1990 Mercedes-Benz 350SDL Antitheft System Diagnosis & Testing Cost

Most vehicles are factory equipped with some sort of technology to prevent or deter theft of the vehicle and it's contents. These systems get the generic name of anti theft systems, but there are great differences between systems depending on year make and model. For instance, older vehicles have used keys with embedded resistors, and many modern vehicles use transistors and infrared sensors on keys to prevent unauthorized starting of the engine. But the systems are much more intricate than just a secure key.

Anti Theft systems work by adding components to the vehicle to detect the proper key at every position of use, and making the system redundant to avoid bypassing security systems. Most modern vehicles first have a wireless control module which must accept the key fob before unlocking, locking, remote starting, starting with the key, or starting the vehicle with the push button start feature. This module will interact with the key at various locations in the vehicle and outside of the vehicle when within a certain distance. To prevent tricking the wireless control module, the module must be checked by one or more other control modules, likely an anti theft system control module, body control module, and engine control module, before it can allow the doors to be opened or the engine to be started. These highly redundant systems nearly eliminate the possibility of accessing the vehicle or starting the vehicle with the incorrect key. Unfortunately, they do not prevent breaking and entering, but will sound an alarm by horn and lights with unauthorized entry.

Attempting to start a vehicle with all of this technology to recognize the key should be seamless and simple. However, in the case of any portion of the anti theft system failing, even intermittently, the vehicle may become impossible to access or start. Some issues will cause the alarm to sound at random, disallow the key fob from accessing anything on the vehicle, sound the alarm anytime the hard key is turned in any lock cylinder, or allow the vehicle to be accessed but prevent operation. Also, some models will prevent the vehicle from being moved from part to any other range in an effort to prevent the vehicle from being driven or moved.

Typically the anti theft system must be repaired before the vehicle can operate at all. There are some failures that will allow the vehicle to operate, but any issue with the anti theft system leaves the driver and passengers at a risk for being stranded or locked out. Lastly, since the wireless control module has access to the door and trunk locks, it may cause the vehicle to become impossible to secure.

Since failure of the battery in smart keys will cause issues with the anti theft system for the vehicle, the anti theft system will experience a fault with most modern vehicles. However, hardware components in the vehicle fail at random, are always unexpected, and the failure will not relate to age or mileage in most cases. Some vehicles are prone to anti theft system failures, but most will be trouble free.

anti theft system failure can be extremely troublesome to diagnose without dealer software, however it is not impossible. When diagnosing the issue, a technician will use a computerized scanning tool to observe conditions within the anti theft system, identify where the miscommunication originates, and service that portion of the system. For instance, if the body control module is receiving correct information to signal the engine control module to start the vehicle, the engine should be able to start. If it does not signal the engine control module to start the vehicle in this case, the circuit, body control module, and engine control module will be tested to find which component is failing to interpret data. This can be a long and difficult process when intermittent issues are present, and, in some cases the dealership will need to perform the repair.

The anti theft system is meant for the protection of the vehicle and its contents. Although there are services which will remove these functions from the vehicle by reprogramming various control modules, this is not recommended. Removing anti theft features places the vehicle at risk of theft or burglary, and reprogramming control modules can cause software failures.

Replacing the anti theft system control module, body control module, or engine control module, even if that part is faulty, will not resolve an anti theft system issue alone. These control modules are keyed to the vehicle VIN, so they must agree when communicating. Some modules come ready to accept the parameters of the control module(s) they are in contact with, and this is a valid solution. Before purchasing these parts, ensure you understand whether the control module will need to be programmed by a professional after installation.

Repair of the anti theft system is best left to professionals. This is an intricate system which is designed with redundancies to prevent tampering. Also, many service manuals will not include some vital information needed for pinpoint testing of the system, so there may be a bit of assumed knowledge involved. Also, more simplistic diagnosis will require a scan tool or factory anti theft diagnostic software that is not typically available to consumers. Unless the issue is related to a lock cylinder, dead key battery, or fuses, leave repair and diagnosis to a professional technician.