Light it Up

Down the trouble-shooting rabbit hole to find and fix a fault with an unlit A124 license lamp

Article and Images Scott Brinkley

Now it was perfect. Almost. The 1993 A124 E320 Cabriolet my Toronto Section colleague had located for me was in my garage and all mine at last. After a couple of weeks addressing some Convenience-relay issues, seatbelt presenters in need of lubrication, and a broken power-antenna actuator gear, I finally noticed the one dashboard warning light illuminated in the car: the lamp indicator warning, telling me one of my lights wasn’t working.

It appeared immediately after switching on the parking lamps, so that was a context clue. The Lamp Control Module (part No. 126 542 01 32), an innovation from the W126 era, the system monitors voltages on the left- and right-hand sides of the car. Should there be a difference between the two sides, the system will illuminate the dashboard indicator lamp. Three things can cause this condition.

Obviously, the first thing to check for are bulbs that are not illuminating as expected. Not knowing the intimate details of the previous owners’ bulb-replacement schedule, I took a shotgun approach and carefully replaced each lamp in the parking-lamp circuit with the OEM-spec bulb, making sure pairs matched and that I had bought the correct voltage and base of bulb. The Owner’s Manual does foreshadow how picky the system can be if DIYers get creative in purchasing bulbs at the discount store. For good measure, I obtained some contact cleaner from my local music store (often used it on electric guitars and DJ equipment), and cleaned the bulb sockets before installing replacements.

The second thing to troubleshoot is the fuse box (Image 1). Knowing I had replaced most of the torpedo fuses in my 124-chassis fuse box, I merely reinspected these to ensure the contacts for fuses 3, 5, and 8 were clean and their fuses intact. The old-style fuse boxes, fuses, and copper contacts can degrade over time; fortunately my car is practically  perfect preserved.

Third, there is the bane of all Mercedes-Benz vehicles built in the early to mid-1990s: the wiring. While my engine wiring is in good shape, there is a common spot on the 124 chassis that needs extra attention. It’s actually more of an area – the entire rear end of the car.

Having already inspected and replaced the entire set of rear-end bulbs, I disassembled each rear lamp (thankfully, a 10-second, no-tools job on each side), cleaning it with contact cleaner on the metal bars that not only held each bulb in place, but also completed the electrical circuit that lights them. These can otherwise corrode, and the ground wires attached can easily become separated or poorly attached.

Unfortunately (or fortunately), this was not my problem. Glad that my car had not been exposed to the moisture that precipitates corrosion, I moved back up front to the fuse box. A friendly member of a Mercedes-Benz online forum I frequent pointed me to the aforementioned lamp control module. More specifically, I was interested in the connectors exposed once the module box has been removed from the relay box behind the 124 chassis fuse box (Images 1 & 2). Removing the rain gutter underneath the windshield helped me access the box.

Suspicious of all the rear lamps – research and inspections of the entire car’s exterior lighting system made me so –  I focused on the 10-pin X2 connector of the three connectors present for inspection. With my multimeter set to 20K Ohms, I went to work measuring resistances on the pins, from pin to ground; each one, except for No. 4, registered an 18.9K Ohm readout. According to the forum poster’s table, this lead me to conclude that something was awry within the wiring going to the left license lamp.

Sure enough, careful disassembly of the cover and sheathing on the wiring harness following the left-hand trunk hinge lead to the discovery of one broken and one exposed wire (Image 3) in the harness for the third brake light, license lamps, and trunk interior lamp switch; I had found my smoking gun! I was now only a solder repair and some electrical-tape mummification away from putting the one big flaw in my A124 to bed.

Little did I know at the time that I was killing two birds with one stone. That the license lamps were illuminating with the wiring in this condition indicated a short somewhere. I had verified that the trunk lamp had not been staying on by shutting my smartphone in the trunk with the video recorder operating. The previously suspected license-lamp circuit must have been the culprit in my previously undiagnosed (and assumed-normal) battery-drain condition.

Hasta la vista, trickle charger!