A couple of weeks ago I drove my 1987 300D Turbo (W124, OM603) at night for the first time in awhile and realized that only some of the dash lights were working. The cluster and some of the illuminated controls were fine, others dead. All the fuses were OK and the dimmer worked too. I couldn't figure out a pattern and ended up copying the wiring diagram for the cluster and console lights and marking "good" in green and "bad" in red (well, "pink").





Except for the heated rear window switch, all of the "bad" lamps were powered off of one contact, which got its power from "N40," [marked in yellow] the "instrument illumination control unit." N40 lives behind the instrument cluster, next to the warning buzzer control. This thing was added by Mercedes to take some of the load off of the dimmer and keep it from burning out as much as in the older cars. It's like a relay with a volume control. Pin 30 is power and 31 is ground. Pin 58d is the control wire from the dimmer and 58D powers a bunch of the lights.

N40 itself turned out to be fine -- I have two spares, neither worked -- but I discovered that pin 30 was not getting any power from fuse 9. That fuse was OK and everything else that worked off of fuse 9 works fine -- radio, etc. Apparently I have a broken wire somewhere between fuse 9 and N40 pin 30.

I temporarily connected pin 30 to the clock wire and all the lights started working. I'll make a permanent fix later, running a new wire to fuse 9. Today I pulled the fuse box apart and checked all the wires. None seem to have any problem at all. Maybe the break is in a connector that doesn't show on the diagram or even worse, in the middle of a wire somewhere. This is very strange -- these wires don't get flexed like B-pillar power window wires.

Has anyone ever had a problem like this? Thank goodness for the wiring diagrams. Even with them it was a puzzle until I figured out how N40 worked.

[Notes: (1) The heated rear window switch is probably a fluke -- someone rewired my car or the diagram is wrong. (2) There are other lights that work off of the same circuit but I left them out to save space.]

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