1990 Mercedes-Benz 300D Body Control System Diagnosis & Testing Cost

Older vehicles had few accessories and electrical components, so computerization was not needed. Manufacturers were able to use a small wiring loom, or wire harness, to make every connection to every electrical component in the vehicle. While newer vehicles have many of the same systems, we demand more from our vehicles in terms of convenience, security, performance, emission reduction and functionality, so hundreds of additional electronic components are now added to each and every vehicle manufactured. The majority of vehicles see this increase in the engine bay, and these components are likely emissions related. However, when comfort, performance, functionality and security are added, the list of electronics piles up quickly - as does the need to manage these systems. Enter the body control system. It is comprised of all functions of the vehicle, devices to link those functions, and processing units to control those functions.

This system, for all intents and purposes, controls every function of the vehicle in some way. For instance, the doors may lock from a button, but they also lock when the vehicle hits a certain speed or unlock when the shifter is placed in park. This is accomplished because the body control module has information from the transmission position sensor, door lock switch, vehicle speed sensor, and the individual door locks. So when the doors are locked with the switch, the body control module is taking the same action it does when the speedometer reaches a select speed. This being the case, it must be understood that each function of the vehicle which cause the doors to lock, including pressing the switch, is actually a communication to the body control module. When the body control module gets this signal, from any of its' sources, it will take the action to lock or unlock the doors based on its programming. This process is the same for all aspects of the vehicle. Some of which may be the headlights, anti-theft system, ignition switch, and many others.

The body control system, given its scope of operation, can have any number of symptoms. Warning light activations, functions not working properly or not turning off, failure to start the vehicle, exterior or interior light failure, driveability issues, or constant anti theft system alerts are to name a few. Remember, this system controls or communicates to all aspects of the vehicle, so symptoms will range greatly. It should be mentioned that the symptoms may or may not point a skilled technician in the right direction, and some luxury vehicle body control system issues go unresolved without manufacturer engineer intervention.

Depending on the symptom, the vehicle can be left in disrepair indefinitely or cease all function. In order to decide whether or not a body control system failure is detrimental to operating a vehicle, a professional technician should evaluate the vehicle, provide a diagnosis, and alert the owner to the level of urgency.

Most electronic failures will call for body control system diagnostics. They are not commonly considered body control system diagnostics because the body control system is broken down into much smaller components. In general, all vehicles will have some part of the body control system fail during their useful service life, and most likely more than once.

For all components of the body control system, a diagnostic scanner can be connected to the vehicle to scan the body control module. While scanning the body control or additional accessory control modules, a technician can command and observe the operation of specific components, and observe the result of operations. This will generally lead to a diagnosis or, at the very least, a path for testing.

Body control modules, as well as many other accessory control modules, can fail, and this is common on some vehicles. We do not recommend replacement of a control module with a used unit in most cases. Some can be replaced with new or used units, however, many, such as Anti Theft control modules, can only be installed on one vehicle since the VIN number is permanently stored. Some control units can be reprogrammed, and this may come with a charge from the dealership or authorized repair center for that brand of vehicle.

Arching from the battery, jumping a vehicle incorrectly, or servicing electronic components without removing the negative battery cable can damage multiple body control system components by sending positive current through the ground connection for those components.

Most DIY mechanics will not have the experience to service or diagnose severe body control system issues. However, fuses, relays, switches, grounding wires and straps, or bulbs are easily diagnosed and repaired by most people. However, body control system problems are generally called electrical problems, and these are typically left to professional technicians.