
1985: Does not have this problem

Any ideas about what to do next? Especially regarding item 4. above (seems like a problem).

Thanks as always--

Battery light is not a bad sign. Alternators have to be "kicked" on and often the idle speed is not enough to do it. Mine does this every once in a while

FWIW, I had a similar issue that turned out to be a degraded electrical connection at the alternator. The wires inside the plastic connector were corroded and were not adequate to keep up with the load. I discovered that the voltage varied when I wiggled the cable to the alternator. Also, check all the grounds.

Will check the alternator electrical connector
 

FWIW: I've noticed that with the 1985 300CD-T, it *never* has this battery-light-on-at-idle-right-after-starting issue.

We've owned the 1983 300CD-T for about 24 years and this battery-light-on situation is a new one (relatively speaking, as I mentioned previously in this thread).

I'm going to pull the electrical connector on the alternator and see if I can find some corrosion or other possible problem(s) with it.

I've been really thorough about confirming that all of the grounds are secure and corrosion free.

Thank you-

Electrical connector ok
 

Pulled the electrical connector from the back of the alternator and it looked ok. Opened it up and used some cleaner on the contacts.

Started the engine and the battery light remained on until I reved the engine. Let the engine idle for about 5 minutes.

Turned off the engine and immediately re-started it. Interestingly, this time, the battery light went off as soon as the engine started - no need to rev the engine to make the light go off.

Let the car sit for about five minutes.

Started the engine again and this time, the battery light stayed on until I reved the engine.

???

Re: Battery Drain Problem
 

Recap:

Ok, if I remember correctly, a completely disconnected battery should have just over 12V. Check the condition of the battery - make sure the electrolyte in each cell is topped off to puckering with distilled water. Measure with a battery hydrometer to rule out any big differences in specific gravity between cells.
From your post, you said the battery is pretty new.

With negative cable disconnected, the 10milliamp draw you reported sounds ok to me. Is this variable? You previously mentioned 5A draw.

When an alternator is charging the battery, there should be around 15V across the battery. At least more than 12V so the battery will actually charge.

An alternator pulls energy from the battery to energize the electromagnet/rotor. As the shaft spins, a current is induced in the stator. The produced AC is then rectified to DC and fed back into the electrical system of the car. The regulator varies the amount of current to the rotor in order to govern the current production of the alternator. (my original post had this backward)

The bad rectifier idea of the battery draining is one that I had not thought of. Yet, you've replaced the alternator and the problem persists.

Because there is a substantial drain on the battery to energize the alternator AND the charge light is sometimes on/sometimes off, I have to second Craig's assertion of bad connectors at the alternator. Maybe a short in the wiring?

Codifex
1981 240D SkyBlue
1983 300DTurbo w/sunroof

Dogguy, where in Cali do you live? I notice you do not put on many miles.

Battery replacements:
2001: 228,600 mi.
2002: 231,800 mi.
2004: 240,800 mi.
2006: 242,400 mi.
2008: 244,900 mi.

Is the car not used for extended periods and exposed to cold temps during the winter?

At the moment, the charge between the battery terminals (measured with both cables disconnected) is 12.37 volts. Same reading, incidentally, after reconnecting both cables.

Don't have a hygrometer - I'll pick one up. What means, "...sg between cells."?

As for the previously mentioned 5 amp parasitic draw. Explanation: A failed fuse in my DMM. My bad. Fixed now.

Actually, the wife and me (and dog) live in Phoenix, Arizona. As for the mileage on this vehicle:
We have two cars. I drive one one week and the other one the next week, etc. Only about 150 mi. a week total to get to and from my job. She works at home. No issue with cold temps. here in Phoenix (even during winter). The car is garaged at our house. Never left outside.

Thanks for the ideas. I'm going to go back and check the wiring at the alternator's connector. The wires are slightly frayed. I'm going to jiggle them with the battery light on and see if anything happens.

Wiring appears to be ok
 

I started the engine and let it idle. The battery light remained on. Had the wife watch the light while I really jiggled the wires at the alternator connector. The Light never flickered or turned off.

Any other ideas fellas?

I'm sure you've done this; verify there is 0V between the negative battery terminal and both the engine and a body ground (with the engine running), just to make sure the grounds are all OK.

Also, have you checked the voltage (to ground) at the terminal on the right fender to make sure it's the same as the battery voltage? I still suspect you have a bad connection or wire someplace.

If you measure the current draw from the battery terminal to the battery cable with all electrical off. You will find some current flow if there is a problem. I wouild pull the fuse for the electric clock or make an allowance for it's continious draw.

You seem to have changed the alternator about the time this problem started. Any small reverse leakage through a diode in that alternator will pull the battery down. If you still draw current with all fuses removed the alternator is a suspect in my mind.

You are also going through far too many batteries as deep cycling them is reducing their life span seriously. I imagine on a quick output check at a parts store that alternator could have passed muster. If disconnecting the alternator eliminates all draw after doing the fuses you have it.

Actually, no. I have not done this yet.
I have the vehicle's front wheels de-installed presently. I'll probably be able to do this test on Wednesday and then I'll post the results.

I don't completely understand what you mean when you say, "...at the terminal on the right fender". Could you please clarify?

Thanks--