GM Prizm/Nova 1985-1993 Repair Guide

General Description

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The Supplemental Restraint System (SRS) helps supplement the protection offered by the driver's seat belt by deploying an air bag from the center of the steering wheel. The air bag deploys when the vehicle is involved in a frontal crash of sufficient force up to 30 degrees off the centerline of the vehicle. To further absorb the crash energy there is a knee bolster located below the steering column for the driver. The steering column continues to be collapsible.

COMPONENT DESCRIPTION



Center Sensor Assembly

Do not open the center sensor assembly case for any reason. Touching the connector pins or soldered components may cause electrostatic discharge (ESD) damage. Repair of a faulty center sensor assembly is by replacement only.

The center sensor assembly is designed to perform the following functions in the SRS:

  1. Energy Reserve - The center sensor assembly maintains a 6 volt reserve power supply to provide deployment energy when the vehicle voltage is low or lost in a frontal crash.
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  3. Fault Detection - The center sensor assembly performs diagnostic monitoring of SRS electrical components.
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  5. Fault Recording - The center sensor assembly provides SRS diagnostic trouble code (DTC) information through the air bag indicator.
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  7. Driver Notification - The center sensor assembly warns the vehicle driver of SRS faults by controlling the air bag indicator.
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  9. Safety Circuit - The center sensor assembly prevents SRS operation by opening the power feed to the arming sensor when an SRS fault is detected.
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  11. Arming Sensor - The arming sensor is contained within the center sensor assembly.
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  13. Passenger Compartment Sensor - The passenger compartment sensor is contained within the center sensor assembly.
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The center sensor assembly is connected to the vehicle harness by a 10-cavity connector. This harness connector uses a shorting bar across two terminals B7 to B8 in the contact area. This shorting bar shorts the inflator module circuits (D+ and D-) together when the center sensor assembly connector B is disconnected. This will prevent accidental deployment of the air bag. The center sensor assembly is also connected to each forward sensor by two 2-cavity connectors.

Air Bag Indicator

Battery voltage is applied to the air bag indicator at all times. The center sensor assembly controls the indicator by providing ground with a lamp driver. The air bag indicator is used in the SRS to do the following:

  1. Verify indicator and center sensor assembly operation by illuminating steadily for approximately 6 seconds when the ignition switch is first turned to ACC or ON .
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  3. Warn the vehicle driver of SRS electrical system faults which have affected the operation of the SRS. When the center sensor assembly detects an SRS fault, the safety circuit opens the power feed to the arming sensor and prevents SRS operation.
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  5. Provide diagnostic information by flashing diagnostic trouble codes (DTCs) when the Flash Code diagnostic mode is enabled.
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The air bag indicator is the key to driver notification of SRS faults.

Arming Sensor

The arming sensor is a protective switch located on the power feed (high) side of the deployment loop in the center sensor assembly. The arming sensor is calibrated to close at low level velocity changes (lower than the other three sensors). This ensures that the inflator module is connected directly to the 6 volt reserve power supply, ignition voltage or accessory voltage when either the LH forward sensor, RH forward sensor or center sensor closes.

The arming sensor consists of a sensing element and normally open switch contacts. The sensing element closes the switch contacts when the velocity of the vehicle changes at a rate indicating potential need for deployment.

Forward Sensors

There are two forward sensors in the SRS. The LH forward sensor is located inside the left wheelwell and the RH forward sensor is located inside the right wheelwell. The forward sensors are protective switches wired in parallel on the ground (low) side of the deployment loop. These sensors are calibrated to close with velocity changes which are severe enough to warrant deployment.

Each forward sensor consists of a sensing element, normally open switch contact and a diagnostic resistor. The sensing element closes the switch contacts when the vehicle velocity changes are server enough to warrant deployment.

A diagnostic resistor is connected in parallel with the switch contacts and connector within each of the forward sensors. These parallel resistors supply ground path for the current passing through the deployment loop during normal, nondeployment operation. This current flow results in voltage drops across each component within the loop. The center sensor assembly monitors these voltage drops to detect circuit or component faults. In order for the diagnostic resistor to function properly, the forward sensor connectors must be fully engaged to the sensors. There is an electrical check mechanism inside of the connector which completes the diagnostic resistor circuit.

Passenger Compartment Sensor

The passenger compartment sensor is located inside the center sensor assembly, mounted to the floor under the center console. The passenger compartment sensor is a protective switch wired in parallel on the ground (low) side of the deployment loop. This sensor is calibrated to close with velocity changes which are severe enough to warrant deployment. The passenger compartment sensor is a semiconductor (solid state) sensor.

