From the W116 and W126 models, Mercedes Benz began incorporating sophisticated electronic systems to control various parts of their vehicles. If you own a W124, you're probably familiar with the advanced electronics it boasts. In fact, the W124 took these innovations further, introducing even more sophisticated and widespread use of electronics across different systems. This model marked a pivotal transition for Mercedes, paving the way for the high-tech, electronically controlled vehicles we see today. However, the W124 is also known for something a bit more infamous: its biodegradable soy-based wiring harness, which, unfortunately, became a treat for rodents! 😋 Jokes aside, the W124 is packed with advanced systems like the Comfort Control Unit (CCU), which manages electronic windows, cabin lights, central locking, the sunroof (if equipped), and other optional features. The CCU is tucked under the rear seat, in the left-hand corner. To access it, you'll need to remove the rear seat, followed by partial removal of the thick sound-dampening material underneath. The module is secured by two bolts (10mm and 8mm) and connected by four connectors—two of which have an ingenious sliding tab mechanism to release them. Now, getting to my recent experience (and the reason why I am ranting about the CCU is...): about six months ago, my front left electronic window started malfunctioning. It would roll down but refuse to go back up. The only fix was disconnecting and reconnecting the battery, after which the window would work again (only once). Since the window switches are located in the central console, my first suspicion was the power switch itself (was not obvious considering the battery-connection recycling factor). I checked it, and it was functioning correctly. After that, I didn’t do much, except for disconnecting the window motor and leaving it be for six months. Fast forward a couple of months, and a friend of mine, @Prithvi Nath Tagore, drove my car. His complaint? "Your windows don’t work!" (He meant the front left one). His remark finally pushed me to take the issue seriously, and after another month of procrastination, I decided to tackle the problem. First, I checked the fuse box located under the hood. On RHD cars, it’s at the right rear near the firewall (on the opposite side for LHD models). The box has two compartments—one for fuses and another for relays. The relay controlling the power windows fits the socket labeled "A." I inspected the 30-year-old relay, but it seemed to be in good shape. The Relay Next up—the CCU! After removing the rear passenger seat and the two nuts securing the CCU, I disconnected the battery (always a good practice when dealing with electronics) and unhooked the unit from its harness. With the cover removed (a bit of prying around the center locks does the trick), I inspected the internals. The unit was clean, free from corrosion, bad solder joints, or corroded relay contacts. Comfort Control Unit (unbolted) I reconnected the CCU, keeping it on an insulated base (to prevent shorting), and operated the window switches. After a quick test, I found that the relay controlling the upward motion of the front left window was staying latched even after the switch was released—there’s the culprit! Or is it? The real issue wasn’t the relay itself, but whatever was causing it to remain latched. So, the unit was removed and brought to my workbench for further inspection. CCU in operation.mp4 CCU - prepping for diagnostics & surgery! Stay tuned for the next steps!