'84 300D - no power to front interior and visor lights

[rgnprof]

Have searched and can't find anything about my particular issue...I have no power at the door switch on both front doors (the rear doors are fine and both trigger the rear dome light to come on), nor do I have power at the visor clips, and no power at the wires that connect to the front dome light.

Any ideas?

Thanks, Ryan

[jay_bob]

Fuse F2 on the 123 is the constant hot fuse. It runs both the front and back dome lights, so if you have rear dome light then your fuse is not the problem.

From the bottom of F2, there are 2 wires. One goes to the front dome light, the other to the rear dome switch on the console. Since your rear light works we know that leg is ok.

The front dome light gets power (solid red pin 30) and ground (brown pin 31) through the connector on the dome light. The front door switches apply ground to the TK pin on the light module (via brown-gray wire). The door switches are not between the battery and bulb like you might expect.

The visor lights get power through a loop through connection (brown-red pin 30) in the dome light assembly. If the plug is disconnected from the dome light you will not have any front dome lights nor visor lights.

First I would pull down the dome light and check for voltage at the solid red wire (pin 30). Then go tracing from there.

[rgnprof]

Thanks for the reply...

I have already pulled the dome light - no power at any of those wires that connect to the light. I have been reading over the wiring diagrams and I'm thinking the seat belt/buzzer relay may be the problem...

ryan

[rgnprof]

Does anyone know if a bad seat belt/buzzer relay would cut power to the front dome light?

ryan

[rgnprof]

I installed a new(used) seat belt relay - I now have 12v when I open the front doors ONLY! Shouldn't I have 12v all the time - at some connections?

Visor lights still don't work...

[jay_bob]

Ok back to basics. Do you have access to a voltmeter or test light? If not go pick up a cheap meter from the big box store.

Let's start with the connector on the light assembly. You should have 4 points on the connector:
Pin 30 - red - power from F2
Pin 30 - red/brown - power to vanity lights
Pin 31 - brown - ground
Pin TK - brown/gray - door switch (Tür Kontakt or Door Switch in German)

Take your meter/test light and see if you have voltage between the solid red 30 and the brown 31.
If you don't get anything, try solid red 30 to the car body. If that has voltage, the ground from 31 to the body is bad.

Also see if you have voltage from the red/brown 30 to the brown 31 with the light plugged in. If no voltage, see if the jumper clip between the 30s on the light is present. The vanity light power passes through this clip.

Now with the light plugged in, take a piece of jumper wire and jumper pins 31 and TK. That should make the light turn on.

Try all that and let me know what you find...

[jay_bob]

And another thing, make sure the switch is on. There are 3 positions on the switch.

The center is off. The half moon symbol with sun rays is "on all the time" and the plain half moon is "on only with the door open". Can't remember on a 123 which way is which.

If you have the fancy model with the reading light the symbol of the book is reading light mode.

[rgnprof]

Jay-Bob,

Thanks for the extremely helpful post! I have followed your steps as outlined - everything checks out except for the light coming on when I jumper pins 31 and TK. All the voltages check out and I even have 12.5v at the two visor connections - Just NO lights. I know the bulb is good as I connected it straight to a 12v source...and yes, I made sure the switch was ON.

I am assuming this means a bad light itself (nifty looking circuit board in this thing) - $140!!! Which I didn't really want to have to replace. Any way of checking the circuitry in the light? Bad connections? I can't even really see how to get in there to clean anything w/o breaking it.

Again, thanks!
Ryan

[jay_bob]

There is an electronic circuit in there. I took mine apart and fixed it. The switch had fallen apart.

I posted the results on W123 Dome Light Switch fix - Benzworld.org - Mercedes-Benz Discussion Forum

I would also hit the p-n-p if you need a replacement. If you take along a 12 V cordless drill, a known good bulb, and a couple test leads in your bag you should be able to test it in the yard using the drill battery.