SRS Control Unit

WARNING: This page is about a different car, the 1993 Mercedes-Benz 500SEL and 1993 Mercedes-Benz 500E. However, it is still accessible from the selected car via links, so may be relevant.

SRS control unit is located under center console, forward of shifter, and is bolted to transmission tunnel. SRS control unit incorporates an electrical sensor to detect deceleration rate, a mercury switch to override threshold evaluation switch, a microprocessor and a memory system to process and retain information (even after main power supply has failed).

SRS control unit monitors, detects and evaluates front-to-rear vehicle deceleration and acceleration rates, and processes this information through an internal dual threshold evaluation switch. When a preset threshold value (maximum deceleration rate) is exceeded, appropriate air bag deployment is activated according to evaluated seat belt sensor status information (regardless of whether or not the passenger-side seat belt is buckled).

The SRS control unit monitors and records the combination of "G" force and change in speed (deceleration and acceleration). All of the following are required to activate SRS: high "G" force, closed mercury switch, and very rapid deceleration or acceleration. Consequently SRS control unit cannot be activated by a single hammer blow or other similar influence, which only produces high "G" force.

The sharp deceleration produced by a collision causes the mercury switch to close, while SRS control unit's electrical sensor simultaneously determines whether or not the impact is sufficient to trigger gas generator. Activating SRS requires that both the mercury switch and electrical sensor detect a collision.

Two types of SRS control units are used. One unit contains a 12-pin connector to be used on vehicles without passenger-side air bag. The other unit contains a 16-pin connector to control driver-side and passenger-side air bags. Both SRS control units function as a diagnostic monitor, which continuously monitors operation of complete SRS system and records any faults, regardless of ignition switch position or engine operation.

SRS control unit will activate SRS warning light in the instrument panel until a fault is corrected and SRS memory is cleared. SRS control unit is powered by energy accumulator (capacitor) if normal power source (battery voltage) is interrupted at any time.