The absence of lighting at the transmission quadrant of the W126 chassis due to bulb burnout is a serious problem. Here's what to do to restore lighting to allow correct shifter positioning at night:

1) Remove the ashtray, and its support assembly underneath, removing the plug to the lighter, and the lamp and socket for tray lighting.

2) Remove the radio.

3) Remove the upper panel above the radio that carries the hazard warning switch and others. This panel is held by two small screws along its bottom edge that pass through the radio's DIN frame and into the hard plastic of the upper console framework. The top edge is held by two tabs that engage the bottom edge of the dash just forward of the dash vinyl covering. Carefully pull downward and then rearward to remove.

4) Unplug and remove all the switch connectors with their wiring harnesses from the switches on this upper panel, taking special care with the thin fiber optic cables (jackets are black in color and are about the diameter of a thin spaghetti strand.) The fiber optic cables have a brass end; pull this out from the switch body using long-nose pliers; not much force is needed. All switches have unique connectors making confusion during reassembly impossible. Set the panel aside to avoid damage.

(When later reassembling these components, first route each fiber optic cable through its hole in the connector and insert the brass end into the switch until it bottoms, using long-nose pliers. Then plug the connector onto the switch.)

5) The light source for the transmission quadrant is a single 1.2 watt miniature bulb, held in a uniquely-shaped black plastic bulb holder. This bulb holder is about the diameter of a half-dollar coin and is mounted vertically on a flat panel about 5 to 6 inches straight forward from the edge of the dash just below the center air outlets. It has several fiber optic cables entering at uniformly spaced points around its circumference. The opening for the bulb faces the rear of the car. To locate the bulb holder, visualize a level imaginary line starting at the bottom of the dash above the hazard warning switch position and just below the midpoint of the two center dash air outlets. Project this line forward toward the engine area about 5 to 6 inches.

SUGGESTION: Assemble a miniature worklight using a low-wattage taillight bulb having a brass base, soldering two pieces of stranded insulated wire (18 to 20 AWG) each long enough to reach outside the right front door to a voltage source; one to the brass base, the other to the center contact. Insulate the connections with heat shrink tubing. Before searching for the bulb holder as described above, connect this worklight to 12 volts DC or to a battery charger's leads and position the worklight inside the dash area above the ACC module as a light source. It makes work possible in this dark area and lessens frustration. Do not use a brake light or backup light bulb as their wattage is excessive and heat will be produced that can soften insulation and plastic.

The wiring insulation for the several 1.2 watt miniature bulbs and sockets that you will be working with is of light brown and medium blue color. There is a socket for the rotary temperature selector, several for the pushbuttons across the ACC center, one for the fan operation control, and the important one you are after - the socket for the fiber optic light source.

After you have found the wiring to the fiber optic bulb holder and replaced the bulb, I suggest you then shrink on a piece of 3/8" x 4" long heat shrink tubing over the blue and brown wires that go to this socket for the fiber optic cables. This will stiffen this part of the harness, making it easier to reinsert the socket and bulb into the holder. (Use care when directing the heat gun's air blast to avoid contact with dash surfaces.) During replacement, holding the outer end of the heat shrink tubing with fingers of the left hand while guiding the socket and bulb toward the holder opening using an electrician's screwdriver with thin blade as a probe and guide works well. A long surgeon's forceps with the latch ground off would be ideal for this step.

A bothersome problem that I found with my 1982 W126 while doing this job was (1) miniature bulb lead contact with socket contacts is often erratic, (2) the wiring harness is stiff and unforgiving, and (3) the socket retaining clips on the fiber optic bulb holder, and those on the ACC module, no longer reliably hold the sockets in place. Heat from years of use has reformed the clips such that they no longer perform their intended function.

During reassembly of the upper console components, it is therefore necessary to carefully route the bulb wiring harness and the switch cables so that the replacement of the upper switch panel does not dislodge the bulb sockets from their holders. It took me many attempts at this before being successful. Desiring light at the transmission quadrant made persistence easier. Good luck and remember that patience is a virtue...

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Respect, protect, and maintain complex systems - whether natural or of German design, to benefit from their full potential.

1982 300SD W126.120 with an OM617.950 from a W116.120
1984 300SD W126.120 with an OM617.951