was the exact same as yours. Same conditions under same circumstances.

Replaced the old alternator with a used one off flea-bay and all problems went away.

Note: when you swap alternators be sure and search for the writeups. I got a ton of help from this forum. Also get a set of stubby gear wrenches. You definately need the 17mm.

Good luck,
PJ

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1985 300 D - turbo
258k miles and still ticking!
Black/palamino
Alda dustcap modification
Eastern NC -

As suggested, I tried running my jumper cable from the (-) on the battery to the engine. Measured voltages before and after, with no change. It was late last night when I tried it, so I may not have had a good connection to the engine so I will try it again this time with more enthusiasm. If this doesn't work, I am back to the alternator plug/wiring harness, or a bum rebuilt alternator.

Thanks for all of the advise, will keep you posted.

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1989 300E
103,500 miles

1982 300SD
Sold but not forgotten

I finally found the ground strap, after finding it I can't see how I could have missed it before. Anyway I have a solid engine ground of that I am sure. I read resistance from engine to frame and got all zeros. Did the jumper from battery neutral to engine block again and again got the same results.

I pulled the plug on the back of the alternator and did a continuity check between it and the battery cable and got good continuity. I tried tracing down the small blue wire on the back of the alternator plug but I lost track of it. I am assuming that this is the field wire ?? I have continuity from the plug to the terminal strip, but after that??

I took a finger nail file and cleaned up the terminals on the plug as best I could and plugged it back in, rehooked up the battery and started it up. Again, still not charging, and the check battery light does not come on even when turning on the ignition switch. Taking to the shop tomorrow, unless anyone else has any other ideas.

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1989 300E
103,500 miles

1982 300SD
Sold but not forgotten

was the battery replaced after the new alternator was installed? I just went through this when I replaced my alternator. All my electrics died and I charged the battery and it died again the next day, so I found a used alternator from a local mercedes indy, (great guy btw if youre ever broken down around Harrisburg/Reading PA. Tim Reinhart) anyway, the newly installed alt. also wasn't putting anything out, and my bat. indicator light on the dash wouldn't light up. Turned out that my original battery was bad, the new alternator, specifically the voltage regulator inside it, was shutting down any amperage coming out, I guess it was reading the dead battery as a fully charged one. Bought a new battery and haven't had any problems since.

A bad alternator can kill a new battery in a matter of a couple days.

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1985 300D Gretchen (Astral Silver) 220k
1983 240D 4-speed Evelyn (Orient Red) 203k


TANSTAAFL

Thanks for the idea, but I replaced the battery with a new one at the time I replaced the alternator. Battery will hold a charge when charged from my charger.

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1989 300E
103,500 miles

1982 300SD
Sold but not forgotten

The first time I read this, I thought that this was not my problem since I had just replaced my battery. Now I think about it this may be my problem. When I replaced the battery, I only changed the VR on the old alternator. With the old alternator/new VR I could get some charge, but not much. About a week later, (plenty of time to kill my battery) I replaced the alternator with a rebuilt one.

I took my car to Pep Boys today and had them check the alternator and battery. The battery checked as fully charged, but the there was no charge from the alternator. The check battery light will also not come on even when first turning the ignition.

I am going to pull the battery and take it to Checkers tomorrow to have it tested outside the car. Will try to get a replacement to see if makes any difference.

When you had your problem did you have to replace the VR after replacing the battery?

Thanks again

__________________
1989 300E
103,500 miles

1982 300SD
Sold but not forgotten

Before I pulled my battery to have it checked, I checked to see if was getting a field voltage to the alternator. As mentioned before, the alternator light on the dash does not come on even when turning on the ignition switch. I understand the filed goes through the light, and if the light is out, no field and no charge.

I ran a jumper wire from the terminal box that the alternator field wire terminates at, and attached the other end to the positive battery cable. Alternator light comes on and stays on, but alternator still not charging.

I then removed the battery and had it tested. Battery tested good. Put battery back in, removed the field wire jumper. No alternator light, and no charge. I can understand maybe why I don't get an alternator light with the car running and a good battery, but why doesn't the light come on at all??

Measured voltage to ground at each of the connections on the alternator terminal strip (battery, starter and alternator wires all seem to terminate there). There are four sets of terminals in this strip. Starting at the terminal that is the farthest towards the front of the car, I read same voltage to ground as at battery, same was true for the next one (alternator and battery terminal) and the next one (field wire) but the one furthest away from the front only read about 1/2 as much as measured at the battery. Is this significant?

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1989 300E
103,500 miles

1982 300SD
Sold but not forgotten

sorry to get back so late, hope you've resolved this by now. If not I would try just putting a different battery in, not necessarily going out and buying another one, maybe borrow one from a friend even if its not the same size. It just sounds so much like what happened to me, and after checking everything in sight with the meter, I put an old battery out of a non-running 300D as a last resort and bingo.

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1985 300D Gretchen (Astral Silver) 220k
1983 240D 4-speed Evelyn (Orient Red) 203k


TANSTAAFL

If the battery holds a charge, and tests good with battery tester (not just volt meter), not sure if swapping with another battery would help. I have it narrowed down to one of two possibly three issues; bad alternator cable, bad alternator, or possibly burnt out brake pad light. The last one seems pretty far fetched, but I read a post that the field is routed through this and the battery light. Who knew???

Anyway sincere thanks, and will let you know what I find when I finally figure it out.

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1989 300E
103,500 miles

1982 300SD
Sold but not forgotten

I picked up a 84 300sd about 2 months ago. I put about 1,000 miles around town. I decided to drive it on a 400 mile trip. Just as I was reaching my destination, the radio and airconditioner quit.. No headlights either. Parked it at my destination then shut it off. When I tried to restart it, - no glow plug light and no starter solenoid action.

AAA ran a test and said my alternator was bad (battery is less than 6 months old).
Drive to Advanced Auto and changed the alternator in their parking lot.. Their test then indicated bad voltage regulator. No voltage reading was above 12.6 - running or not.
Installed a second one and their test indicates a bad diode. With the bad diode test, I have readings similar to the ones listed above. I show right at 13 volts at 2,000 rpm with AC on.

I am also not seeing any ground straps from the chassis to the engine on either side of the engine. Checked at Advanced Auto and also at Lowes but neither has any ground strap braid.

Sounds like to me, I was just lucky enough to get two bad alternators since they show different problems.

Is the voltage regulator built into the alternator? Does anyone have the part numbers for the diodes and components of the Voltage Regulator?

Thanks

RS in Va

After reading several posts, I think I got a bum alternator, so I will pull it this weekend and test. As far as the voltage regulator, mine was internal. If you look at the back of the alternator you will see two screws holding a small white looking bracket. You can change this out without pulling the alternator. Got mine at a local parts store for about $30.

Good Luck,

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1989 300E
103,500 miles

1982 300SD
Sold but not forgotten

Finally pulled my newly rebuilt alternator, took it to an alternator shop, and sure enough its not charging. Picked up another on at Myers, put it in, and everything works now. Even the door locks work now, although there seems to be a vacuum leak.

Thanks for everyone's help and advise. Now onto fixing my AC .

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1989 300E
103,500 miles

1982 300SD
Sold but not forgotten

Glad yours is fixed.

I am showing 28 volts AC accross the battery at an idle. Just cchecked my chevrolet and it showed even higher.

Does anyone know the partnumber of the 6 stator diodes internal to the alternator or have any equivalent / replacement diode part numbers?

If I can locate a set of replacement diodes, I will pull mine out of the alternator and test them. Since they are so static sensitive and heat sensitive, I don't want to unsolder them without a replacement set.

Thanks In Advance.

RS in Va