Bosch regulator used by Mercedes.
You can see the green resistor.

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Per what you just described, ignition switch is fine (edit: fine only at the time you observed it to have voltage... it could still be intermittent). The brake wear warning lamp is grounded on the firewall behind the cluster, which is a different ground than the battery lamp, which is grounded through the alternator (via the regulator carbon brushes to the field coil of the alt to the alternator frame to ground. They are on separate circuits and should be unrelated. Can 3 alternators be bad in the same way? Unlikely. Does the starter motor work well? There is a ground strap that has to make a good connection. If the battery lamp is not lighting then the ground path from the alternator connector onward to battery negative is suspect.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

That's a lot of work. There are easier ways to isolate where the problem is. Besides most stores are closed today and he needs to drive the car.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Good point about checking the alternator case ground. Sometimes we overlook basics.
When you grounded the blue wire and the bulb lit, did you ground to the alternator case or somewhere else?
If to the case then your ground is probably OK.

I'll make this simple. You have a bad voltage regulator or your brushes are worn beyond the use-by date. Either way, replace the regulator. If that doesn't do it, then the alternator has an open field or bad slip rings and needs a full rebuild.

There seems to be intense confusion about how this circuit works. There are no fuses involved. What happens is that the lamp is directly powered from the battery when the switch is in the on position. The blue wire grounds it through the d+ contact, which terminates on the alternator field coil. Because it's wired this way, voltage flowing through the bulb creates a magnetic field in the field coil, which is why the alternator starts producing power once the engine is running. Once the alternator is turning, it will supply it's own power to the D+ contact. At that point, the bulb will be powered on both legs. Since the potential across both bulb leads is zero, the bulb goes out. If you understand this, you can understand from the schematic diagram how the brake warning lights work as well.

It was the alternator..... I have a different view on 'remanufactured' parts now by unknown or shady parts companies.

That is unbelievable! Two bad rebuilt alternators in a row! Next time you buy an alternator, new or rebuilt, have it bench tested before leaving the store.

In all my years of driving, I have never bought an alternator. I replace the brushes on the regulator and the 2 ball bearings when they go bad.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I too experienced the dreaded issue with non-charging alternator and located the issue behind the cluster. I will include photo to help anyone else with similar problem. So my issue was same as posted in a few forums...

Turn key and left bank lights wouldn't illuminate. Upon further investigation my findings took me to the pin connector behind the cluster. I re-soldered the pin and presto- back in business.

Another thing- I have located numerous pin connector wire diagrams and on my 1983 300D they have proven to be INCORRECT- I finally located drawing of wire diagram that was correct... PIN 7 was my BLUE wire not 10 as notated in other diagrams.

Hope this helps someone as it took me a full day to determine- ahhhh the therapy this provides me is endless though.

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I hope no one minds if i ask a question about my car, since this thread seems to follow exactly what my issue is, and it seems the OP has his problem fixed.

My w126 (1991 350SDL) is having similar or exactly the same issue. In my case the left bank of lights light up slowly in direct correlation to throttle pedal or RPM. It is noticeable mostly at night. I have already swapped out the voltage regulator and that changed nothing, and i have pulled the cluster. I had three bolts sticking out of the pcb in the back of the left side of the cluster and i ran 3 new ground lines to the chassis under the dash. That changed nothing as well. I was stumped until i found this thread, and now i am considering doing 2 things.

1. take the connector apart and find my blue wire. I will either re-solder just that one of all of them.

2. Check and see if I need an Alternator replacement?

Any other suggestions. i have not been able to find the wiring diagram for this w126, as was shown earlier in the thread for the w123. thanks for the help.

__________________

Grey '91 350SDL 214k Dad's car
Beige '81 240D 4 Speed 254k SOLD
Blue '82 300D 225k SOLD
White '95 E300D 46k SOLD
Blue '87 190D 2.5 Turbo 315k SOLD
Brown '80 240D 4 Speed 716k SOLD
Beige '80 300D N/A 119k SOLD
Blue '85 300D Model 186k T-Boned

I'm not clear what you mean by "left bank of lights". Are you talking headlights? instrument panel?
If you're talking headlights, there is a grounding point on the fender, just behind the headlights. You may have some mild corrosion in the area or slightly loose connection.
Here is the FSM with diagrams:
https://www.startekinfo.com/StarTek/outside/12253/program/matrix.htm
https://www.startekinfo.com/StarTek/outside/12253/program/ETM/126from86.pdf

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )

Replace the battery charge indicator bulb its part of the circuit .No bulb then no charge simple ..

I have had this issue , I have copied the post I put up.

Check your alternator voltage regulator .

Remove it and check the brushes

There is a fair chance that the brush nearest the center of the alternator is making contact and the other one nearest the rear does but not very often.

Here is why,

My other half's 89 300d om603 non turbo UK car,,no glow plug light and hence no start ??

Inspection reveals ignition switch (electrical portion) and relays to be ok ???

Ignition switch changed and no difference ,,but !! letting the engine crank see's the glow plug lamp come back on ??

Now there was an issue with other dash warning lamps and eventually the warning lamps consisted of alternator and ABS only and these dimmed when engine turned over,I hasten to add that the battery was fully charged .

So realizing that the engine turning over would bring them back to normal function I checked the voltage regulator to find it worn out,it was replaced and instantly everything was back to normal.

I find the warning signs of the regulator having worn out brushes is flickering lights at idle at night ,anyway do a search for 'no dash warning lamps ' and the common theme seems to be the voltage regulator

I am sorry that i was not clear. i am talking about the lights in the bottom left of the instrument cluster. the Lights are "Brights", Brake pad sensor warning, coolant, battery charging, etc. These will all light up, but only the left ones. the right side like the glow plug, ABS, etc will not slowly light up.

__________________

Grey '91 350SDL 214k Dad's car
Beige '81 240D 4 Speed 254k SOLD
Blue '82 300D 225k SOLD
White '95 E300D 46k SOLD
Blue '87 190D 2.5 Turbo 315k SOLD
Brown '80 240D 4 Speed 716k SOLD
Beige '80 300D N/A 119k SOLD
Blue '85 300D Model 186k T-Boned

The Alternator is charging the battery and i am not having a battery drain issue. Also the bulb works, and i checked it by power it with an alternate 12v source. The bulb also shows to be working when the issue i am describing is happening. The Left bank of lights will All slowly light up as the throttle is increased.

__________________

Grey '91 350SDL 214k Dad's car
Beige '81 240D 4 Speed 254k SOLD
Blue '82 300D 225k SOLD
White '95 E300D 46k SOLD
Blue '87 190D 2.5 Turbo 315k SOLD
Brown '80 240D 4 Speed 716k SOLD
Beige '80 300D N/A 119k SOLD
Blue '85 300D Model 186k T-Boned

The voltage regulator was the first item that i checked and thought of. I have put 2 different voltage regulators in it and nothing changed. Unless i go by the order of events from the PO, should i consider the entire Alternators to be causing this, and not just the Voltage Regulator?

__________________

Grey '91 350SDL 214k Dad's car
Beige '81 240D 4 Speed 254k SOLD
Blue '82 300D 225k SOLD
White '95 E300D 46k SOLD
Blue '87 190D 2.5 Turbo 315k SOLD
Brown '80 240D 4 Speed 716k SOLD
Beige '80 300D N/A 119k SOLD
Blue '85 300D Model 186k T-Boned