Maybe just change the idiot bulb to a new correct one before you really start over thinking things? Incandescent bulbs age and resistances increase.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

Check your idle speed. If the idle speed is too low, even with a good flash signal the alternator won't build adequate voltage until the RPM comes up enough to support itself.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Ahhh. Yes I bought the BNR auto parts 120A as a replacement.

If you’re running 13.8 the light shouldn’t glow. That’s nuts. Looking at the diagram you probably want to get it idling, check the voltage across the bulb...of course you have voltage or it wouldn’t be on. Then you need to figure out why.

It is possible the connection between D+ and B+ around the lamp is bad and it is finding a circuit through one of the little diodes in the cluster or warning module. It may be something as silly as a corroded contact that needs a little current to establish a connection.

All theories aside, the way to fix this is find out where the electrons are coming from and where they’re going. It’s just a matter of starting the car and getting it in the bad state and looking for where and why one side of the bulb is at a lower voltage with a voltmeter referenced to battery negative. At V battery 13.8 both sides should be the same. ...and we know they are not on your car because of some crazy short or diode leakage or such.

Good luck chasing it down.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles

I am wondering if you have a 2 watt bulb in the system and if a 3 watt bulb might fix your problem.

Additionally I am showing that things that I did indeed changed the output voltage since I got the care even though I did not have your specific problem.

Anywhere there is a system that has to operate together one of more faults in the system can be the cause of an issue.

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84 300D, 82 Volvo 244Gl Diesel

I wasn’t clear here -the lights go out once I blip the throttle. Blipping the throttle is what gets the alternator generating. When it’s generating, no lights of course.

I’ll pull the binnacle later, and it’s been awhile so I can’t remember, but aren’t the indicator bulbs 1.2w grain bulbs? And the main cluster illumination 5w?

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1981 240D 4sp manual. Ivory White.

The idiot lights are low wattage bulbs. The alternator light is a high wattage bulb, I think 3W or something. It's physically larger. There is a low wattage bulb that fits the same holder, if it's been swapped in, that could be part of your problem. The high wattage is necessary to pass enough current to the exciter wire to adequately flash the alternator.



All of the other idiot lights feed through diodes to the exciter wire as a "backup" flash path in case the alternator bulb dies.


Again, check your idle speed. If too low, the behavior you're having is expected. The throttle blip can raise the RPM high enough for the alternator to build itself up and maintain charging.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Oh this might be the issue...

Noticed my battery light does not illuminate with the other idiot lights when key is in #2 position. I only have brake wear indicator and the brake light.

Re: idle, I’m at 850. For kicks I’ve tried turning the gorilla knob up all the way. Seems I need at least 1500 to excite the alternator.

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1981 240D 4sp manual. Ivory White.

Found the break.

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1981 240D 4sp manual. Ivory White.

1. Fascinating thread to read.

2. My question: Given that most W123s are now 36-44 years old, is there any value in replacing an original battery to ground cable? Stated another way: Does such a cable significantly degrade over time and thus impede performance?

Thanks-

That's a question for someone else, but given the state of the remaining soft metals, I wouldn't be surprised.

Our current epoch will no doubt bring a slew of unique issues purely due to this level of vintage (decrepitude).

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1981 240D 4sp manual. Ivory White.

In theory you are supposed to be able to do a voltage drop test.
If you put your volt meter on the negative battery terminal and you ground the other lead good to the Chassis and have someone crank the Engine you are not supposed to see any voltage on your meter.

The same test works on the positive cable but it is harder to get at. You put one end of the Voltmeter on the positive batter terminal and the other end on the stud and nut that holds the cabal end onto the Starter and have someone crank. Again you should see no voltage on the meter.

Well when I changed the negative battery cable it was because I needed a different length of cable and I got a surprised when with the slight voltage increase. So in my case there must have been some issue with the Cable.

When I worked as a Mechanic I found that ends of the battery cables with the crimped eye on the the copper cable inside of the crimp and inside and of course the inside of the tube on the Eye would get corroded over time.

Replacing the whole cable on a a big rig truck due to a bad crimped connector is a whole lot of work and expensive and I was there as a maintenance mechanic so my Boss nor myself was making money off of that sort of job.

I would take a propane torch and heat up the crimped end and fill it with Solder and that restored the good connection of the cabal inside to the crimped on eye. A fast and simple fix. (I also did this to the ground strap/cable on the Mercedes.)

Of course you could get a chisel and remove the crimped on Eye clean the cable ends and crimp it back on but when I did that I still filled it in with Solder.

With a bad terminal clamp end if it is brass you can often do the same where the cabal inserts into it. Stick the cable end in baking soda and water to deal with any acid and to a small extent with corrosion. Heat it with the propane torch and fill it in with solder.

If the terminal clamp is lead all you can hope is the cable is long enough you can cut the end off and install anther terminal clamp.

The pic is of my original Mercedes positive cable. In the pic I put red dashed lines where I took a hacksaw and cut a slot into the Terminal Clamp. I used a Screwdriver in the slot and spread it out enough to pull out the cable.
Once the cable end and the hole the cable goes into were all cleaned of the white corrosion I shoved it back in and again got out the Propane Torch and filled it in with Solder.
I got no extra charging voltage out of that. But it saved me from having to buy another Cable.

Attached Thumbnails

 

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84 300D, 82 Volvo 244Gl Diesel

Is the socket it goes into also corroded?

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84 300D, 82 Volvo 244Gl Diesel

Lots of educational material and great direction here. Sincere thanks for sharing all of this. I now have some new M-B research and probably fun work in my future!

Yes and no. The crimped connections at the battery terminal and the eyelet that connect to the body are your common failure points. Leaking battery acid, water, and salt are your enemies. If the cable looks crusty, it has a corroded terminal on either end, or you see green/blue "patina", the cable probably needs replacement.

More importantly, check the engine ground strap from the transmission to the body. Every W126 I've owned has needed this one replaced. I do not live in an area that salts the roads, but all of them had degraded to the point that they fell apart when removed. This is the ground source for the entire engine - your starter motor, your temperature senders, your glow plugs, and the alternator.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Good to know and places I'll check.

I've kept an eye on this over the years and all seems well. On the other hand, it sounds like it might be useful to loosen and gently remove it and see if perhaps it starts to fall apart in the process.

Thanks-