My dome light turns 'on' when front passenger door is open but will not turn
'on' when driver door is open.
The push button switch from the driver door has tested okay by hooking this on the passenger side. So both push button switches are okay.
I suspect a grounding fault, and if it is, I wouldn't know how to fix it.
Does anybody know the solution to this? Will appreciate any suggestions.

W123 dome light works by grounding the lead coming from the light to the door switch.

The driver side switch lead passes through the seatbelt/headlights on/key in switch warning box. There is a coupling diode in there that isolates the left side switch for the logic in the box so that the buzzer only sounds if the door is open, key off, and either lights on or key left in switch.

I would check to see if someone unplugged the buzzer box. That would stop the left switch from turning on the dome light.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Thanks for your prompt reply.

Kindly guide me where I will find the location of the buzzer box.
I am not too good with these things.

I will check back to see your answer after running a pressing errand.
Thanks again.

Buzz box under the dash, drivers side, near the parking brake.

Online wiring manual is located at startekinfo.com

Click on MB workshop resources, look for 123 service info in the results, about 3 pages in.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Hi Jay Bob,

Thanks for all these data. I have a lot of studying to do to make sure
I'll do this correctly.

Hi Jay_bob,

I checked startekinfo.com and they didn't have the info on cars older than 1994, my 300D is 1984.
Once our weather gets warm enough, I'll be tracing where the pushbutton wires will take me. Hopefully I'll find the end of it merely unplugged.

Meantime, I wonder if the dome light will work by wiring the driver side switch directly to the passenger side switch, bypassing the warning buzzer box. ANyway, I am only interested in getting the dome light to turn on each time I open the driver door.

Eager for a warmer weather.

I know "old post" (back off trolls). Found this thru a google search.

Exactly my problem and brilliant answer. I was trying to understand the 1984 wiring schematic in my Haynes manual which shows both L & R switches in direct parallel can ground the dome lamp. It does show the buzzer coming off the L switch, but as a separate "sense" lead (?). Also, the wires on the car didn't match the Haynes schematic, so I was stumped, but appears the schematic isn't quite right.

Last time under the dash I removed the buzzer box to better pull down the fuse box, and left it out thinking it was the alarm box which I removed from my 1985 (maybe 1984 too) because evil and bothersome. I also noticed I didn't get a warning buzzer a few days ago when I left the headlamps on (good I noticed), which should have clued me. All works now, but not before I hacked into the sheath where the L switch branches to poke around (fixed w/ RTV tape) and shattered the door switch by closing the door with switch pulled out (but rear switch fit). But most of my repairs suffer some collateral damage.

__________________
1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Funny I just bought this car referenced at the beginning of this thread via online auction. I then found this thread as I was researching the problem and you guys are spot on. It turned out to be a bad diode in the warning device. I replaced the diode and the dome light works again. The black plastic parts to the right are the old diode pieces and used to be located third position from right. See pointer.

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