GM Storm/Spectrum 1985-1993 Repair Guide

General Information


The air bag system used on the Storm is referred to as the Supplemental Inflatable Restraint (SIR) system. The SIR system provides additional protection for the driver, if a forward collision of sufficient force is encountered. The SIR assists the normal seatbelt restraining system by deploying an air bag, via the steering column. A knee bolster, located beneath the driver's side instrument panel, also aids in absorbing the collision's impact. The steering column, as in previous design, still continues to be collapsible.


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 (36VLR) 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.


See Figure 1

Diagnostic Energy Reserve Module

The Diagnostic Energy Reserve Module (DERM) is designed to perform 5 main functions. It maintains an energy reserve of 36 volts for several seconds and can maintain sufficient voltage to cause a deployment for up to 10 minutes after the ignition switch is turned OFF and the battery disconnected. 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.

The DERM is connected to the system with a 24 pin 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

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Fig. Fig. 1: Air bag component locations - Storm

The 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.

Arming Sensor

The arming sensor, mounted at the center of the upper dash panel, 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 connected to the 36VLR output of the DERM or ignition 1 voltage.

The sensor consists of a sensing element, a 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.

When the arming sensor is located in the same housing as the passenger compartment discrimination sensor, the assembly is referred to as the dual sensor.

Discrimination Sensor

There are 2 discriminating sensors wired in parallel on the low side of the deployment loop. The forward sensor is located in front of the radiator and the passenger sensor is located on the floor ahead of the transmission selector lever. These sensors are located on the low side of the deployment loop and are calibrated to close with velocity changes which are severe enough to warrant air bag deployment.

The sensors consist of a sensing element, 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.

SIR Coil Assembly

The coil assembly, or clockspring, connects the air bag to the DERM. It is attached to the steering column and allows the rotation of the steering wheel, while maintaining continuous electrical continuity to the air bag in case a deployment is needed.

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.

Air Bag Module

The SIR air bag module is located in the steering wheel. It includes the air bag, inflator and initiator. When the vehicle is in an accident of sufficient force, current is passed through the deployment loop. The current passing through the deployment loop ignited the material in the inflator module and produces a gas which 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 the DERM. The resistor 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 Pad

The knee pad is used to absorb energy and control the driver's forward movement during an accident by limiting leg movement.