The air bag system used on many vehicles is referred to as the Supplemental Inflatable Restraint (SIR) system. The air bag is designed to deploy when the vehicle is involved in a front end collision of sufficient force, up to 30 degrees off center line of the vehicle. The steering column still continues to be collapsible, the same as vehicle without an air bag.
The SIR system contains a deployment loop and a Diagnostic Energy Reserve Module (DERM). The function of the deployment loop is to supply current through the inflator module in the steering wheel, which will cause air bag deployment during a severe accident. The DERM supplies the necessary power, even if the battery has been damaged.
The deployment loop is made up of the arming sensors, coil assembly, inflator module and the discriminating sensors. The inflator module is only supplied sufficient current to deploy the air bag, when the arming sensors and at least 1 of the discriminating sensors close simultaneously. The function of the DERM is to supply the deployment loop a 36 Volt Loop Reserve (36 VLR) to assure air bag deployment for seconds after ignition voltage is lost during an accident.
The DERM in conjunction with the resistors make it possible to detect circuit and component faults within the deployment loop. If the voltages monitored by the DERM fall outside expected limits, the DERM will indicate a fault code through the storage of a malfunction code and turning ON the INFLATABLE RESTRAINT lamp.
Diagnostic Energy Reserve Module (DERM)
The DERM is designed to perform 5 main functions. It maintains an energy reserve of 36 volts for several seconds. The DERM performs diagnostic monitoring of the SIR system and records malfunction codes, which can be obtained from a hand scan tool or the INFLATABLE RESTRAINT lamp. It warns the driver of a malfunction by controlling the INFLATABLE RESTRAINT lamp and keeps a record of the SIR system during a vehicle accident. Air bag deployment can still take place without the DERM connected, if adequate voltage is present at the arming sensor or dual pole sensor.
The DERM is connected to the system with a 24-way connector. This harness has a shorting bar across certain terminals in the contact areas. The shorting bar connects the INFLATABLE RESTRAINT lamp input to ground when the DERM is disconnected causing the lamp to light when the ignition switch is ON .
The DERM does not need to be replaced after each air bag deployment. After 4 deployments the DERM will register a Code 52. The Code 52 informs that the accident memory is full and the DERM must be replaced.Inflatable Restraint Indicator
The INFLATABLE RESTRAINT indicator lamp is used to verify the DERM operation by flashing 7-9 times when the ignition is first turned ON . It is also used to warn the driver of a SIR malfunction. For certain tests it can provide diagnostic information by flashing the fault code when the fault code diagnostic mode is enabled.
There can be 2, 3 or 4 discriminating sensors. The forward, left or right sensors are located in the engine compartment. The passenger sensor, if equipped, is located behind the right side of the instrument panel or the center of the console. These sensors are calibrated to close with velocity changes which are severe enough to warrant air bag deployment.
The sensors consist of a sensing element, a normally open switch and a diagnostic resistor. The diagnostic resistor is wired in parallel with the switch within each sensor. They provide a ground for current to pass during normal non-deployment operation. The DERM measures this current to determine component faults.
When the arming sensor is located in the same housing as the passenger compartment discriminating sensor, the assembly is referred known as a dual sensor.Arming Sensor
This sensor is found on vehicles with a single air bag.
All sensors are specifically calibrated to each series vehicle and keyed to the mounting brackets. Great care must be taken to mount the correct sensors on the vehicle being serviced. The sensors, mounting brackets and wiring harness must never be modified from original design.
The arming sensor is a protective switch in the power feed side of the deployment loop. It is calibrated to close at low level velocity changes. This insures that the inflator module is connect to the 36 VLR output of the DERM or ignition 1 voltage.
The sensor consists of a sensing element, normally open switch a diagnostic resistor and 2 steering diodes. The resistor is connected in parallel with the switch and allows a small amount of current to flow through the deployment loop during normal non-deployment operation. The DERM monitors this voltage to determine component faults.
The arming sensor is located in the same housing as the passenger compartment discriminating sensor. The assembly is referred to as the dual sensor and is located behind the right side of the instrument panel.Dual Sensor
On single air bag vehicles so equipped, the dual sensor is a sensor that combines the arming sensor and the passenger compartment discriminating sensor into the same unit.SIR Coil Assembly
The coil assembly consists of 2 current carrying coils. They are attached to the steering column and allow rotation of the steering wheel, while maintaining continuous contact of the deployment loop through the inflator module.
There is a shorting bar on the lower steering column connector, which connects the SIR coil to the SIR harness. The shorting bar shorts the circuit when the connector is disconnected. The circuit to the module is shorted in this way to help prevent unwanted deployment of the air bag, while performing service.Inflator Module
The inflator module is located in the steering wheel hub under the vinyl trim. It includes the air bag, inflator and initiator. When the vehicle is in an accident, current is passed through the deployment loop. This current passing through the deployment loop ignites the squib in the inflator module. The gas produced rapidly inflates the air bag.
There is a shorting bar on the lower steering column connector, which connects the SIR coil to the SIR harness. The shorting bar shorts the circuit when the connector is disconnected. The circuit to the module is shorted in this way to help prevent unwanted deployment of the air bag, while performing service.Resistor Module
The resistor module is in the SIR harness between the inflator module and DERM. This allows the DERM to monitor the deployment loop for faults and also allows the DERM to detect if the air bag has been deployed.
The resistors in the resistor module are balanced with the resistors in the arming and discriminating sensors to allow the DERM to monitor voltage drops across the circuits. These resistors also help reduce the possibility of unwanted deployment in the case of wiring harness damage.Knee Bolster
The knee bolster is used to absorb energy and control the driver's forward movement during an accident by limiting leg movement.Wiring Harness and Connectors
The wiring harness and connectors for the SIR system are of special design. Any wiring repairs should be done using tool kit J-38125, or equivalent. Always use the crimp and seal splice sleeves contained in this wiring repair kit. If damage is done to a component pigtail the component must be replaced.
DISARMING THE SYSTEM
See Figure 1
- Read all service precautions.
- Turn the steering wheel so wheels are facing straight ahead and turn the ignition switch to LOCK .
- Disconnect the negative battery cable and tape the end of cable to avoid possible terminal contact. Remove the SIR fuse.
- Remove the left side sound insulator and courtesy lamp, as needed.
- Remove the Connector Positive Assurance (CPA) lockpin on the yellow 2-way connector at the base of the steering column.
- Disconnect the yellow 2-way connector.
- The inflator module or steering can now be serviced.
ENABLING THE SYSTEM
- Reconnect the yellow connector at the base of the steering column.
- Reinstall the CPA lockpin on the yellow connector.
- Install the sound insulator and lamp, if removed.
- Install the SIR fuse and connect the battery cable.
- Turn ignition switch to ON and check that the INFLATABLE RESTRAINT lamp flashes 7-9 times and then goes out.