I am wondering if you have a ground issue? Have you removed the ground cable from the battery to the chassis and cleaned the contact? How does your positive battery cable look? They tend to swell with corrosion over time, its also possible to have broken strands of copper near the battery clamp. Did you take the battery in for a charge and testing after you replaced the alternator? I drove my 300d for over a month with the alternator not charging the battery....it seems strange that you can't go a mile before its dead...its also possible the starter could be causing the drain...

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The battery is brand new, I charged it overnight after the short in the bad alternator drained it out. Now that I've replaced the alternator, the new battery is holding charge overnight. So that's great. The cables are in OK condition, obviously not brand new, but they're tight and unbroken. If I had a grounding issue, wouldn't I have read something more than 0.01 OHM load somewhere in my testing between chassis, engine and negative terminal? I guess I haven't tested from the pos terminal to the starter, but it's starting just fine as long as there's juice in the battery.

even the best batteries BRAND NEW will be junk if they are discharged to zero. automotive batteries are NOT designed to be pulled below 60% charge. if they are, they WILL be damaged. if completely discharged, special procedures have to be followed to recharge and desulfate the plates... not something most of us have the equipment to do.

take the new battery back to the store and have it tested. replace if it's partially low on CCA.

it's also possible that whatever killed your first alternator is still causing problems.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

When you turn the key on, the diodes within the alternator provides the path to ground to turn the charge light on . You said you were able to turn the light on by grounding the blue wire after removing the connector.

Possible causes:

1. Alternator diodes blown open circuit (all 9 of them).

2. Connector not making good contact.

3. Alternator not grounded well.

4. Cluster traces damaged. This can happen if the ground strap on the bell housing is not making good contact while you start the engine.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

"Alternator makes a rhythmic rattly banging noise during operation (only with wires connected)"

This is a weird one and raises a red flag. Did this happen with the new alternator? Did you find the cause?

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Where are the diodes located and how can I test them? That round black thing on the regulator, or the separate black box? Or elsewhere perhaps.

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83 SD

84 CD

Thank you, I'm going to test ohm load between alt housing and battery negative terminal. That should prove out if the alt is grounded well or not. I know the connector is making good contact, as I've separated it and manually connected each lead. The cluster traces you speak of are the traces in the dashboard instrument cluster, yes? When I had the cluster out I noticed my oil pressure gauge is weeping slightly. Do you know if engine oil is electrically conductive? This could be a problem.

The new alternator is quiet, I was quite happy when I first fired it up and tested. It gave me 14.4 on the battery with smooth quiet operation from the alt. The old one sounded like someone banging on crap with a hammer. It's still quiet now, it's just not charging....and yes it has a belt.

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Own:
1970 Mercedes 250c, 1976 International Scout II, 1982 Mercedes 240d, 1997 Honda Accord Wagon, 1998 Acura Integra Type-R, 2009 Mazda5.
Owned:
1974 International 100 pickup, 1975 International Scout II, 1981 Mercedes 240d, 1985 Mercedes 300SD, 1988 Honda Accord, 1988 Ford Thunderbird, 1991 Honda Accord, 1996 Geo Metro, 1996 Ford F150, 1996 Dodge Grand Caravan, 1997 Honda Accord, 2003 Acura TL-S, 2003 Yamaha V-star.

83 300SD electrical issue

I'm going through similar issues...

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83 SD

84 CD

That electrical plug for the alternator is pretty dirty. I would pop it open, make a note of the orientation of the wires, clean them really well with electrical cleaner...make sure they are nice and shiny, re tension the wire connectors and confirm all the solder joints are good. Its possible the plug is staying taught in the alternator, but the individual plugs aren't making contact.

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Had no clue it came apart. I mean, I looked at it, but couldn't see that it had a split anywhere. I'll give that a try, thanks! Perhaps hooking it up gingerly without the plastics of the plug itself might give insight.

After watching your video, I'm leaning towards cooljay's idea. I might also be having a touch of the same thing with my connector.

It's cleanin' time!

It's also been pointed out to me that the voltage regulator can be damaged by connecting and disconnecting the circuit while the engine is running. YMMV

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Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)

Just got done taking it apart, it wasn't too cruddy inside, but it wasn't great either. I need to replace the plastics now, and it wouldn't hurt to crimp on some new connectors as the insulation is damaged a bit. I was able to hook up the leads without the plastics in the way to ensure good contact and test. Unfortunately the warning lights still aren't coming on. Curiously I'm reading 2vdc on the blue wire at the alt with the key off and the blue wire lead connected to it's respective home on the alternator. I'm guessing that's going to drain my battery overnight. I'm also guessing this newly installed alternator is already fried. You'd think buying the Duralast Gold (which has a lifetime warranty) would have resulted in an alternator that was going to last at minimum 50 miles. Although perhaps my car is doing something specifically to kill them?

I am running out of ideas. The only other thing I would suggest, is to pull the fuse box cover and check all the fuses...remove each fuse and inspect the ends...Reading a lot of threads on the matter, they say it is possible to have a fuse melt inside. I would also pull the cluster and clean the bundle of grounds behind it, and do a visual inspection of the cluster. All the gauges connect together and that is how they are grounded, by touching one another.....I would also clean and check the wires on the starter...I suppose its possible to have a bad solenoid and that cause your issues, my starter went bad and killed my battery.

To see if you have a bad ground connection on the block/alternator. You can take the negative side of a jumper cable, clip it on the negative side of the battery then the other end to the engine block or alternator.

I read a thread where the glow plug relay can also be blamed, you can pop it open.....remove the connectors then see if your battery light comes back on.

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I would think the fact that the Battery Light and Brake Light not working at the same time would have some meaning - like the driver's side front window and passenger side rear window circuit diagnostic.

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Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)

My new good alt wouldnt charge so I ran a 10 gauge wire from the alt housing to where the neg battery cable bolts to the chassis. Alt gives 13.3 amps now. A simple fix to try

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2009 E320 Bluetec 117,000
1995 E300D 306,000 Sold
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1985 300TD Still 225,000 Sold
2016 Ford Fusion 24,900