Pulled the schematic on the 'warning module' and traced several wires.

OK, original problem, no buzzer, lights or key and driver's door would not operate interior lights. Ironically, my indy never found the unplugged 'warning module'....(I happened to find it after pulling the underdash soundproofing to look at the parking brake cable)

I did pull the #2 from the socket, which, according to the All-Data drawing came from the key warning switch. Oh, after plugging in the 'warning module', everything worked BUT the buzzer kept going after the starting the car. All the other items seem to trace with out a short. I honestly thought that removing the key warning would eliminate the problme.

Thoughts ?? ideas ??

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87 300SDL - 215K Miles !!
99 F-350CC Dually PSD - 190K
86 300SDL - 189K
All on B-100

Not knowing how much similarity there is between your SDL and my '87 300D Turbo (W124, OM603) . . . my schematic shows that pin 2 on the warning module connects via the key warning switch to +12 Volts. Unfortunately, it doesn't indicate whether the switch is open or closed when the key is in the ignition switch.

You could try connecting pin #2 directly to 12 Volts and see if that stops the buzzer. Shouldn't hurt since that pin gets 12 Volts anyway. Maybe the key warning switch is bad.

Also double-check the fuse that feeds that switch, make sure it doesn't have a hairline crack. In the 300D it is fuse #9, no idea what it is for your SDL.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

I'd try to source another buzzer module. When I got my SDL someone had slammed the door on one of the entry lights, shorting it out and blowing up a trace on the board of the buzzer (the entry lights ground through the door switches, and the driver's door switch is connected directly to the buzzer module).

I would venture a guess that the older cars use the same buzzer unit, they simply didn't hook up the seatbelt warning light to it on the 86+ cars.

-Jason

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.