Air Bag Control Module: Description and Operation
SRSCMThe SRS airbag system has sophisticated electrical and electronic components. Therefore the airbag operating components should be handled very carefully.
SRSCM (Supplement Restraint System Control Module)
SRSCM determines when to deploy the airbag module by sensing the frontal and side impact force through the built in sensor SRSCM.
1. DC/DC convertor : The DC/DC convertors of the power supply includes a step up and a step down converter, which provide the firing voltage for six firing circuits and the internal operating voltage. If the internal operating voltage falls below a defined threshold a reset is executed.
2. Arming sensor/safing sensor : The arming/safing sensor built in to the airbag firing circuit has the function of arming the airbag circuit under all required deployment conditions and maintaining the airbag firing circuits unarmed under normal driving conditions. The safing sensor is a dual-contact electromechanical switch which closes if it experiences a deceleration exceeding a specified threshold.
3. Back-up power: The SRSCM has emergency energy reserves to provide deployment energy for a short period when the vehicle voltage is low or if lost in a vehicle frontal crash.
4. Malfunction detection : The SRSCM continuously monitors the current SRS operation status while the ignition key is turned on and detects a malfunction of the system. The malfunction can be displayed in the form of diagnostic trouble code using Hi-scan.
5. MIL (Malfunction Indication Lamp) notification : If any fault is detected, the SRSCM sends a signal to the indicator lamp on the cluster to warn the vehicle's driver. The MIL indicator is the key to driver notification of SRS faults. Verify lamp and SRSCM operation by observing the flashinging 6 times when the ignition- switch is first turned on.
6. Malfunction recording : Once a fault occurs in the system SRSCM records the fault in the memory in the form of DTC and the DTC is erased by Hi-scan.
7. Data link connector : The SRSCM memory stored data are linked through this connector located at the underneath of driver side crash pad to an external output device such as Hi-scan.
8. After firing the airbags once, the SRSCM cannot be used again and must be replaced.
9. Crash output
The crash output is used to control an external device which will unlock the doors in case of a crash event. The crash output is specified as follows : 0-200 micro A in OFF mode and 200 mA in ON mode. In case of an unlock command the switch is closed for 200 mS.
SRS HARNESS
The SRS harness is wrapped in yellow tube to identify it from other system harnesses. A shorting bar is included inside the wiring connectors of DAB, PAB, DSAB, PSAB and BPT inflator side. The shorting bar shorts the current flow when the connectors are disconnected. The circuits to the inflator module are shorted in this way to help prevent unwanted deployment of the airbag when serving the airbag module.
SRSCM INDEPENDENT LAMP ACTIVATION
The SRS malfunction indicator lamp (MIL) is located on the instrument cluster to provide information about SRS operating conditions.
There are certain fault conditions in which the SRSCM (SRS Control Module) cannot function and thus cannot control the operation of the lamp. In these cases, the lamp is directly activated by circuitry that operates independently of the SRSCM, as follows:
1. Loss of ignition voltage supply to the SRSCM : lamp turns on continuously.
2. Loss of internal operating voltage: lamp turns on continuously.
3. SRSCM not connected: lamp turns on through shorting bar in wiring harness connector.
MIL Operating Method:
MIL OPERATING METHOD
CLOCK SPRING
The clock spring (coil spring) consists of two current carrying coils. It is attached between the steering column and the steering wheel. It allows rotation of the steering wheel while maintaining continuous contact of the deployment loop through the inflator module.
The steering wheel must be fitted correctly to the steering column with the clock spring at the neutral position, otherwise cable disconnection and other troubles may result.
PPD (PASSENGER PRESENCE DETECTION)
The Passenger Presence Detection system detects the presence of passenger in the front passenger seat. The system is designed primarily to prevent replacement of airbag components that fire needlessly in an accident. The PPD system consists of a weight sensor unit.
The threshold for the PPD is : occupied (15 kg, 33 lb): not occupied (0.6 kg, 1.3 lb). The sensor characteristics are:
RP ≤ 50 k Ohms for W ≥ 15kg
RP > 50 k Ohms for W < 0.6 kg
(RP : Resistance of force sensing resistor)
(W : Mass of weight)