Mercedes-Benz SL400 Headlamp Control Module Replacement Cost

The light control modules in a vehicle should last as long as the vehicle itself. Most lighting problems stem from issues with the bulbs, fuses, relays, or wiring. But a computer module can be at fault; they do occasionally go bad.

Unless a vehicle is displaying a diagnostic trouble code, either as a warning light on the dashboard or by way of a special scan tool, a technician will have to rely on a process of elimination to diagnose a faulty light control module. Many of the symptoms of a bad LCM (or other module) can be attributed to other causes. Intermittent headlights can be caused by a corroded ground at the wire harness. Rapidly blinking turn signals can be a result of a burned out bulb. Starting with the fuses, relays, and bulbs, a technician will rule out the simplest and most common lighting failures first. A series of tests for voltage and resistance with a multimeter will help to locate problems with the wiring or the connections in the system. A qualified technician is skilled at pinpointing a specific problem, targeting the electrical circuit where the problem presents itself, and honing in on the root cause of the problem. Special knowledge, tools, and vehicle-specific information is necessary for proper diagnosis.

Accurate diagnosis is usually the most difficult part of light control module replacement. The procedure for replacing a module is specific to each vehicle because it can be located in a number of places. An LCM is usually located below the steering wheel in the lower dashboard, but it can be located elsewhere: behind the glovebox or in the engine compartment near the fuse block or on the firewall. Other modules that control specific adaptive headlamp functions, such as auto-dimming, auto-leveling, and auto-tuning functions, may be mounted on the headlamp assembly along with the HID igniter module. Disconnecting and removing an old module, and replacing it with a new one, is a matter of unplugging the old and plugging in the new. It is not complex. Accessing the module may be a different story. If the module is located in the dashboard, interior trim panels will need to be disassembled. If the module is mounted on the headlamp, the headlamp assembly - and likely other components - will need to be removed. Again, vehicle-specific repair information is essential. And the new module will probably need to be programmed before it will work.

Some repair operations can be done without too much difficulty. On many vehicles, a tail lamp bulb or a headlamp bulb can be replaced by the vehicle owner. RepairPal recommends always using a replacement headlight bulb (or any other bulbs) of the same style and wattage as the original bulb. Bulbs outside the range recommended by the vehicle manufacturer may cause a bulb warning light to come on and even damage wiring or the headlamp control unit. Care should be taken to avoid touching a headlamp bulb during replacement. Oil from the skin will shorten the life of a halogen or HID bulb.

A faulty light control module may need to be reprogrammed. The replacement for a bad module may need to be programmed before use. This requires special equipment and software. The lighting system will not operate properly otherwise.

If there is no doubt that a light control module needs to be replaced, an intermediate to advanced DIYer can tackle the repair. But the new module will probably still need programming. And the chances of installing an incorrect module, not to mention one that was not bad in the first place, is high. Diagnosis and repair of the lighting system that goes beyond replacement of the bulbs, or maybe a lamp assembly, should probably be left to a qualified technician at a trusted repair shop.