GM: Electra/Park Avenue/Ninety-Eight 1990-1993

Generation 1 Corporate SIR System


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Fig. Fig. 1 Sir senors schematics

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Fig. Fig. 2 Sir deployment loop

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Fig. Fig. 3 Sir system components

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Fig. Fig. 4 View of the knee bolster


The air bag system used on all vehicles covered in this guide except the 1990 Ninety Eight 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.

This system has an energy reserve, which can store a large enough electrical charge to deploy the air bag for 10 minutes after the battery has been disconnected or damaged. This system must be disabled if any service is to be performed on the sir system or steering wheel assembly.

The gen 1 system requires the replacement of all sensors and inflator module after a deployment. This system has selfdiagnostic ability. Some diagnostic codes can be obtained by counting flashes of the inflatable restraint lamp, but other codes can only be obtained or cleared by using a scan tool.

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.


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.

Discriminating Sensor

There can be 2, 3 or 4 discriminating sensors. The forward, left or right sensors are located in the engine compartment. 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

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 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.

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.

Passenger Compartment Discriminating Sensor

This sensor can either be part of the dual sensor or can be a separate sensor altogether.

Dual Sensor

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.


To avoid personal injury 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. Instructions must be followed in detail to avoid deployment.

Never disconnect any electrical connection with the ignition switch on unless instructed to do so in a test.
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.
Sensor mounting and wiring must never be modified.
Never strike or jar a sensor, or deployment could occur.
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 away from your body.
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 discarded.
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.
A Code 51 requires, at minimum, replacement of the arming sensor, passenger compartment discriminating sensor, forward discriminating sensor and the inflator module.
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 use a battery or A.C. powered test light or tester on the sir system or deployment could occur.
Never use an ohmmeter on the sir system, unless instructed to do so, or deployment could occur.
Never bake dry paint on vehicle or allow to exceed temperatures over 300°F, without disabling the sir system and removing the inflator module.
Sensors are not interchangeable between models or years.
Never allow welding cables to lay on, near or across any vehicle electrical wiring.
Avoid extension cords for power tools or droplights to lay on, near or across any vehicle electrical wiring.


  1. Disconnect the negative battery cable and isolate the cable end with high-quality electrical tape.
  3. Allow at least 15 minutes to expire prior to entering the interior of the vehicle. The sir system is designed to maintain sufficient energy to deploy the air bag for a short period of time even after battery voltage has been cut off.
  5. Remove the sir fuse from the interior fuse panel below the left side of the instrument panel.
  7. Remove the left side sound insulator.
  9. Remove the Connector Positive Assurance (cpa) lock from the yellow 2-way sir harness connector at the base of the steering column, then disconnect the connector.


  1. Connect the yellow 2-way sir connector at the base of the steering column and insert the Connector Positive Assurance (cpa) lock.
  3. Install the left side sound insulator.
  5. Install the sir fuse in the fuse panel.
  7. Connect the negative battery cable.
  9. Enter the vehicle from the passenger side and turn the ignition switch to the run position and observe the supplemental inflatable restraint lamp on the instrument cluster. The lamp should flash 7, 8, or 9 times, then stay off. If the lamp does anything else, have the system checked.


Proper operation of the sensors and sir system requires that any repairs to the vehicle structure return it to its original production configuration. Deployment (which will set a code) requires replacement of at least the inflator module, arming sensor, discriminating sensors or dual sensor, if equipped. Any visible damage to the sensors and/or their respective mounting brackets requires replacement, whether a deployment has occurred or not.

All components, including harnesses and brackets, must be inspected. Any wiring damage must only be repaired using repair kit J-38125-A. If the air bag was deployed, inspect the coil assembly for scorching or melting damage and replace if necessary. Never attempt to repair any parts. Service is by replacement only. Never interchange parts from another vehicle.

The steering column jacket assembly should be visually checked for damage and collapsing or felt for looseness when pushed or pulled by hand. The steering shaft should be checked for shearing of the injected plastic, requiring the replacement of the steering shaft. The steering shaft runout must be checked if the collision caused frame damage that could have bent the steering shaft.