SRS Coil Assembly

The SRS coil assembly consists of current-carrying coils. The SRS coil assembly is combined with the combination switch and mounted together on the steering column, allowing rotation of the steering wheel while maintaining continuous contact of the deployment loop to the inflator module.

There is a shorting bar on the lower steering column connector which connects the SRS coil assembly to the wiring harness. This shorting bar shorts the circuits of the SRS coil assembly and inflator module when the lower steering column connector is disconnected. The circuit to the inflator module is shorted in this way to help prevent unwanted deployment of the air bag when servicing the steering column or other SRS components.

Inflator Module

The inflator module is located in the center of the steering wheel. The inflator module consists of an inflatable bag and an inflator (a canister of gas-generating material and an initiating device). When the vehicle is in a frontal crash of sufficient force to close the arming sensor and at least one of the other three sensors, current flows through the deployment loop. Current passing through the initiator thus ignites the material in the inflator module. The gas produced from this reaction rapidly inflates the air bag.

There is a shorting bag on the inflator module side of the upper steering column connector which connects the SRS coil assembly to the inflator module. The shorting bar shorts across the inflator module circuits when the upper steering column connector is disconnected. The circuit to the inflator module is shorted in this way to prevent unwanted deployment of the air bag when servicing the inflator module or other SRS components.

Steering Column

The steering column is energy absorbing and is designed to compress in a frontal crash to decrease the chance of injury to the driver.

Knee Bolster

The knee bolster is used to absorb energy and control the forward movement of the driver during a frontal crash, by limiting leg movement.

SERVICE PRECAUTIONS




CAUTION
The procedures in this section must be followed in the order listed to temporarily disable the Supplemental Restraint System (SRS) and prevent false diagnostic trouble codes (DTCs) from setting. Failure to follow procedures could result in possible air bag deployment, personal injury or otherwise unneeded SRS repairs.

The center sensor assembly can maintain sufficient voltage to cause a deployment for up to two minutes after the ignition switch is turned to LOCK or the battery is disconnected. Many service procedures require disconnection of the IGN fuse, CIG & RADIO fuse and the steering column circuit from the deployment loop to avoid an accidental deployment.

DISABLING THE SRS



See Figures 1 and 2

  1. Turn the steering wheel so that the vehicle's wheels are pointing straight ahead.
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  3. Turn the ignition switch to LOCK .
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  5. Disconnect IGN fuse and CIG and RADIO fuse from junction block 1.2. Disconnect the connector position assurance (CPA) and disconnect the lower steering column (yellow two cavity) connector at the base of the steering column.
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Click image to see an enlarged view

Fig. Fig. 1: Junction block 1



Click image to see an enlarged view

Fig. Fig. 2: Lower steering column connector

ENABLING THE SRS



  1. Turn the ignition switch to LOCK .
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  3. Connect the lower steering column connector and secure with CPA.
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  5. Connect IGN fuse and CIG and RADIO fuse to junction block 1.
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  7. Turn the ignition switch to ACC or ON and verify that the air bag indicator illuminates steadily for approximately six seconds and then turns off. If it does not operate as described, SRS Diagnostic System Check will have to be performed by an authorized dealer.
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HANDLING/INSTALLATION/DIAGNOSIS



  1. Inflator modules should not be stored at temperatures above 150°F (65°C).
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  3. Forward sensors, inflator modules and center sensor assemblies should not be used if they have been dropped from a height of 91.4 cm (3-feet) or more.
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  5. When a forward sensor is replaced, it MUST be oriented with the arrow on the sensor pointing toward the front of the vehicle. It is very important for forward sensors and the center sensor assembly to be located flat on the mounting surface, parallel with the vehicle datum line. It is important that the sensor mounting surface is free of any dirt or other foreign material.
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  7. Do not apply power to the SRS unless all components are connected or a diagnostic chart requests it. Powering up the SRS with components disconnected will set a diagnostic trouble code (DTC).
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  9. The SRS Diagnostic System Check must be the starting point of any SRS diagnostics. The SRS Diagnostic System Check should be performed by an authorized dealer and will verify proper air bag indicator operation and will lead you to the correct chart to diagnose any SRS faults. Bypassing these procedures may result in extended diagnostic time, incorrect diagnosis and incorrect parts replacement.
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