GM Metro/Sprint 1985-1993 Repair Guide

General Information


The air bag system used on GEO vehicles is referred to as 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 sea belt restraining system by deploying an air bag, via the steering column. A knee pad, located beneath the the driver's side instrument panel, also aid in absorbing collision 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 Reverse (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 Figures 1 and 2

Click image to see an enlarged view

Fig. Fig. 1: Supplemental inflatable restraint (SIR) system component locations

Click image to see an enlarged view

Fig. Fig. 2: Steering wheel connector used to disarm the system

Diagnostic Energy Reserve Module (DERM)

The DERM is designed to perform 5 main functions. It maintains an energy reverse 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 turn 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

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 connect to the 36VLR 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.

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

Discriminating 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. The passenger sensor is located under the console near the DERM. These sensors are located on the low side of the deployment loop and are calibrated to close with velocity changes which are sever 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 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. 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 ignites 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 Bolster

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


To avoid deployment when servicing the SIR system or components in the immediate area, do not use electrical test equipment such as battery or A.C. powered voltmeter, ohmmeter, etc. or any type of tester other than specified. Do not use a non-powered probe tester. To avoid personal injury, all precautions must be strictly adhered to.

Never disconnect any electrical connection with the ignition switch ON unless instructed to do so in a test.
Before disconnecting the negative battery cable, make a record of the contents memorized by each memory system like the clock, audio, etc. When service or repairs are completed make certain to reset these memory system.
Always wear a grounded wrist static strap when servicing any control module or component labeled with a Electrostatic Discharge (ESD) sensitive device symbol.
Avoid touching module connector pins.
Leave new components and modules in the shipping package until ready to install them.
Always touch a vehicle ground after sliding across a vehicle seat or walking across vinyl or carpeted floors to avoid static charge damage.
The DERM can maintain sufficient voltage to cause a deployment for up to 10 minute, even if the battery is disconnected.
All sensors are specifically calibrated to each series vehicle's series. The sensors, mounting brackets and wiring harness must never be modified from original design.
Never strike or jar a sensor, or deployment could happen.
Never power up the SIR system when any sensor is not rigidly attached to the vehicle.
Always carry an inflator module with the trim cover facing away.
Always place an inflator module on the workbench with the trim cover up, away from loose objects.
The inflator module is to be stored and shipped under DOT E-8236 flammable solid regulations.
The inflator module must be deployed before it is scrapped.
After deployment the air bag surface may contain sodium hydroxide dust. Always wear gloves and safety glasses when handling the assembly. Wash hands with mild soap and water afterwards.
Any visible damage to sensors requires component replacement.
Wire and connector repair must be performed using kit J-38125-A, or equivalent. Use special crimping tools, heat torch and seals.
Absolutely no wire connector, or terminal repair is to be attempted on the arming sensor, passenger compartment discriminating sensor, forward discriminating sensor, inflator module or SIR coil assembly
Never bake dry paint on vehicle or allow to exceed temperatures over 300°F, without disabling the SIR system and removing the inflator module.
Do not interchange sensors between models or years.
Do not install used SIR system parts from another vehicle.
Never allow welding cables to lay on, near or across any vehicle electrical wiring.


  1. Turn the ignition switch OFF .
  3. Remove the SIR IG fuse from the SIR fuse block.
  5. Remove the access cover at the back of the steering wheel to gain access to the 2-way SIR connector.
  7. Remove the Connector Position Assurance (CPA) lock and disconnect the yellow 2-way connector inside the inflator module housing.


  1. Turn the ignition switch OFF .
  3. Reconnect the yellow 2-way connector at the back of the steering wheel and install the CPA lock.
  5. Install the SIR IG fuse.
  7. Refit the rear plastic access cover to the inflator module housing.
  9. Turn the ignition switch ON and observe the inflatable restraint indicator lamp. If the lamp does not flash 7 to 9 times and then remain OFF, perform the SIR diagnostic system check.