Mystery solved! No dome light when driver's door is opened - PeachParts Mercedes-Benz Forum

Benz Mondi · #1 10-31-2015, 11:44 AM

It finally got to me so I had to fix it.

1990 300E originally purchased as my son’s college car, we soon realized that it had too much work to be done, major and nuisance, for him to deal with while in school at SoCal.I brought it home and he now has my Grand Cherokee.I’m one of those guys that like to have his car working properly in every respect, if it’s at all practical to do so. Relatively easy fix for this nuisance. Yes, checked all the other possible causes like "a break in the wiring at the door".

Symptoms:
1. Open driver's door dome light will not turn on, OK on passenger side
2. Light at bottom of door doesn't turn on
3. Seat belt buzzer doesn't turn on (some of you are already guessing, but wait for the answer)

What I found:
The door light is controlled by the door jam plunger switch. It supplies the ground contact, power is supplied when the parking lamps are turned on.
The dome light gets its ground signal from the same door jamb switch.
The door jamb switch simply connects the ground connection from one side of the switch to other when the door is opened. One side has two brown wires. The other has a single wire, brown/black with grey dots, (the signal wire that was not communicating with the dome light). Door switch is fully functioning. Simply jumping the switch did not turn the light on.

The work around... door light
I simply ran a 2nd wire from the door switch to the door light. I disconnected the existing ground wire from the door light and the single wire side of the door switch. Connected the new wire it to the light and to the switch- door light works as designed.

The work around... dome light
I did some testing between the wiring to the door switches from the driver's and passenger side, and from examining the plug at the right side of the dome light (slide the dome light to the right and gently pull the left side toward you to remove). The typical great MBZ designers allows for the plug cover to be removed and tilted up from one side. The signal wire from the driver's side is connected to the socket at the upper right hand corner of the plug.Tested the continuity and it passed.My attempt at jumping a new wire, from the new ground wire at the door switch, after taking enough fascia off to run a wire from the light, down the left side of the windshield, down the side of the dash and into the existing opening to the door switch, up to the did not work.So I just jumped the ground wire connection for the passenger side (the bottom right corner of the plug). Now the dome light turns on and off when the driver’s door is opened and closed.

But wait… that’s not all…

The Proper Fix
The clue is that the seat belt buzzer doesn’t sound still.
A relay is the problem. In the back of the dash, after you have pulled out the speedo cluster, you will see two relays mounted below a bracket (8mm on the left, 10mm nut at the rear).The one on the left is the lighting control relay (panel lighting dimmer), the one on the right is the offending relay (seat belt buzzer, dome light timer, and power seat control).I didn’t mention that one of the fixes on my To-Do list is to figure out why when I open the door, without putting the key in the ignition, I can move the seat on the passenger side,, but I can’t on the driver’s side- a ha!

At the time I did the alternate wiring, I actually hadn’t figured it out. It was only while at the Pick N Pull salvage yard yesterday that I took the time to pull out these two relays that I had always seen, but didn’t know anything about.I did a parts search on eBay.Low and behold… you now know the rest of the story.

So you have two ways to solve it: (without taking the speedo out and having the relays) run alternate wiring, and the proper fix.

You don't have to disassemble the car as much as I did in the pictures. I was doing some other fixes at the same time. See my other post for the Check Engine Light. In one of the pictures you see the offending signal wire that show my previous attempt at jumping the alternate wire. I just covered it with tape, but will properly seal it when I go back in and restore the original connections after I replace the relay.

AL

Benz Mondi · #2 10-31-2015, 12:18 PM

Here's what they look like together.